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push vs pull vis a vis rudders
Intuitively, most people sense that water "pulled" over a rudder will cause a
rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. Why? Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. |
push vs pull vis a vis rudders
Net, net, you wanna steer with a rudder backing up, prop forward of the
rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
actually, there is a difference. If you put the tranny in forward and push the
throttle forward you can turn the boat one way or the other. Because of this, some people believe you can do the same in reverse. You can't. In addition, I have met people who had trouble backing up their boats who in some cases were told that if they were to spend $XX,XXX's by moving the prop closer to the rudder they would get better turning ability on the boat. They would not. Indeed, in a recent thread "Thrust Vectoring" many people insisted that pulling water over a rudder would turn a boat. When I said it would not, several people wanted to argue about it (including one dumb cluck who claims to have a degree in physics, really dumb because this problem in physics is a classic taught to early semester students). A couple people asked for an explanation as why pulling water over a rudder had no effect and all, and I said I would write one up today. If one wants to steer by rudder backing up, one needs to start the boat moving backwards slowly, otherwise prop walk will twist the boat sideways. Start slowly until the boat is moving and the rudder becomes effective, depending on how much the boat is moving through the water. Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
Subject: push vs pull vis a vis rudders
From: "Doug Dotson" Actually, I'm not sure how the subject of steering astern got into the discussion as it was not a consideration of the original post about "thrust vectoring". However, on a twin screw boat (and this was what I believe he was discussing) you don't need to have headway or sternway to alter your heading. This can be done with props alone, or more easily and quickly, with props and rudders. In various maneuvering situations, this can be a great advantage, and just to throw in a kicker, the advantage can work with single screw as well. Shen Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
This all fits with my expierence. In reverse, propwalk is dominant
until some way is acheived. Then rudder control is possible. Doug s/v Callista "JAXAshby" wrote in message ... actually, there is a difference. If you put the tranny in forward and push the throttle forward you can turn the boat one way or the other. Because of this, some people believe you can do the same in reverse. You can't. In addition, I have met people who had trouble backing up their boats who in some cases were told that if they were to spend $XX,XXX's by moving the prop closer to the rudder they would get better turning ability on the boat. They would not. Indeed, in a recent thread "Thrust Vectoring" many people insisted that pulling water over a rudder would turn a boat. When I said it would not, several people wanted to argue about it (including one dumb cluck who claims to have a degree in physics, really dumb because this problem in physics is a classic taught to early semester students). A couple people asked for an explanation as why pulling water over a rudder had no effect and all, and I said I would write one up today. If one wants to steer by rudder backing up, one needs to start the boat moving backwards slowly, otherwise prop walk will twist the boat sideways. Start slowly until the boat is moving and the rudder becomes effective, depending on how much the boat is moving through the water. Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
I responded to an original post entitled "push vs pull vis a vis rudders".
Not familiar with the thread about Thrust Vectoring. doug S/v Callista "Shen44" wrote in message ... Subject: push vs pull vis a vis rudders From: "Doug Dotson" Actually, I'm not sure how the subject of steering astern got into the discussion as it was not a consideration of the original post about "thrust vectoring". However, on a twin screw boat (and this was what I believe he was discussing) you don't need to have headway or sternway to alter your heading. This can be done with props alone, or more easily and quickly, with props and rudders. In various maneuvering situations, this can be a great advantage, and just to throw in a kicker, the advantage can work with single screw as well. Shen Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
If we turn the rudder to port,
the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. Except when examined with a high powered light and magnifying glass, that statement is just plain wrong. One *could* make a case that the stern moves to starboard when the rudder is hard aport, but the "boat" itself will move to port because of the headway. |
push vs pull vis a vis rudders
JAXAshby wrote in message ... Intuitively, most people sense that water "pulled" over a rudder will cause a rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. To remove possible confusion - Actually, the *stern* moves to starboard and (until the boat is moving forward) this causes: a. The boat to yaw port and b. the Cof G to move starboard Once you gather way the boat will move to port due to keel lift. These points don't affect your argument though. Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure More correctly, it accelerates under differential pressure. There's quite a strong drop in pressure on the input side of each prop blade, and the whole volume of water on the input side is characterised by a pressure gradient, low by the prop, ambient at an infinite distance. You could calculate the pressure at any point if you knew the speed of the water relative to ambient - conservation of energy. You could calculate the water speed at any point if you knew the shape (cross sectional area) of this input 'plume' and it's gradients. There's a nice equation hiding here. (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. As you say . . . YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. We start to part company. You're implying that the pressure gradient varies directly with distance from prop, irrespective of obstacles to the water flow . . . now this may be true, but you haven't yet persuaded me. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Bit rash. The water will flow along the rudder surface in the direction of whatever pressure gradient exists, starting at the tail of the rudder with the same input conditions as the water travelling on the other side. Exit pressures (therefore velocities) would be the same too, except that the pressure gradient now calls for a sharp left turn into the prop. This change in momentum has to be caused by a force. My thesis is that this force is created because the water travelling around the starboard side of the rudder has to travel a longer distance (ie, faster) round the bend. And if it's going faster, it's at a lower pressure (back to conservation of energy). As an aerofoil. Your thesis implies that the starboard side water actually travels slower, unlike flow around an aerofoil. This is, of course, possible, but I don't see the mechanism at the moment. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. JimB |
push vs pull vis a vis rudders
schlackoff, the rudders don't control anything in reverse, unless the boat is
also moving backwards. They can't. However, on a twin screw boat (and this was what I believe he was discussing) you don't need to have headway or sternway to alter your heading. This can be done with props alone, or more easily and quickly, with props and rudders. In various maneuvering situations, this can be a great advantage, and just to throw in a kicker, the advantage can work with single screw as well. Shen Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. Not sure what the point of steering if you are not moving would be? Forward or reverse. If you are not moving, steering has no meaning. Doug s/v Callista |
push vs pull vis a vis rudders
you are correct, the "back of the boat" (i.e. rudder) moves to starboard.
If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. Except when examined with a high powered light and magnifying glass, that statement is just plain wrong. One *could* make a case that the stern moves to starboard when the rudder is hard aport, but the "boat" itself will move to port because of the headway. |
push vs pull vis a vis rudders
jim, the explanation was dirt simple and without the mathematical and physical
nuances to gladden the hearts of physicists. It is, however, accurate. zero rudder control going backwards until the boat is actually going backwards. the prop affects the rudder not at all in reverse. it can't. Feynman the physicist had so many people argue so hard with his statement he actually made a movie of the "under water lawn sprinkler" to show that drawing in water the sprinkler head moved not at all. Intuitively, most people sense that water "pulled" over a rudder will cause a rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. To remove possible confusion - Actually, the *stern* moves to starboard and (until the boat is moving forward) this causes: a. The boat to yaw port and b. the Cof G to move starboard Once you gather way the boat will move to port due to keel lift. These points don't affect your argument though. Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure More correctly, it accelerates under differential pressure. There's quite a strong drop in pressure on the input side of each prop blade, and the whole volume of water on the input side is characterised by a pressure gradient, low by the prop, ambient at an infinite distance. You could calculate the pressure at any point if you knew the speed of the water relative to ambient - conservation of energy. You could calculate the water speed at any point if you knew the shape (cross sectional area) of this input 'plume' and it's gradients. There's a nice equation hiding here. (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. As you say . . . YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. We start to part company. You're implying that the pressure gradient varies directly with distance from prop, irrespective of obstacles to the water flow . . . now this may be true, but you haven't yet persuaded me. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Bit rash. The water will flow along the rudder surface in the direction of whatever pressure gradient exists, starting at the tail of the rudder with the same input conditions as the water travelling on the other side. Exit pressures (therefore velocities) would be the same too, except that the pressure gradient now calls for a sharp left turn into the prop. This change in momentum has to be caused by a force. My thesis is that this force is created because the water travelling around the starboard side of the rudder has to travel a longer distance (ie, faster) round the bend. And if it's going faster, it's at a lower pressure (back to conservation of energy). As an aerofoil. Your thesis implies that the starboard side water actually travels slower, unlike flow around an aerofoil. This is, of course, possible, but I don't see the mechanism at the moment. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. JimB |
push vs pull vis a vis rudders
Subject: push vs pull vis a vis rudders
From: (JAXAshby) NO CRAP, Dipsquat. Would you try and learn to read a post for actual content and not just what you want the content to be ! Once again, the original post on thrust vectoring was talking about rudder use when "kicking the engine AHEAD" ... NOT when kicking the engine astern and all this has developed from there ..... try to follow along, as basically, all your longwinded dissertations about astern have had nothing to do with the subject at hand ..... typically. Shen schlackoff, the rudders don't control anything in reverse, unless the boat is also moving backwards. They can't. |
push vs pull vis a vis rudders
As much as it pains me to defend JAX, I think the fact
that he started a new thread to discuss this topic is legitimate. If you are not interested in this topic which primarily deals with moving astern, then don't participate. Doug s/v Callista "Shen44" wrote in message ... Subject: push vs pull vis a vis rudders From: (JAXAshby) NO CRAP, Dipsquat. Would you try and learn to read a post for actual content and not just what you want the content to be ! Once again, the original post on thrust vectoring was talking about rudder use when "kicking the engine AHEAD" ... NOT when kicking the engine astern and all this has developed from there ..... try to follow along, as basically, all your longwinded dissertations about astern have had nothing to do with the subject at hand ..... typically. Shen schlackoff, the rudders don't control anything in reverse, unless the boat is also moving backwards. They can't. |
push vs pull vis a vis rudders
"Surely You Must Be Joking, Dr. Feynman". More folks should read
his stuff. Maybe some Buckmeister Fuller as well but he make my brain hurt :) Doug s/v Callista "JAXAshby" wrote in message ... jim, the explanation was dirt simple and without the mathematical and physical nuances to gladden the hearts of physicists. It is, however, accurate. zero rudder control going backwards until the boat is actually going backwards. the prop affects the rudder not at all in reverse. it can't. Feynman the physicist had so many people argue so hard with his statement he actually made a movie of the "under water lawn sprinkler" to show that drawing in water the sprinkler head moved not at all. Intuitively, most people sense that water "pulled" over a rudder will cause a rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. To remove possible confusion - Actually, the *stern* moves to starboard and (until the boat is moving forward) this causes: a. The boat to yaw port and b. the Cof G to move starboard Once you gather way the boat will move to port due to keel lift. These points don't affect your argument though. Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure More correctly, it accelerates under differential pressure. There's quite a strong drop in pressure on the input side of each prop blade, and the whole volume of water on the input side is characterised by a pressure gradient, low by the prop, ambient at an infinite distance. You could calculate the pressure at any point if you knew the speed of the water relative to ambient - conservation of energy. You could calculate the water speed at any point if you knew the shape (cross sectional area) of this input 'plume' and it's gradients. There's a nice equation hiding here. (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. As you say . . . YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. We start to part company. You're implying that the pressure gradient varies directly with distance from prop, irrespective of obstacles to the water flow . . . now this may be true, but you haven't yet persuaded me. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Bit rash. The water will flow along the rudder surface in the direction of whatever pressure gradient exists, starting at the tail of the rudder with the same input conditions as the water travelling on the other side. Exit pressures (therefore velocities) would be the same too, except that the pressure gradient now calls for a sharp left turn into the prop. This change in momentum has to be caused by a force. My thesis is that this force is created because the water travelling around the starboard side of the rudder has to travel a longer distance (ie, faster) round the bend. And if it's going faster, it's at a lower pressure (back to conservation of energy). As an aerofoil. Your thesis implies that the starboard side water actually travels slower, unlike flow around an aerofoil. This is, of course, possible, but I don't see the mechanism at the moment. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. JimB |
push vs pull vis a vis rudders
Would you all agree that in areas of dispute the truth may be revealed by an
experiment? Please try the following: Take a fan, say a large house cooling fan (that's your propellor). Take a flat surface, for example a stiff lightweight book (thats the rudder). Turn the fan on and hold the rudder at an angle on the outflow side (transmission in forward). Does the flow exert a torque (turning effect) on the rudder? Let go one corner and see. Is there a sideways thrust that you have to oppose to keep the rudder in position? Repeat the experiment with the "rudder" on the inlet side of the fan (transmission in reverse). Is there a turning effect (torque) or not? Is there a sideways thrust on the "rudder"? You tell me - I just did it. The answers to all four questions is yes. Aero/hydrodynamic lift/drag is determined by the flow patterns over surfaces (Bernoulli effects, etc), not by the simple minded pseudo-science that is being thrown around here. It's a VERY complex situation. We all agree that in practice the effect is much, much weaker in reverse but it is still present. (The reason that it is weaker is that only a small fraction of the in-flow to the propellor actually passes over the rudder in reverse.) "JAXAshby" wrote in message ... Intuitively, most people sense that water "pulled" over a rudder will cause a rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. Why? Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. |
push vs pull vis a vis rudders
Subject: push vs pull vis a vis rudders
From: "Doug Dotson" This thread started because Jax couldn't or wouldn't understand what the original poster of "thrust vectoring" was saying about rudder usage "when kicking an engine ahead". Since I've been basing my comments on the original post and bouncing between both threads, my comments are in relation to that original post which Jax can't seem to understand, so I find myself having to correct his misconceptions regarding these post. Simply stated, if you don't like what I have to say to Jax, feel free to skip any post from me, on the subject. The subject of steering astern, is of great interest to me, as I frequently get involved with doing it, coupled with making use of propwalk. Shen As much as it pains me to defend JAX, I think the fact that he started a new thread to discuss this topic is legitimate. If you are not interested in this topic which primarily deals with moving astern, then don't participate. Doug s/v Callista |
push vs pull vis a vis rudders
sclackoff, nice flip-flop.
NO CRAP, Dipsquat. Would you try and learn to read a post for actual content and not just what you want the content to be ! Once again, the original post on thrust vectoring was talking about rudder use when "kicking the engine AHEAD" ... NOT when kicking the engine astern and all this has developed from there ..... try to follow along, as basically, all your longwinded dissertations about astern have had nothing to do with the subject at hand ..... typically. Shen schlackoff, the rudders don't control anything in reverse, unless the boat is also moving backwards. They can't. |
push vs pull vis a vis rudders
quote:
feel free to skip any post from me unquote Shen |
push vs pull vis a vis rudders
dude, don't try to metaphor the answer. metaphor is metaphor, not science.
Feynman made a movie of the exact issue people were arguing with him about. The movie showed, as it would, nothing happens when fluid is pulled past a rudder/lawn sprinkler. If you don't know what Feynman did for a living, do a google on his name. Would you all agree that in areas of dispute the truth may be revealed by an experiment? Please try the following: Take a fan, say a large house cooling fan (that's your propellor). Take a flat surface, for example a stiff lightweight book (thats the rudder). Turn the fan on and hold the rudder at an angle on the outflow side (transmission in forward). Does the flow exert a torque (turning effect) on the rudder? Let go one corner and see. Is there a sideways thrust that you have to oppose to keep the rudder in position? Repeat the experiment with the "rudder" on the inlet side of the fan (transmission in reverse). Is there a turning effect (torque) or not? Is there a sideways thrust on the "rudder"? You tell me - I just did it. The answers to all four questions is yes. Aero/hydrodynamic lift/drag is determined by the flow patterns over surfaces (Bernoulli effects, etc), not by the simple minded pseudo-science that is being thrown around here. It's a VERY complex situation. We all agree that in practice the effect is much, much weaker in reverse but it is still present. (The reason that it is weaker is that only a small fraction of the in-flow to the propellor actually passes over the rudder in reverse.) "JAXAshby" wrote in message ... Intuitively, most people sense that water "pulled" over a rudder will cause a rudder to change direction of a boat in much the same way as water "pushed" over a rudder does. However, intuition misses some things along the way. First, let's take a boat sitting in the water, not moving the prop not turning. The water pressure on each side of the rudder is the same, so turning the rudder one way or the other does not cause the boat to turn at all. Now, let's put the transmission in forward and turn the prop. The prop pushes water aft. With the rudder centered, the water moving backward passes by the rudder with the pressure the same on each side. If we turn the rudder to port, the water being pushed back by the prop strides the port side of the rudder (and NOT the starboard side) and the boat moves starboard. Why? Because the impact (pressure) of the water (molecules) on the port side of the rudder was greater than the impact (pressure) on the starboard side. What happened was that the water flowing past the rudder was *diverted* from its path and the energy in the water was used to *divert* the rudder the other direction. Remember the law of physics, "For each and every action there is an equal and opposite reaction". The water went to port, rudder went to starboard. Absolutely neccessary for the rudder to force the back of the boat to starboard is that the rudder forced water (from the prop stream) to port. "Equal and opposite" Now, let's take the same boat sitting in still water and put the transmission in reverse and turn the prop. What happens? Well, the prop pushes water forward. Where does it get "new" water from? Aft. Now, here is the part where intuition comes apart. so, let's going slowly. the water fills into the prop from aft because it is under pressure (i.e. water pressure, or "water runs down hill"). the closer to the prop, the faster the water fills. YET -- and here is the big part -- at all points aft and the same distance from the prop have the same pressure pushing water towards the spinning prop. THAT means that the pressure on one side of the rudder **is the same** as the pressure on the other side. net, net, you can turn the rudder any way you wish, but nothing happens because the pressure is the same on each side, just as it is when the prop is not turning and the boat is not moving. Still have a hard time with that? Well, let's look at it from another view. The prop is in reverse and is drawing water into its circle and pushing that water forward. Let's turn the rudder to port and see what happens as the water streams by the rudder. Water hits the now aft side (former starboars side) of the rudder? Kinda, but lets assume that it does. Which way is the water stream deflected? Towards starboard? Then the rudder would push the boat (aft end) to port. However, the water drawn over the rudder's port side hits that side and is deflected towards port. Then the rudder would push the boat (after end) to starboard. And equal and opposite reaction. Net, net, the boat does not turn. The pressure on each side of the rudder is equal. Nada. Net, net, you wanna steer with a rudder backing up, prop forward of the rudder, you MUST be moving. |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
Derek Rowell wrote:
Would you all agree that in areas of dispute the truth may be revealed by an experiment? Yes, if the experiment is setup properly, and the data is *accurately* analyzed and interpreted. Please try the following: Take a fan, say a large house cooling fan (that's your propellor). Take a flat surface, for example a stiff lightweight book (thats the rudder). Turn the fan on and hold the rudder at an angle on the outflow side (transmission in forward). Does the flow exert a torque (turning effect) on the rudder? Let go one corner and see. Is there a sideways thrust that you have to oppose to keep the rudder in position? Clearly. f=ma, i.e. force is the product of Mass times acceleration. In this case the mass of accelerated air will apply disproportionate force to the 'book' as a function of aspect ratio. Basically, the book will "weathervane" until the force applied to each side equalizes. Repeat the experiment with the "rudder" on the inlet side of the fan (transmission in reverse). Is there a turning effect (torque) or not? Is there a sideways thrust on the "rudder"? You tell me - I just did it. The answers to all four questions is yes. And...irrelevant! YES there is force on the 'vane', just as there was on the downstream side. Less due to the the wider flow pattern on the suction side, but still significant. However, in the context of the boat model, this force is virtually irrelevant. In the first case, water is forced over the rudder at an angle to the boat centerline. It's a simple vector equation. Water moves to starboard, reaction force is thus to port, the boat turns starboard. In REVERSE, the water flow, *irrespective of rudder position* is along the centerline of the boat, thus the reaction force is parallel to the keel line, and the boat moves straight back (ignoring the precessional forces that result in 'prop walk' that is). It's only when the boat moves through the water that the rudder can have an effect - another simple vector equation. There must be additional non-parallel force applied in order to produce a turn, and that is caused by *additional* water flow past the rudder caused by boat movement. Aero/hydrodynamic lift/drag is determined by the flow patterns over surfaces (Bernoulli effects, etc), not by the simple minded pseudo-science that is being thrown around here. Lift and drag are irrelevant. Consider, what is the effect of lift on the rudder? Heeling action, not turning action. You appear to be confusing hdyrodynamics with simple force/vector equations. It's a VERY complex situation. Not at all. Oh the precessional effects are definitely complex, all the more so when hydrodynamic effects are added in the equation, but precession, while important to why your stern always drifts one way in reverse (i.e. force applied to a spinning object - the propeller - will be translated 90° in the direction of rotation, creating a turning force), is not a factor in why the rudder is ineffective in reverse. We all agree that in practice the effect is much, much weaker in reverse but it is still present. (The reason that it is weaker is that only a small fraction of the in-flow to the propellor actually passes over the rudder in reverse.) Sorry, but that's just not accurate. Keith Hughes |
push vs pull vis a vis rudders
ubject: push vs pull vis a vis rudders
From: (JAXAshby) Date: 03/27/2004 15:41 Pacific Standard Time Message-id: sclackoff, nice flip-flop. I'll give you credit for one thing. You can come up with more ways of saying "duh I ain't got no intelligent response and wouldn't know the answer anyway". Shen |
push vs pull vis a vis rudders
OK. You caught me again! Damn! Yep, I'm the fraudulent plumber who can't
read from yesterday. I spent a long time fabricating that email address. It gives my ego a huge boost. Just to prove that the address is fraudulent - go to the MIT web site (http://web.mit.edu) and do a "people" search on my name. I simply gave you a simple experiment to do - and you attack me personally. (Do it yourself, and draw your own conclusions - takes about 5 minutes) That's not how we do business in science and engineering. We calmly look at a situation, make hypotheses and conjectures and then think of a set of experiments to disprove or prove our ideas. We invite others to disprove our theories, and rejoice when they do, because we learn something. That's the end of this discussion. No more! "JAXAshby" wrote in message ... derek, your fricken fraud. I just now noticed your fiticious email address of mit.edu. NObody from MIT would write what you wrote. geesh, dude. get a life. From: "Derek Rowell" |
push vs pull vis a vis rudders
brian, Feyman thought it easy. why do you suppose a cyber clown with a fake
MIT address would find it so difficult. brian, this is EASY stuff. Not intuitive for many people, perhaps, but still EASY for those who think it through for a couple minutes. however for sure, brian and derek, should EITHER of you wish to back up a ruddered boat using the engine to steer, please feel free to do so, as long as you are not near my boat. Bang into boats and docks and pilings and blame the current for all we care. Just don't hit don't do it near my boat. I'll ridicule you in front of the bimbo you have onboard. idiots. you believe you knew the entire universe of knowledge by the time you finally got out of junior high school. Haven't learned a thing since. Derek, or perhaps as a courtesy I should say Professor Rowell, why would you think a physics graduate (as described) with a reading knowledge of some Feynman stuff be impressed by the opinions of an MIT prof of Mech Engineering? Don't be discouraged. People try to avoid the crack pots as far as possible, and still get lots of value here. Brian W On Sun, 28 Mar 2004 00:35:03 GMT, "Derek Rowell" wrote: OK. You caught me again! Damn! Yep, I'm the fraudulent plumber who can't read from yesterday. I spent a long time fabricating that email address. It gives my ego a huge boost. Just to prove that the address is fraudulent - go to the MIT web site (http://web.mit.edu) and do a "people" search on my name. I simply gave you a simple experiment to do - and you attack me personally. (Do it yourself, and draw your own conclusions - takes about 5 minutes) That's not how we do business in science and engineering. We calmly look at a situation, make hypotheses and conjectures and then think of a set of experiments to disprove or prove our ideas. We invite others to disprove our theories, and rejoice when they do, because we learn something. That's the end of this discussion. No more! |
push vs pull vis a vis rudders
derek, since when do ME's deal with "fluid flow"?
you fraud. MIT is going to find out a janitor is posting using the MIT info structure. Be prepared to pay the required taxes on using educational resources for personal gain. Derek, or perhaps as a courtesy I should say Professor Rowell, why would you think a physics graduate (as described) with a reading knowledge of some Feynman stuff be impressed by the opinions of an MIT prof of Mech Engineering? Don't be discouraged. People try to avoid the crack pots as far as possible, and still get lots of value here. Brian W On Sun, 28 Mar 2004 00:35:03 GMT, "Derek Rowell" wrote: OK. You caught me again! Damn! Yep, I'm the fraudulent plumber who can't read from yesterday. I spent a long time fabricating that email address. It gives my ego a huge boost. Just to prove that the address is fraudulent - go to the MIT web site (http://web.mit.edu) and do a "people" search on my name. I simply gave you a simple experiment to do - and you attack me personally. (Do it yourself, and draw your own conclusions - takes about 5 minutes) That's not how we do business in science and engineering. We calmly look at a situation, make hypotheses and conjectures and then think of a set of experiments to disprove or prove our ideas. We invite others to disprove our theories, and rejoice when they do, because we learn something. That's the end of this discussion. No more! |
push vs pull vis a vis rudders
Subject: push vs pull vis a vis rudders
From: (JAXAshby) So, Jax, from this, I see you didn't understand another simple, concise comment on the subject. Shame you don't have any practical experience to back up and explain your typical pontificating. EG wanna try the inboard/outboard turning prop subject ..... didn't think so Shen schlackoff, knock it off. you gibber worse than a gas station attendant trying to claim degree from MIT. |
push vs pull vis a vis rudders
[snip all but the important stuff from schlackoff's post]
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push vs pull vis a vis rudders
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push vs pull vis a vis rudders
From: (Shen44)
[snip] |
push vs pull vis a vis rudders
schlackoff, knock it off. you gibber worse than a gas station attendant trying
to claim degree from MIT. Derek, you may be able to show a "force" being exerted on the rudder in this way. However, connect that rudder to a boat, and although this "force" may be sufficient to pull that rudder further over (seen that), it will, EG almost always, never be sufficient to act as a steering force for the boat to any degree that is useable. My apologies if I don't launch into some longwinded scientific dissertation. Shen |
push vs pull vis a vis rudders
You caught me again! Damn! Yep, I'm the fraudulent plumber
|
push vs pull vis a vis rudders
I simply gave you a simple experiment to do
your silly "experiement" didn't hardly match up with Feynman's. who are we to believe? |
push vs pull vis a vis rudders
Derek, or perhaps as a courtesy I should say Professor Rowell,
why would you think a physics graduate (as described) with a reading knowledge of some Feynman stuff be impressed by the opinions of an MIT prof of Mech Engineering? Don't be discouraged. People try to avoid the crack pots as far as possible, and still get lots of value here. Brian W On Sun, 28 Mar 2004 00:35:03 GMT, "Derek Rowell" wrote: OK. You caught me again! Damn! Yep, I'm the fraudulent plumber who can't read from yesterday. I spent a long time fabricating that email address. It gives my ego a huge boost. Just to prove that the address is fraudulent - go to the MIT web site (http://web.mit.edu) and do a "people" search on my name. I simply gave you a simple experiment to do - and you attack me personally. (Do it yourself, and draw your own conclusions - takes about 5 minutes) That's not how we do business in science and engineering. We calmly look at a situation, make hypotheses and conjectures and then think of a set of experiments to disprove or prove our ideas. We invite others to disprove our theories, and rejoice when they do, because we learn something. That's the end of this discussion. No more! |
push vs pull vis a vis rudders
I simply gave you a simple experiment to do - and you attack me
personally. because you are a lying sob, a cyber clown. |
push vs pull vis a vis rudders
That's not how we do business in science and engineering. We calmly look
at a situation, make hypotheses and conjectures and then think of a set of experiments to disprove or prove our ideas. We, eh? *you* AND Feynman? you are a fraud, dude. |
push vs pull vis a vis rudders
and rejoice when they do, because we learn something.
*if* that were true, you would have "learned something" several decades ago. This is not new stuff, though it is obviously foreign to you, oh great janitor at MIT. |
push vs pull vis a vis rudders
On 27 Mar 2004 23:50:22 GMT, (JAXAshby) wrote (with
possible editing): derek, your fricken fraud. I just now noticed your fiticious email address of mit.edu. NObody from MIT would write what you wrote. geesh, dude. get a life. From: "Derek Rowell" oops! He's a professor there! -- Larry Email to rapp at lmr dot com |
push vs pull vis a vis rudders
Jax,
I accept that 'suction' will not create a force. Any forces come from the new exit momentum of a fluid (point the hosepipe where you will . . . .) presumably this was Feynman's case. I haven't met the guy, Fine. The effect is easy to observe on VTOL aircraft. They suck from forward, but don't have to lean their jet output forward to cancel any 'sucking' force when hovering static. Here we agree. I'm looking for an explanation of the phenomenon I thought I had seen on a very old tug, also on an old trawler, neither of which had any significant prop walk in astern, though both had big props. The phenomenon was that rudder deflection with engine in reverse (boat static) could be used to create a little yaw. The explanation given to me was that 'flow over the rudder' created this effect. I rationalised this explanation (perhaps wrongly) by assuming the rudder changed the momentum of the water ingested by the prop. ie, water speed along the inside of the rudder is faster than water speed over the backside of the rudder; a very simple 'hydrofoil in free stream' effect. As a result of this thread I am re-examining this assumption and my observations. Now, it could be that my observations were wrong, and the phenomenon did not occur. I was, perhaps, seeing yaw caused by another effect - inertia due to a previous action maybe. And perhaps my observations were clouded by the pre-conception planted in my mind that it worked. But I'm afraid your explanation (paraphrased very crudely) 'you're wrong because Feynman says so' doesn't help me. Also, Derek Rowell's experiment shows that there is some effect which needs explanation - rather than dismissal. If you could demonstrate, prove or explain why water speed should be identical along each side of the rudder (which I assume would porve that pressure on each side is identical), irrespective of rudder deflection, when the boat is static with engine in reverse, I'd happily accept your thesis that rudder has no effect. As in many of these cases, it may help to explain this for an extreme case; a balanced spade rudder at 70degrees deflection close to the prop? If the rudder suffers some net pressure, then I'd like to understand what mechanism cancels it. Can you help without appealing to higher authorities? JimB |
push vs pull vis a vis rudders
maybe *some* derek rowel is a professor at MIT, but the clown posting as he
here most definitely is not. NO professor at MIT would write the tripe he wrote. hell, you know the poster is a fraud. he both claims to be a Mechanical Engineer and an expert in fluid flow. the poster claiming to be derek rowell is probably the janitor. the guy who learned his "fluid flow" knowledge cleaning toilets. derek, your fricken fraud. I just now noticed your fiticious email address of mit.edu. NObody from MIT would write what you wrote. geesh, dude. get a life. From: "Derek Rowell" oops! He's a professor there! -- Larry Email to rapp at lmr dot com |
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