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Propeller walk
What is the primary cause of propeller walk - torque or thrust?
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Propeller walk
On 5 Dec, 22:37, "Charles Momsen" wrote:
What is the primary cause of propeller walk - torque or thrust? Who cares? The important thing is that you know how your boat will perform. Regards Donal |
Propeller walk
On Dec 5, 4:37*pm, "Charles Momsen" wrote:
What is the primary cause of propeller walk - torque or thrust? Good question. I would think thrust. Touque would only tend to roll the boat or cause it to tip. A cavitating prop will not walk. Joe |
Propeller walk
wrote in message ... On 5 Dec, 22:37, "Charles Momsen" wrote: What is the primary cause of propeller walk - torque or thrust? Who cares? The important thing is that you know how your boat will perform. Regards Donal Go sit in the corner with Ganz. |
Propeller walk
"Charles Momsen" wrote in message ... What is the primary cause of propeller walk - torque or thrust? Propeller walk is a term used to describe the results of the bottom of the propeller operating in a more dense environment than the top of the propeller. The primary factor is neither of the above. Drag, due to the propeller's interaction with a denser medium, is increased at the bottom half of the propeller. A prop turning clockwise as the vessel proceeds forward as seen from astern will cause the vessel's stern to move to starboard due to increased drag. Whereas thrust in a vessel is a description of fore and aft forces, prop walk is a description of transverse forces caused by drag differential. I hope this helps clarify what is a muddled discussion point in most cases - especially where I do not deem to impart my brilliance. Wilbur Hubbard |
Propeller walk
It's actually not a particularly "good" question.
Both forces are at work, along with water density, angle of drive shaft, differences in drag in forward vs. reverse, etc. For a RH (clockwise) prop movement in forward, the front of the boat tends to turn to port, all things being equal. In reverse, the stern turn to port. On my Sabre, they designed the engine (and shaft) to be mounted off center (shaft exists the boat slightly to the port) in order to reduce/eliminate forward prop walk. From the manual: OFFSET PROPELLERS: Sabre yachts are built with propeller shafts offset to port or starboard depending on engine rotation. Engines with clockwise rotation (right hand) have shafts offset to port. After extensive research it was found that for this hull type an offset propeller benefited engine performance and handling characteristics. Sabre rudders are partially balanced with the rudder shaft about 15% aft of its leading edge. This feature reduces the load on the helm and makes it more comfortable to hold while under sail. The powerful wash of a centerline propeller causes the helm to veer to one side or the other after a turn is initiated. The offset propeller wash to flow to the side of the rudder. A second benefit is that the angle between the shaft and the fore and aft axis of the boat is less than would be possible with a centerline installed shaft. Here's a decent link that describes the forces involved: http://www.sailingislife.com/id28.html. "Joe" wrote in message ... What is the primary cause of propeller walk - torque or thrust? Good question. I would think thrust. Touque would only tend to roll the boat or cause it to tip. A cavitating prop will not walk. Joe -- "j" ganz @@ www.sailnow.com |
Propeller walk
"Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... What is the primary cause of propeller walk - torque or thrust? Propeller walk is a term used to describe the results of the bottom of the propeller operating in a more dense environment than the top of the propeller. The primary factor is neither of the above. Drag, due to the propeller's interaction with a denser medium, is increased at the bottom half of the propeller. A prop turning clockwise as the vessel proceeds forward as seen from astern will cause the vessel's stern to move to starboard due to increased drag. Whereas thrust in a vessel is a description of fore and aft forces, prop walk is a description of transverse forces caused by drag differential. That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. .. Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. |
Propeller walk
"Edgar" wrote in message
... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. -- "j" ganz @@ www.sailnow.com |
Propeller walk
"Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? |
Propeller walk
On Dec 6, 2:02*pm, "Wilbur Hubbard"
wrote: "Charles Momsen" wrote in message ... What is the primary cause of propeller walk - torque or thrust? Propeller walk is a term used to describe the results of the bottom of the propeller operating in a more dense environment than the top of the propeller. The primary factor is neither of the above. Drag, due to the propeller's interaction with a denser medium, is increased at the bottom half of the propeller. A prop turning clockwise as the vessel proceeds forward as seen from astern will cause the vessel's stern to move to starboard due to increased drag. Whereas thrust in a vessel is a description of fore and aft forces, prop walk is a description of transverse forces caused by drag differential. I hope this helps clarify what is a muddled discussion point in most cases - especially where I do not deem to impart my brilliance. Wilbur Hubbard Trust is thrust fore and aft port and stbd. Please don't muddy the waters. To truly walk a whole boat sideways you need twin screws, 3 is best. The drag differentail is caused by varying thrust off the blades... the angle of attack so to speak that thrust is transfered to the shaft. Its up and port walk for a right hand screw, and down and starboard for a left handed screw. Due to the weight of water it's easier to push/slice down than up causing the pressure and thrust difference off the fluke. It's the path of least resistance. Joe |
Propeller walk
"Edgar" wrote in message
... "Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? Not an engineer. However, there is a difference in the "pressure" between the high and low of the prop. I have a hydrostatic PFD (vs. the old pill melting) autoinflate. It's a good choice if you go offshore or sail dinghies, and you don't want the spray to trigger the PFD. I believe it's activated in 2' of water, but don't quote me. I had the pill melt on me once while sailing a dinghy, and I don't want to wear the bulky PFDs. My understanding is that it's only a small part of the effect. As most of us know, the prop wash can be observed on the opposite side from the direction the prop walk forces the stern (usually, the prop wash is observed most strongly on the starboard side, thus the stern moves to port). This is much like a fire hose that forces water out the business end and pushes against the person holding the hose. In the case of the boat, the stern moves in the opposite because there's nothing but some water to oppose it. I think Donal said it best, that it really doesn't matter, since the issue we're all concerned with is how to either use it or deal with it. I find pulsing the engine works vs. actually putting the engine in neutral. I get 90% of the benefit with 1/2 the hassle. -- "j" ganz @@ www.sailnow.com |
Propeller walk
"Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... "Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? Not an engineer. However, there is a difference in the "pressure" between the high and low of the prop. I have a hydrostatic PFD (vs. the old pill melting) autoinflate. It's a good choice if you go offshore or sail dinghies, and you don't want the spray to trigger the PFD. I believe it's activated in 2' of water, but don't quote me. I had the pill melt on me once while sailing a dinghy, and I don't want to wear the bulky PFDs. Yes, OK that is hydrostatic pressure but nothing to do with prop walk. My understanding is that it's only a small part of the effect. As most of us know, the prop wash can be observed on the opposite side from the direction the prop walk forces the stern (usually, the prop wash is observed most strongly on the starboard side, thus the stern moves to port). This is much like a fire hose that forces water out the business end and pushes against the person holding the hose. In the case of the boat, the stern moves in the opposite because there's nothing but some water to oppose it. A hose stays in one position and certainly produces a reaction force that you have to resist in order to hold it still. But the prop is rotating and consequently the sideways force it generates is also rotating. So, as I said, one has to explain why the force to one side produced while the blade is at the bottom is not balanced by the opposite force produced while it is at the top. This is what I tried to explain in my first post. Neal's explanation was quite wrong and I am quite sure he knows it.. I think Donal said it best, that it really doesn't matter, since the issue we're all concerned with is how to either use it or deal with it. I find pulsing the engine works vs. actually putting the engine in neutral. I get 90% of the benefit with 1/2 the hassle. I fully agree with that. Pulsing is the best way to make use of prop walk. Now Joe has just said that to really walk a boat you need twin screws and that is not right either. With twin screws you are turning the boat by regulating the thrust by going ahead on one and astern on the other as necessary. However, I have seen fishermen in heavy deep draught traditional boats with big single screws that could park them perfectly alongside in a narrow gap using prop walk while carrying on a conversation with the guy in the next boat! I have handled such a boat and can vouch for the usefulness of the effect once you are used to your boat's characteristics. My own boat is 38' and has a short(ish) central keel, no skeg and a spade rudder. You can spin her on the proverbial dime ahead or astern once you have steerage way but backing out of my marina slot needs great care because prop walk when you first engage reverse from rest is a real problem especially if a cross wind makes it worse, and I always have to be careful that my pulpit does not gouge the side of the power boat in the next slot before I have steerage way. |
Propeller walk
"Edgar" wrote in message
... "Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... "Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? Not an engineer. However, there is a difference in the "pressure" between the high and low of the prop. I have a hydrostatic PFD (vs. the old pill melting) autoinflate. It's a good choice if you go offshore or sail dinghies, and you don't want the spray to trigger the PFD. I believe it's activated in 2' of water, but don't quote me. I had the pill melt on me once while sailing a dinghy, and I don't want to wear the bulky PFDs. Yes, OK that is hydrostatic pressure but nothing to do with prop walk. Actually, I think it does, although prop walk is also related to Newton's 3rd law, the angle of the shaft and the pitch of the prop. There *is* a hydrostatic pressure difference between the top and the bottom. Not being an engineer, I don't know how to calculate the diff or the effect. Another way to look it is, it seems to me, is to look at a limiting case. Take the paddle wheel. The paddle wheel pushes the water to cause the boat to go forward (or reverse, whatever) by resistance against the water (N's 3rd), but there is pressure against the paddles above the surface of the water... minimal though it is. I wonder what the effect would be if the paddle wheel were tilted? It seems to me that that would be similar to the situation approaching a normal sailboat prop. I wonder if this has ever been tried. My understanding is that it's only a small part of the effect. As most of us know, the prop wash can be observed on the opposite side from the direction the prop walk forces the stern (usually, the prop wash is observed most strongly on the starboard side, thus the stern moves to port). This is much like a fire hose that forces water out the business end and pushes against the person holding the hose. In the case of the boat, the stern moves in the opposite because there's nothing but some water to oppose it. A hose stays in one position and certainly produces a reaction force that you have to resist in order to hold it still. But the prop is rotating and consequently the sideways force it generates is also rotating. So, as I said, one has to explain why the force to one side produced while the blade is at the bottom is not balanced by the opposite force produced while it is at the top. This is what I tried to explain in my first post. Neal's explanation was quite wrong and I am quite sure he knows it.. See previous. My continued understanding it that's a combination of several things. I think Donal said it best, that it really doesn't matter, since the issue we're all concerned with is how to either use it or deal with it. I find pulsing the engine works vs. actually putting the engine in neutral. I get 90% of the benefit with 1/2 the hassle. I fully agree with that. Pulsing is the best way to make use of prop walk. Now Joe has just said that to really walk a boat you need twin screws and that is not right either. With twin screws you are turning the boat by regulating the thrust by going ahead on one and astern on the other as necessary. I believe twin screws turning in opposite directions is one solution. Another is restricting the prop blades inside a tube. Again, the limiting case is a long, narrow tube with a prop inside. You don't get prop walk with this arrangement, but it's expensive to make/use. However, I have seen fishermen in heavy deep draught traditional boats with big single screws that could park them perfectly alongside in a narrow gap using prop walk while carrying on a conversation with the guy in the next boat! I have handled such a boat and can vouch for the usefulness of the effect once you are used to your boat's characteristics. I've seen such but have never been on one, at least not in a position to observe the helm maneauvers. My own boat is 38' and has a short(ish) central keel, no skeg and a spade rudder. You can spin her on the proverbial dime ahead or astern once you have steerage way but backing out of my marina slot needs great care because prop walk when you first engage reverse from rest is a real problem especially if a cross wind makes it worse, and I always have to be careful that my pulpit does not gouge the side of the power boat in the next slot before I have steerage way. Yes, I have the same issue. Fortunately, I'm able to basically let her go in reverse and use the prop walk to make a big U turn in reverse to get out. I have my students keep the wheel hard to starboard until the bow is clear, then gently turn back to port. The starboard wheel will keep her straight long enough to get moving in a straight line, and I could keep backing straight with the pulsing technique, but I'd run out of room in the fairway. -- "j" ganz @@ www.sailnow.com |
Propeller walk
On Dec 7, 12:50*pm, "Edgar" wrote:
"Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... "Capt. JG" wrote in message areasolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? *Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. *I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? Not an engineer. However, there is a difference in the "pressure" between the high and low of the prop. I have a hydrostatic PFD (vs. the old pill melting) autoinflate. It's a good choice if you go offshore or sail dinghies, and you don't want the spray to trigger the PFD. I believe it's activated in 2' of water, but don't quote me. I had the pill melt on me once while sailing a dinghy, and I don't want to wear the bulky PFDs. Yes, OK that is hydrostatic pressure but nothing to do with prop walk. My understanding is that it's only a small part of the effect. As most of us know, the prop wash can be observed on the opposite side from the direction the prop walk forces the stern (usually, the prop wash is observed most strongly on the starboard side, thus the stern moves to port). This is much like a fire hose that forces water out the business end and pushes against the person holding the hose. In the case of the boat, the stern moves in the opposite because there's nothing but some water to oppose it. A hose stays in one position and certainly produces a reaction force that you have to resist in order to hold it still. *But the prop is rotating and consequently the sideways force it generates is also rotating. So, as I said, one has to explain why the force to one side produced while the blade is at the bottom is not balanced by the opposite force produced while it is at the top. This is what I tried to explain in my first post. *Neal's explanation was quite wrong and I am quite sure he knows it.. I think Donal said it best, that it really doesn't matter, since the issue we're all concerned with is how to either use it or deal with it. I find pulsing the engine works vs. actually putting the engine in neutral. I get 90% of the benefit with 1/2 the hassle. I fully agree with that. Pulsing is the best way to make use of prop walk.. Now Joe has just said that to really walk a boat you need twin screws and that is not right either. With twin screws you are turning the boat by regulating the thrust by going ahead on one and astern on the other as necessary. *However, I have seen fishermen in heavy deep draught traditional boats with big single screws that could park them perfectly alongside in a narrow gap using prop walk while carrying on a conversation with the guy in the next boat! I have handled such a boat and can vouch for the usefulness of the effect once you are used to your boat's characteristics. My own boat is 38' and has a short(ish) central keel, no skeg and a spade rudder. You can spin her on the proverbial dime ahead or astern once you have steerage way but backing out of my marina slot needs great care because prop walk when you first engage reverse from rest is a real problem especially if a cross wind makes it worse, and I always have to be careful that my pulpit does not gouge the side of the power boat in the next slot before I have steerage way. *- Hide quoted text - - Show quoted text - If you put a single screw in reverse it will pull to one side or the other depending on rotation. Your understanding of a twin screw is pivoting a boat, not walking it, big difference . With a single screw you can only pivot then counter with a trust. With a twin you can check the bow movement and move sideways. A Captain Ron style landing is about the best you can do with a single screw http://www.youtube.com/watch?v=8alNxLjCBJc i suspect this is what you refer to the fisherman doing in narrow canals. That is using wheel walk, but is not walking a boat. Walking a boat is moving it sideways not having and fore and aft movement at all ...and no turning. this can only be done and distance or period of time with a twin screw vessel with inboard rotating screws. Joe |
Propeller walk
"Joe" wrote in message
... On Dec 7, 12:50 pm, "Edgar" wrote: "Capt. JG" wrote in message easolutions... "Edgar" wrote in message ... "Capt. JG" wrote in message areasolutions... "Edgar" wrote in message ... troll sh*t removed That is the easy answer that seems to satisfy most people but if you think about it do you really believe that water gets measurably denser because it is about 18" (say) deeper at the bottom of an 18"(say) propeller than the water at the top? Of course it does not. Water is hardly compressible at all except at very high pressures. . Density will only be a factor if air is being entrained from the surface. You will find boats displaying propeller walk even if the propeller is too deep for air entrainment to occur. My boat does. So the question is 'why does the opposite direction of the blade at the top not cancel the walk created by the blade at the bottom'? I suggest that this is because as the blades rise upward towards the top the water impinges on the hull and creates a reaction force which acts in the same direction as the walk force created at the bottom. This is because, in yachts especially, the blades pass very close to the hull which usually is somewhat vee shaped and deflects the water sideways. In your example water would be deflected to port but the reaction on the hull would be a starboard force. Think hydrostatic pressure. Also, Neal is wrong. Yes, he certainly is. I have tried your suggestion about thinking 'hydrostatic pressure' but it is the 'static' bit that is somewhat puzzling me because propeller walk is a dynamic phenomenon. Can you expand on this a bit? Not an engineer. However, there is a difference in the "pressure" between the high and low of the prop. I have a hydrostatic PFD (vs. the old pill melting) autoinflate. It's a good choice if you go offshore or sail dinghies, and you don't want the spray to trigger the PFD. I believe it's activated in 2' of water, but don't quote me. I had the pill melt on me once while sailing a dinghy, and I don't want to wear the bulky PFDs. Yes, OK that is hydrostatic pressure but nothing to do with prop walk. My understanding is that it's only a small part of the effect. As most of us know, the prop wash can be observed on the opposite side from the direction the prop walk forces the stern (usually, the prop wash is observed most strongly on the starboard side, thus the stern moves to port). This is much like a fire hose that forces water out the business end and pushes against the person holding the hose. In the case of the boat, the stern moves in the opposite because there's nothing but some water to oppose it. A hose stays in one position and certainly produces a reaction force that you have to resist in order to hold it still. But the prop is rotating and consequently the sideways force it generates is also rotating. So, as I said, one has to explain why the force to one side produced while the blade is at the bottom is not balanced by the opposite force produced while it is at the top. This is what I tried to explain in my first post. Neal's explanation was quite wrong and I am quite sure he knows it.. I think Donal said it best, that it really doesn't matter, since the issue we're all concerned with is how to either use it or deal with it. I find pulsing the engine works vs. actually putting the engine in neutral. I get 90% of the benefit with 1/2 the hassle. I fully agree with that. Pulsing is the best way to make use of prop walk. Now Joe has just said that to really walk a boat you need twin screws and that is not right either. With twin screws you are turning the boat by regulating the thrust by going ahead on one and astern on the other as necessary. However, I have seen fishermen in heavy deep draught traditional boats with big single screws that could park them perfectly alongside in a narrow gap using prop walk while carrying on a conversation with the guy in the next boat! I have handled such a boat and can vouch for the usefulness of the effect once you are used to your boat's characteristics. My own boat is 38' and has a short(ish) central keel, no skeg and a spade rudder. You can spin her on the proverbial dime ahead or astern once you have steerage way but backing out of my marina slot needs great care because prop walk when you first engage reverse from rest is a real problem especially if a cross wind makes it worse, and I always have to be careful that my pulpit does not gouge the side of the power boat in the next slot before I have steerage way. - Hide quoted text - - Show quoted text - +If you put a single screw in reverse it will pull to one side or the +other depending on rotation. + +Your understanding of a twin screw is pivoting a boat, not walking it, +big difference . + +With a single screw you can only pivot then counter with a trust. +With a twin you can check the bow movement and move sideways. + +A Captain Ron style landing is about the best you can do with a single +screw +http://www.youtube.com/watch?v=8alNxLjCBJc +i suspect this is what you refer to the fisherman doing in narrow +canals. +That is using wheel walk, but is not walking a boat. + +Walking a boat is moving it sideways not having and fore and aft +movement at all ...and no turning. +this can only be done and distance or period of time with a twin screw +vessel with inboard rotating screws. You'll still have prop walk in a twin screw if the props are turning in the same direction. If you have separate controls on the engines, then it isn't an issue, since as you said, you can compensate. If you have props turning in opposite directions without dual controls, you can also eliminate prop walk. The Capt. Ron video is cool of course. Prop walk is quite useful, e.g., in narrow situations, with the back and fill technique, where you use prop walk in reverse to both stop forward momentum and move the bow/stern in the direction you want to complete the turn in a small area. -- "j" ganz @@ www.sailnow.com |
Propeller walk
You'll still have prop walk in a twin screw if the props are turning in the same direction. If you have separate controls on the engines, then it isn't an issue, since as you said, you can compensate. uhhh.. Have you ever seen in your life, a twin screw that did not have "Separate" controls? I curious.... what was the application? If you have props turning in opposite directions without dual controls, you can also eliminate prop walk. That would be counter acting, not eliminating. To eliminate it you need a Voith set-up. Joe The Capt. Ron video is cool of course. -- "j" ganz - Hide quoted text - - Show quoted text - |
Propeller walk
"Joe" wrote in message
... You'll still have prop walk in a twin screw if the props are turning in the same direction. If you have separate controls on the engines, then it isn't an issue, since as you said, you can compensate. uhhh.. Have you ever seen in your life, a twin screw that did not have "Separate" controls? I curious.... what was the application? Never said I did. I said that if you had props turning in the opposite direction, the prop walk effect would be eliminated. You can play games with the semantics, but it's still going to be gone. If you have props turning in opposite directions without dual controls, you can also eliminate prop walk. -- "j" ganz @@ www.sailnow.com |
Propeller walk
"Joe" wrote in message
... You'll still have prop walk in a twin screw if the props are turning in the same direction. If you have separate controls on the engines, then it isn't an issue, since as you said, you can compensate. uhhh.. Have you ever seen in your life, a twin screw that did not have "Separate" controls? I curious.... what was the application? If you have props turning in opposite directions without dual controls, you can also eliminate prop walk. That would be counter acting, not eliminating. To eliminate it you need a Voith set-up. Joe The Capt. Ron video is cool of course. -- "j" ganz - Hide quoted text - - Show quoted text - FYI, a quick google: http://www.battleships-cruisers.co.u...htm#HMS%20Arun -- "j" ganz @@ www.sailnow.com |
Propeller walk
"Capt. JG" wrote in message ons... It's actually not a particularly "good" question. Here's a decent link that describes the forces involved: http://www.sailingislife.com/id28.html. That's not a particularly "good" answer. The bulk modulus of water is such that the density differential at 14 inches is neglible. Plus if density was a contributing factor, it occurs in a vertical profile and is zero difference is a horizontal profile, so it would not cause the boat to walk, but rather to pitch. The torque of the propeller does twist the boat and the thrust lateral to the propeller directed toward the hull and redirected by the angle of the hull. A right hand prop leans the boat to the left and the starboard bottom of the hull directs more thrust laterally than downward compared to the port side and hence the walk to the port. The angle of the driveshaft has nothing to do with walk. The asymmetry is vertical, not horizontal, in the thrust difference. |
Propeller walk
This graph:
http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Now if the propeller shaft mounted off centerline reduces the walk, this would indicate that sideways (lateral) thrust from the propeller directed against the hull is a factor. Also with the shaft off center the boat is less likely to "torque" against the propeller. It's always good to find physical evidence to support a conjecture. |
Propeller walk
"Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard |
Propeller walk
"Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? 1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi. http://www.youtube.com/watch?v=u5KKcBQen-Y |
Propeller walk
"Charles Momsen" wrote in message ... "Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? Don't think so. Drag increases to a greater extent than lift. If this weren't the case your average airplane or boat propeller would never cavitate. 1 foot of depth gives a pressure differential of .06 atmospheres or .98 psi. http://www.youtube.com/watch?v=u5KKcBQen-Y Excellent! That is one static psi difference in a foot. Take a three-bladed prop with a surface area of about 600 square inches and you're dragging the blades of the damned thing through 600 psi on the bottom half? Correct? What kind of pressure do you have on the top half? 500PSI? Makes your mere ..98 psi seem suddenly more significant, doesn't it? Wilbur Hubbard So you drag the top of the prop through |
Propeller walk
On Dec 11, 12:00*pm, "Wilbur Hubbard"
wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density *would change .0004% over the diameter (tip to tip) of a 12 inch propeller. *So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? *Don't think so. Think, Momsen, think! *There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe |
Propeller walk
"Charles Momsen" wrote in
: "Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin...ity.html&edu=h igh Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Wouldn't lift increase in the same proportion as drag? Yes, you are absolutely correct. blondie |
Propeller walk
"Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. It was only with the great minds combined here on ASA that such previous unexplained hydrodynamic effects have come to full understanding for the greater benefit of mankind. Yes, ASA is slowly rising from the dismal swamp it had once become to shining paragon of truth, justice and the American way! Good work to all, and let's keep our noses to the grindstone! Admiral Momsen |
Propeller walk
"Charles Momsen" wrote in message ... "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. Already a false assumption. The vortex, like that from a tip of an airplane wing will angle upwards. Why? because water pressure lessens towards the surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. I resent these remarks. As I have shown density and pressure have everything to do with prop walk. It was only with the great minds combined here on ASA that such previous unexplained hydrodynamic effects have come to full understanding for the greater benefit of mankind. Yes, ASA is slowly rising from the dismal swamp it had once become to shining paragon of truth, justice and the American way! Good work to all, and let's keep our noses to the grindstone! Admiral Momsen It will only rise from the abyss when Pansy Ganzy throws in the towel. Wilbur Hubbard |
Propeller walk
"Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. Already a false assumption. The vortex, like that from a tip of an airplane wing will angle upwards. Why? because water pressure lessens towards the surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. I resent these remarks. As I have shown density and pressure have everything to do with prop walk. It was only with the great minds combined here on ASA that such previous unexplained hydrodynamic effects have come to full understanding for the greater benefit of mankind. Yes, ASA is slowly rising from the dismal swamp it had once become to shining paragon of truth, justice and the American way! Good work to all, and let's keep our noses to the grindstone! Admiral Momsen It will only rise from the abyss when Pansy Ganzy throws in the towel. Wilbur Hubbard Have you not noticed that when the dialogue is more sailing related, less political, involves rationality, knowledge and gives less opportunity for personal attacks his participation noticeably wanes? It's a sad, but true observation. Hopefully he can find it within himself to deal with others as equals, rather than from a drippy point of condensation. His mention of prop wash was invaluable in the search for the truth about prop walk. |
Propeller walk
Charles Momsen wrote:
"Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin |
Propeller walk
"Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin If it's the explanation I'm thinking of it involves the total path length traveled by the blade tip on either ascent or descent (with the boat moving forward). I could see the argument there but could not resolve if a longer path resulted in less thrust force or more and the same with the short path length. This same argument applies to retreating blade stall in helicopters. I'll look through Chapman and see what he says. The angle of attack argument only holds if the velocity of water is significant compared to the tangential velocity of the blade. If the water velocity is zero then the angle of attack is the same regardless of orientation. If the blade is turning 500 rpm and is 12 inch diameter the tip is moving at 314 inches/sec or 214 mph. |
Propeller walk
"Charles Momsen" wrote in message ... "Wilbur Hubbard" wrote in message anews.com... "Charles Momsen" wrote in message ... "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. Already a false assumption. The vortex, like that from a tip of an airplane wing will angle upwards. Why? because water pressure lessens towards the surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. I resent these remarks. As I have shown density and pressure have everything to do with prop walk. It was only with the great minds combined here on ASA that such previous unexplained hydrodynamic effects have come to full understanding for the greater benefit of mankind. Yes, ASA is slowly rising from the dismal swamp it had once become to shining paragon of truth, justice and the American way! Good work to all, and let's keep our noses to the grindstone! Admiral Momsen It will only rise from the abyss when Pansy Ganzy throws in the towel. Wilbur Hubbard Have you not noticed that when the dialogue is more sailing related, less political, involves rationality, knowledge and gives less opportunity for personal attacks his participation noticeably wanes? It's a sad, but true observation. Hopefully he can find it within himself to deal with others as equals, rather than from a drippy point of condensation. His mention of prop wash was invaluable in the search for the truth about prop walk. Rarely do I bother reading his posts lately. They lack substance more and more as time passes. I think Dave finally managed to show him the error of his ways. What a beating that was! Wilbur Hubbard |
Propeller walk
"Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin Wrong! Pitch angle doesn't matter. My outboard motor on the back of my bluewater yacht can be put in reverse on a calm day at about 1/8 throttle while I'm tied to my mooring. My yacht will turn slow circles caused by prop walk. The propeller is totally horizontal. Wilbur Hubbard |
Propeller walk
Wilbur Hubbard wrote:
"Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin Wrong! Pitch angle doesn't matter. My outboard motor on the back of my bluewater yacht can be put in reverse on a calm day at about 1/8 throttle while I'm tied to my mooring. My yacht will turn slow circles caused by prop walk. The propeller is totally horizontal. Wrong yourself. Pitch angle contributes. In your case, the vortex is propelled upward on the starboard side, impinging the starboard side of your boat, on the port side the vortex tends be more downward. Pitch angle just adds to the effect. Cheers Martin |
Propeller walk
Charles Momsen wrote:
"Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin If it's the explanation I'm thinking of it involves the total path length traveled by the blade tip on either ascent or descent (with the boat moving forward). I could see the argument there but could not resolve if a longer path resulted in less thrust force or more and the same with the short path length. This same argument applies to retreating blade stall in helicopters. I'll look through Chapman and see what he says. The angle of attack argument only holds if the velocity of water is significant compared to the tangential velocity of the blade. If the water velocity is zero then the angle of attack is the same regardless of orientation. If the blade is turning 500 rpm and is 12 inch diameter the tip is moving at 314 inches/sec or 214 mph. 214 mph? 500rpm * (pi *12)in/rev = 1.885E4 inches/min 1.885E4 in/min * 60 min/hr = 1.131E6 inches/hour 1.131E4 in/hr / 63360 in/mi = 17.85 mile/hr 18 mph is a tad bit slower than 214..... Back to remedial slide-rule class for you! Cheers Martin |
Propeller walk
"Marty" wrote in message ... Wilbur Hubbard wrote: "Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin Wrong! Pitch angle doesn't matter. My outboard motor on the back of my bluewater yacht can be put in reverse on a calm day at about 1/8 throttle while I'm tied to my mooring. My yacht will turn slow circles caused by prop walk. The propeller is totally horizontal. Wrong yourself. Pitch angle contributes. In your case, the vortex is propelled upward on the starboard side, impinging the starboard side of your boat, on the port side the vortex tends be more downward. Pitch angle just adds to the effect. Cheers Martin So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. Wilbur Hubbard |
Propeller walk
Wilbur Hubbard wrote:
"Marty" wrote in message ... Wilbur Hubbard wrote: "Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin Wrong! Pitch angle doesn't matter. My outboard motor on the back of my bluewater yacht can be put in reverse on a calm day at about 1/8 throttle while I'm tied to my mooring. My yacht will turn slow circles caused by prop walk. The propeller is totally horizontal. Wrong yourself. Pitch angle contributes. In your case, the vortex is propelled upward on the starboard side, impinging the starboard side of your boat, on the port side the vortex tends be more downward. Pitch angle just adds to the effect. Cheers Martin So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. I do hope you're not being deliberately obtuse. I specifically mentioned the pitch of the propeller blades and the angle of the shaft in the same post so as to make it clear. Anyway if you like, change my 'Pitch Angle" and yours, to 'Shaft Angle', and all is well. If the shaft is horizontal, blade pitch still causes the vortex to rise on the starboard side, inclining the shaft just adds to the effect Cheers Martin |
Propeller walk
"Wilbur Hubbard" wrote in message anews.com... So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. Wilbur Hubbard You are confused then. Propeller pitch is the distance a prop would screw itself forward in one revolution if there was no slip. I noticed from your earlier post that your 'bluewater yacht' has an outboard on the stern. ROFL |
Propeller walk
"Marty" wrote in message ... Wilbur Hubbard wrote: "Marty" wrote in message ... Wilbur Hubbard wrote: "Marty" wrote in message ... Charles Momsen wrote: "Joe" wrote in message ... On Dec 11, 12:00 pm, "Wilbur Hubbard" wrote: "Charles Momsen" wrote in message ... This graph: http://www.windows.ucar.edu/tour/lin....html&edu=high Shows the density of water as a function of depth. Water density changes from 1.025 gm/cm^3 to 1.026 gm/cm^3 in 250 feet. That's a change of 0.1% in 250 ft. Since that portion of the curve is linear, one can estimate that water density would change .0004% over the diameter (tip to tip) of a 12 inch propeller. So is a .0004% change in water density (in the vertical plane no less) going to walk a boat sideways? Don't think so. Think, Momsen, think! There are very large forces at work when a propeller is turning at speed. You are stuck on static in your thinking. Picture it this way. Let's say you were riding a bicycle at 1mph and you had a ten mile per hour headwind. You would experience an 11mph head wind. Now, if you aren't a girly-man you should be able to sprint up to 35mph. You would then experience a 45mph headwind. Suddenly your inconsequential wind has great consequence. It's the same way with a propeller and the lift vs.drag coefficient. Even a very small density difference results in a significant drag difference between the top half of the prop and the bottom half of the prop. But there is another thing that has a greater effect than density causing density to be only part of the equation. Water density does not vary greatly due to the fact that it doesn't compress easily. What does change significantly with depth is water pressure (divers say 1 atmosphere for every 15 feet?) The more pressure = the more drag for the propeller. I hope this helps. Wilbur Hubbard Let me use examples you may understand Neal. Ever mix paint in a 5 gallon bucket with a paint mixing propellor on a drill? Why is the propellor in the paint pulled off center? A dairy has huge tanks to store milk. They keep the creme mixed in the milk with propellors on long shafts. They hang straight down , he shaft is vertical. When you turn them on the long shafts bend some in the direction of wheel walk. Are you saying that it is pressure difference when the prop is horizional causing the walk? When a propellor flys off an airplane they never go straight, they spin off in the direction of walk. Think path of least resistance to the face of the fluke Hope this helps. Joe ************************************************** ************************************ ************************************************** ************************************ Joe, Thanks to your input, that of Wilbur and others I believe I have come up with the most plausible explanation for propeller walk. This explanation may even impress Blondie! The cause of the prop walk is due to 2 effects, namely the Magnus Effect and the Coanda Effect. They can be found he http://lpmpjogja.diknas.go.id/kc/a/air/airplane.htm The spinning prop creates a vortex of water that is moving relative to water surrounding it, especially if the prop is angled down relative to the water's surface. A right hand prop on a forward moving boat would create a downward angling vortex that had higher relative velocity to the surrounding moving water on the starboard side and lower on the port. Viewing the vortex as a rotating cylinder moving through a fluid, the lift would be generated to port, as is observed. The Coanda Effect would explain the draggging and leakage of water laterally by the prop. I believe these explanations are the simplest and consistent with all observed effects, including paint stirrers in Joe's buckets. I searched and could find no explanation of prop walk using the Magnus or Coanda Effect. Mostly what I found was the same false pablam of water density and other voodoo science mindlessly regurgitated by babbling non-thinking parrots. Read Chapman, he explains it in exactly the same terms, (vortexes) but leaves out calling it either Magnus or Coanda. It's a nice simple explanation with a little drawing explaining why the port side blade on a right hand prop has a lower angle of attack than the right side. If you imagine perhaps the worst case: A prop has a pitch angle of 10 degrees (I know this doesn't happen with a real prop), it's on a shaft inclined by 10 degrees. In this case the port side blade vertical at it ascends or descends and generates no thrust, forward or aft. All the thrust comes from the starboard side blade angled at 20 degrees, you can clearly see there is an unbalance here. Now put it reverse and the prop throws a rising vortex of water at the starboard side of the hull, kicking it to port, while at the same time pulling the prop shaft sternwards from the starboard side only, tending to skew the boat to port. Put both together and you've got port prop walk. Cheers Martin Wrong! Pitch angle doesn't matter. My outboard motor on the back of my bluewater yacht can be put in reverse on a calm day at about 1/8 throttle while I'm tied to my mooring. My yacht will turn slow circles caused by prop walk. The propeller is totally horizontal. Wrong yourself. Pitch angle contributes. In your case, the vortex is propelled upward on the starboard side, impinging the starboard side of your boat, on the port side the vortex tends be more downward. Pitch angle just adds to the effect. Cheers Martin So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. I do hope you're not being deliberately obtuse. I specifically mentioned the pitch of the propeller blades and the angle of the shaft in the same post so as to make it clear. Anyway if you like, change my 'Pitch Angle" and yours, to 'Shaft Angle', and all is well. If the shaft is horizontal, blade pitch still causes the vortex to rise on the starboard side, inclining the shaft just adds to the effect Cheers Martin I see the logic in your thinking. So, in my previous example, what if were to take the motor off the transom and attach it to the end of a hundred foot long pole that was firmly affixed to the stern of the yacht and did the same thing with the motor in reverse. I bet the boat would still turn lazy circles even though no water stream was hitting the hull. Wilbur Hubbard |
Propeller walk
Edgar wrote:
"Wilbur Hubbard" wrote in message anews.com... So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. Wilbur Hubbard You are confused then. Propeller pitch is the distance a prop would screw itself forward in one revolution if there was no slip. I noticed from your earlier post that your 'bluewater yacht' has an outboard on the stern. ROFL I thought it odd also Edgar. Every mariner worth his salt knows that propeller pitch refers to the distance the prop would travel through the water under ideal conditions, usually expressed in inches per revolution, the use of degrees is only for academic purposes as the angle of the blade is not uniform from hub to tip in normal applications. Shaft angle should be rather self explanatory. Cheers Martin |
Propeller walk
"Edgar" wrote in message ... "Wilbur Hubbard" wrote in message anews.com... So you're talking about pitch of the prop as inches per revolution and not pitch as angle of the prop shaft? That's going off on a tangent as those talking about pitch angle in this thread are talking about the angle of the prop shaft from the horizontal or at least that's the impression I got. Wilbur Hubbard You are confused then. Propeller pitch is the distance a prop would screw itself forward in one revolution if there was no slip. I noticed from your earlier post that your 'bluewater yacht' has an outboard on the stern. ROFL Sails, my boy, sails. Ain't no motor sailer in the world that can cross the Pacific under power. A motor needs only be used to get into or out of port when there is no wind. The rest of the time it can be removed from the transom and stowed safely away below. Wilbur Hubbard |
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