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#1
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Propeller walk
What is the primary cause of propeller walk - torque or thrust?
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#2
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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 |
#3
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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 |
#4
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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. |
#5
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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 |
#6
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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 |
#7
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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. |
#8
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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 |
#9
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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? |
#10
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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 |
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