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push vs pull vis a vis rudders
If you could demonstrate, prove or explain why water speed should
be identical along each side of the rudder water speed does not have to be equal or greater or less. This can be a bit confusing because "bernoulli" is often -- though erroneously -- given as the reason sails/wings have "lift". It might be a bit easier to remember that for the rudder to be pushed one way, it (the rudder) must push water the opposite way. If the water is not deflected then there is no force on the rudder. I mentioned Feynman because some clowns on this ng (I speak of schlackoff and jeffies and others) go ape squat when I make a statement, absolutely insisting that if I say it I must be making it up (I make up nothing) will argue for weeks (like sophomores in college wasting afternoons in the student cafeteria as they consider their fourth or fifth major) to prove because they didn't know something prior, no one else could have either. Feynman, a serious physicist, got sick and tired of arguing with the 4th major sophomore types and made a movie of the situation, showing clearing exactly what was expected. I used Feynman's name to shut up schlackoff (fat chance) and jeffies (who became quiet once he goggled the name Feynman). I mentioned the whole issue because I have met boaters who, when the complained about troubles backing up their ruddered boat, had unscrupulous marinas try to sell them a multi-thousand dollar "solution" to the problem by "moving the prop closer to the rudder for better control". Which won't work, of course. |
push vs pull vis a vis rudders
brian, you have been suckered. that was not derek rowell professor of stress
analysis in Mech Eng, but rather someone pretending to be him. NO professor at MIT would write what that clown wrote. None. He would be laughed at the rest of the staff, and some disgruntled student would report him to the president for disciplinary action. Who would pay money to "learn" something that is known so wrong by the entire staff? L. M. Rappaport wrote: From: "Derek Rowell" oops! He's a professor there! Not surprising that he's offering an opinion on fluid dynamics. While supersonic flow is studied more by aero engineers these days, slow speed fluid flow is the province of mechanical engineers. Take a look at "Fluid Mechanics" Fogiel/Cimbala for an example. Cimbala is Prof of Mech Eng at Penn State. Brian W |
push vs pull vis a vis rudders
supersonic flow
supersonic flow was mentioned by no one in this context, and is not important to note in this context. |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
wayne, are you really saying that while Feynman was right regarding the forces
involved in water flow, he wasn't right when water flowed over a rudder? Interesting. Should you be able to show that you can beat sher to the next Nobel prize in physics. Hurry. Time to give it up Jax, you're busted. Fact is Feynman was right, BUT, [Sprinkler Heads] [not equal] [Rudders] Prop flow over rudder in reverse is small but it's there. |
push vs pull vis a vis rudders
Fact is Feynman was right, BUT,
Prop flow over rudder in reverse is small but it's there. nobody said there was no flow -- there is -- but it is stated that the total of the forces on the rudder are zero. that's a fact of physics. accept it or not. your choice. look like intelligent or a Luddite. your choice. |
push vs pull vis a vis rudders
It exerts a force against the rudder,
why is that? Please explain in detail, as the physicists disagree with you. If you are right, you stand to make a fortune on the Nobel prize money alone. |
push vs pull vis a vis rudders
On Sun, 28 Mar 2004 00:35:03 GMT, "Derek Rowell"
wrote: ..... 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. From: "Derek Rowell" Derek, In hopes you didn't give up on this list altogether, here's a little puzzle you might enjoy. There is a demonstration of the Feynman sprinkler puzzle somewhere at MIT. What simple modification could you easily introduce to the nozzle in order to demonstrate a force due to suction as well as that due to pressure? Perhaps I could hint that it would augment the force? :-) Brian W |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
JAXAshby wrote in message ... If you could demonstrate, prove or explain why water speed should be identical along each side of the rudder water speed does not have to be equal or greater or less. This can be a bit confusing because "bernoulli" is often -- though erroneously -- given as the reason sails/wings have "lift". Sails/wings create lift (a force) by altering the momentum of the air passing by. The mechanism creating this lift is a (mean) fluid pressure difference between one side, and the other, of the sail/wing. Any pressure change in a freely flowing fluid will be matched to a change in local fluid speed (barring supersonics, flow breakaway, and the trivial effects of surface viscosity) to conserve energy. This is (presumably) the 'bernouili' bit you claim is often erroneous. Interesting. Do you disagree with the concept of conservation of energy? or do you claim special conditions which make his equations irrelevant? It might be a bit easier to remember that for the rudder to be pushed one way, it (the rudder) must push water the opposite way. If the water is not deflected then there is no force on the rudder. Agree; for the rudder to create yaw, it must deflect water. It must change the momentum of the water. Many ways of saying the same thing. That's where I'm stuck. I see the rudder (prop in reverse, boat static) altering the direction of the water approaching the prop. Now, perhaps it doesn't. Or perhaps there's an opposite effect somewhere else which I haven't yet identified. I'm looking for education here, not stating a flat opinion. JimB |
push vs pull vis a vis rudders
Derek Rowell wrote in message news:E4o9c.108024$1p.1536914@attbi_s54... 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. Yes, in reverse there is a torque on the rudder. But (if I read your hinges correctly) it may be caused by a solely fore and aft force on the rudder. Either would certainly explain the rudder kick I have experienced. What we're actually looking for is a net force at right angles to the centreline of the boat, so the proper hinge for this experiment would be parallel to the boat centreline, above the flow. We'd then look to see if the paper was still deflected. Just nipping off to play with some bits of wire and card . . . JimB |
push vs pull vis a vis rudders
Wayne.B wrote in message ... On 29 Mar 2004 03:27:38 GMT, (JAXAshby) wrote: It exerts a force against the rudder, why is that? Please explain in detail, as the physicists disagree with you. If you are right, you stand to make a fortune on the Nobel prize money alone. ================================================== == If flow deflection takes place (rudder at angle to flow), a force is exerted. Old news to everyone, Nobel prize not likely. Wayne, I can imagine a description of flow over the rudder which would meet Jax's flat (and rather unhelpful) statements, and also square with the obvious deflection that must occur just before entering the prop. Whether it's realistic or not is another matter. Perhaps water approaches the propeller via the deflected rudder through an Ess bend. ie, water approaches a point about a third of the way from the tip of the deflected rudder (lets call it the stagnation point). From one side of this point, water idles off at a steep angle to round the rudder tip, doing a hairpin bend to run back to the prop. From the other side the water moves quickly along the rudder surface to the prop. Well, it's a thought. I'm off to play with bits of card and wire to repeat Derek Rowells experiment, so perhaps I should add a few burning fag ends to the picture? JimB |
push vs pull vis a vis rudders
******************If****************** flow deflection takes place (rudder at
angle to flow), a force is exerted. *IF* is the operative word. The question is why do *you* believe there is deflection? The physicists don't believe that. Why do you? |
push vs pull vis a vis rudders
to demonstrate a force due to suction
there is no force in nature called "suction". none. |
push vs pull vis a vis rudders
There is a demonstration of the Feynman
sprinkler puzzle somewhere at MIT. the link was posted last night. the guy who hijacked the professor's email addy should have taken greater care in whose address he grabbed, for it would seem the real professor at MIT would have long ago known of the demo that any student -- or his mother or even little sisten in grade school -- could walk up to and push the button to see for himself. |
push vs pull vis a vis rudders
This is (presumably) the 'bernouili' bit you
claim is often erroneous. jim, please don't make the mistake of saying that wings lift "because they are round on one side". you can go to any airshow on the planet and see aircraft fly upside down, the round side of the wing towards the ground bernouili had to do with venturi effects and "sounds" scientific to lay ears. a 1st semester aero eng student knows that bernouili does not explain lift. |
push vs pull vis a vis rudders
Sails/wings create lift (a force) by altering the momentum of the
air passing by. yes. for the rudder to create yaw, it must deflect water. yes. It must change the momentum of the water. yes. That's where I'm stuck yes. I see the rudder (prop in reverse, boat static) altering the direction of the water approaching the prop. no, the water pressure of either side of th rudder is the same. Now, perhaps it doesn't it doesn't. perhaps there's an opposite effect somewhere else which I haven't yet identified the water pressure on either side of a rudder is the same for water drawn over the rudder. |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
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push vs pull vis a vis rudders
Wayne.B wrote:
Take a look at flow patterns through props some time. You're assuming that all the flow past the rudder (when on the suction side) is parallel to the keel (center) line, as it *basically* is on the discharge side. The intake side of the prop, however, has a cone-shaped intake pattern, with the prop at the apex. If the rudder is at an angle to the centerline, flow will take the path of least resistance, and to the extent that there is impact pressure on the rudder side with the highest aspect ratio, this will just cause disproportionate flow around the other side, increasing impact pressure on that side, until an equilibrium is reached. Once past the rudder, the flow resumes its 'along the centerline' flow, so there is no net deflection, and all 'thrust' is parallel to the centerline. Keith Hughes If flow deflection takes place (rudder at angle to flow), a force is exerted. Old news to everyone, Nobel prize not likely. |
push vs pull vis a vis rudders
okay, yo-yo. which WAY is the rudder deflected if it is pushed to port?
please explain your reasoning. the water pressure on either side of a rudder is the same for water drawn over the rudder. ==================== Only if the rudder is parallel to the direction of flow. At an angle to the flow, water is deflected, momentum is changed, force is created. It's not very much force in reverse, not enough to be useful for maneuvering, but a force nevertheless. |
push vs pull vis a vis rudders
wtf are you talking about? it's awfully early in the day to be so incoherant
from alcohol. 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. [...] 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. Jox, since you're such an "expert" on Feynman inverse sprinkler problem and how to misapply it to any situation, maybe you can answer a question about it. While it's true that the sprinkler won't turn when water is being sucked in, it's not true that no net force is generated by sucking the water in. In fact, there is a net force generated. It's just not in a direction that will turn the sprinkler. In relation to your discussion about about equal and opposite, net net, no net force, etc., how do you reconcile that with the fact that it's not true for the inverse sprinkler problem? Steve |
push vs pull vis a vis rudders
nice, Keith.
Wayne.B wrote: Take a look at flow patterns through props some time. You're assuming that all the flow past the rudder (when on the suction side) is parallel to the keel (center) line, as it *basically* is on the discharge side. The intake side of the prop, however, has a cone-shaped intake pattern, with the prop at the apex. If the rudder is at an angle to the centerline, flow will take the path of least resistance, and to the extent that there is impact pressure on the rudder side with the highest aspect ratio, this will just cause disproportionate flow around the other side, increasing impact pressure on that side, until an equilibrium is reached. Once past the rudder, the flow resumes its 'along the centerline' flow, so there is no net deflection, and all 'thrust' is parallel to the centerline. Keith Hughes If flow deflection takes place (rudder at angle to flow), a force is exerted. Old news to everyone, Nobel prize not likely. |
push vs pull vis a vis rudders
On Mon, 29 Mar 2004 08:59:59 -0700, Keith Hughes
wrote: this will just cause disproportionate flow around the other side, increasing impact pressure on that side, until an equilibrium is reached. Once past the rudder, the flow resumes its 'along the centerline' flow, so there is no net deflection, and all 'thrust' is parallel to the centerline. ====================================== Point taken and understood. I was assuming a starting condition with the rudder parallel to an established flow, and then turned at an angle causing a deflection and small side force. Given the general weakness of the flow and somewhat unfocused direction, it's quite believable that an equilibrium could be reached. Until that happens I'm still convinced that a small amount of deflection and force would be produced, similar to what the good professor at MIT observed with his fan. |
push vs pull vis a vis rudders
similar to what the good professor at MIT observed with
his fan. what the "good professor at MIT observed" was that starting with an an empty tube there was a tiny movement until the tube filled. wayne, you may have noticed in your travels that water surrounds a boat in the water, so there is no waiting for the tube to fill. you are trying to salavage an untenable position. |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
schlackoff, you are using a constrained airflow? know what that means? know
how it is different from unconstrained. get some sleep, schlackoff, and you will feel better by tomorrow afternoonn. wtf are you talking about? it's awfully early in the day to be so incoherant from alcohol. Bzzzt!!! Wrong answer jox. Try again. It's clear you don't understand the sprinkler problem. While you're cogitating on why you're wrong in applying feynman's sprinkler problem to this arena, here's another, simpler question for you: Say you have a wind tunnel with a rudder mounted at the test point. First case is a blower at one end forcing air though the tunnel and past the rudder at 1mph. You turn the rudder at a 45 degree angle to the airflow. Is there a lateral force generated by the rudder? Second case is a blower at the other end of the tunnel but now it's sucking air through the tunnel past the rudder at 1mph. You turn the rudder at a 45 degree angle to the airflow. Is there a lateral force generated by the rudder? 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. [...] 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. Jox, since you're such an "expert" on Feynman inverse sprinkler problem and how to misapply it to any situation, maybe you can answer a question about it. While it's true that the sprinkler won't turn when water is being sucked in, it's not true that no net force is generated by sucking the water in. In fact, there is a net force generated. It's just not in a direction that will turn the sprinkler. In relation to your discussion about about equal and opposite, net net, no net force, etc., how do you reconcile that with the fact that it's not true for the inverse sprinkler problem? Steve |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
steve, consider just where the air on the "supply side" of the fan blades come
from and consider how that differs from the air on the "demand side". the demand side is more or less a stream that expands. the supply side is more like a hemi-sphere of air (actually, air from the demand side passes back to the supply side as each blade of the fan passes, i.e. tip vortices). Consider, also, that *if* fluid drawn over a rudder by a prop have any effect on the rudder, mariners would all know which direction the stern moved with which rudder position. Even the guys who insist pulled water affects a rudder don't have a clew which way the boat turns. indeed, the "good professor" was reduced to claiming that friction in the rudder bearin made the difference. schlackoff, you are using a constrained airflow? know what that means? know how it is different from unconstrained. Interesting that you think it makes a difference. Ok, try it again but this time with a theoretical infinitely sized wind tunnel, or a physical one large enough that the difference between constrained flow and unconstrained flow is negligable, like a 1 mile diameter wind tunnel and a 1" rudder. In one case the air in an infinite wind tunnel is being pushed at 1mph past the rudder and in the other case it's being drawn past the rudder at 1mph. In both cases, air is flowing past the rudder at 1mph and the rudder is at a 45 degree angle. Does the rudder generate a lateral force in both cases? Steve wtf are you talking about? Bzzzt!!! Wrong answer jox. Try again. It's clear you don't understand the sprinkler problem. While you're cogitating on why you're wrong in applying feynman's sprinkler problem to this arena, here's another, simpler question for you: Say you have a wind tunnel with a rudder mounted at the test point. First case is a blower at one end forcing air though the tunnel and past the rudder at 1mph. You turn the rudder at a 45 degree angle to the airflow. Is there a lateral force generated by the rudder? Second case is a blower at the other end of the tunnel but now it's sucking air through the tunnel past the rudder at 1mph. You turn the rudder at a 45 degree angle to the airflow. Is there a lateral force generated by the rudder? 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. [...] 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. Jox, since you're such an "expert" on Feynman inverse sprinkler problem and how to misapply it to any situation, maybe you can answer a question about it. While it's true that the sprinkler won't turn when water is being sucked in, it's not true that no net force is generated by sucking the water in. In fact, there is a net force generated. It's just not in a direction that will turn the sprinkler. In relation to your discussion about about equal and opposite, net net, no net force, etc., how do you reconcile that with the fact that it's not true for the inverse sprinkler problem? Steve |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
Consider that it doesn't really matter as long as there is flow of fluid
media over the rudder. but it does, because the rudder and prop are hooked together. If the prop were fixed as to direction, the rudder would turn into it until the movement stopped. Consider the fact that fluid drawn over a rudder by a prop may have an effect on how the stern moves, but one that is much less then prop walk. the "good professor" argued that without friction in the rudder bearings rudder would move. I say it doesn't. plainly, a shot of forward throttle with the rudder turned turns the stern, and all (most?) mariners know in which direction the boat will turn from experience. nobody can remember which direction a boat will turn with a shot of reverse throttle because nobody has seen it. |
push vs pull vis a vis rudders
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push vs pull vis a vis rudders
On Mon, 29 Mar 2004 21:00:56 -0700, Keith Hughes
wrote: Steven Shelikoff wrote: Consider that it doesn't really matter as long as there is flow of fluid media over the rudder. Again, here you're assuming laminar (or at least unidirectional) flow. When inserted into a laminar flow stream, and angled surface, such as a rudder, will certainly be subjected to a force related to the mass of the fluid deflected. Fluid flow on the 'suction' side is nowhere near laminar, and will in fact be totally non-uniform around the rudder. All fluid will be redirected immediately upon clearing That's ok. Fluid on the pressure side of the prop is nowhere near laminar either and will in fact be totally non-uniform around the rudder. Yet the rudder still has an effect on the boat's direction. Consider the fact that fluid drawn over a rudder by a prop may have an effect on how the stern moves, It *may* for a brief instant until an equilibrium is reached and the pressure equalizes on both rudder surfaces (remember, water is *not* elastic in the way air is, so you can't create a vacuum in water like you do in air - if you do, you cavitate and dissolve gases come out of solution until the partial pressures equalize and/or until water 'fills in the void' and the gases redissolve). Sure you can create a vacuum in water, just like in air. The only difference is that water doesn't change it's volume (as much, but it does a small amoutn) when the pressure changes. There's still a vacuum though. And you can certainly create a vacuum in water without cavitation. Cavitation only occurs if the pressure of the water drops below it's vapor pressure. There's a whole art/science of creating props that work without cavitation for use with submarines. but one that is much less then prop walk. Many orders of magnitude less IME and IMO. Especially with an angled propshaft. But there nontheless. Steve |
push vs pull vis a vis rudders
Steven Shelikoff wrote: Consider that it doesn't really matter as long as there is flow of fluid media over the rudder. Again, here you're assuming laminar (or at least unidirectional) flow. When inserted into a laminar flow stream, and angled surface, such as a rudder, will certainly be subjected to a force related to the mass of the fluid deflected. Fluid flow on the 'suction' side is nowhere near laminar, and will in fact be totally non-uniform around the rudder. All fluid will be redirected immediately upon clearing the rudder, and the resulting reaction force is parallel to the boats centerline. Consider, also, that *if* fluid drawn over a rudder by a prop have any effect on the rudder, mariners would all know which direction the stern moved with which rudder position. Even the guys who insist pulled water affects a rudder don't have a clew which way the boat turns. indeed, the "good professor" was reduced to claiming that friction in the rudder bearin made the difference. Consider the fact that fluid drawn over a rudder by a prop may have an effect on how the stern moves, It *may* for a brief instant until an equilibrium is reached and the pressure equalizes on both rudder surfaces (remember, water is *not* elastic in the way air is, so you can't create a vacuum in water like you do in air - if you do, you cavitate and dissolve gases come out of solution until the partial pressures equalize and/or until water 'fills in the void' and the gases redissolve). but one that is much less then prop walk. Many orders of magnitude less IME and IMO. Keith Hughes |
push vs pull vis a vis rudders
JAXAshby wrote in message ... jim, please don't make the mistake of saying that wings lift "because they are round on one side". you can go to any airshow on the planet and see aircraft fly upside down, the round side of the wing towards the ground Of course I won't make that mistake. What made you think I would? I repeat the relevant part of my post: "Any pressure change in a freely flowing fluid will be matched to a change in local fluid speed (barring supersonics, flow breakaway, and the trivial effects of surface viscosity) to conserve energy. This is (presumably) the 'bernouili' bit you claim is often erroneous." I said this in response to your statement that pressure change does not have to be related to a speed change in the circumstances we're talking about. This seemed to me to violate the laws of conservation of energy. It was you who called Bernoulli into it, bless his cotton socks. I quote from your post: "water speed does not have to be equal or greater or less. This can be a bit confusing because "bernoulli" is often -- though erroneously -- given as the reason sails/wings have "lift"." You were here responding to my assumption that if there's a (mean) pressure differential over the rudder, than there will be an allied mean change in fluid speed. Just like an airplane wing creating lift. The fluid speed on the low pressure side will be faster (caveats for supersonic flow etc - we are talking boats). I hope you don't disagree with that. JimB |
push vs pull vis a vis rudders
JAXAshby wrote in message ... I see the rudder (prop in reverse, boat static) altering the direction of the water approaching the prop. no, the water pressure of either side of th rudder is the same. Now, perhaps it doesn't it doesn't. the water pressure on either side of a rudder is the same for water drawn over the rudder. Those are statements, not explanations. That's why I'm stuck. How about an explanation of those phenomena for a numerate old thickie? Try third year fluid dynamics instead of first year. It won't kill me. JimB |
push vs pull vis a vis rudders
JAXAshby wrote in message ... similar to what the good professor at MIT observed with his fan. what the "good professor at MIT observed" was that starting with an an empty tube there was a tiny movement until the tube filled. I seem to remember you damned the professor for using a metaphor .. . . JimB |
push vs pull vis a vis rudders
Steven Shelikoff wrote: That's ok. Fluid on the pressure side of the prop is nowhere near laminar either and will in fact be totally non-uniform around the rudder. Yet the rudder still has an effect on the boat's direction. No, it's not laminar, it is unidirectional along one axis. Unidirectional flow can be diverted creating a thrust vector, unlike the non-unidirectional flow on the suction side where the rudder provides pressure drop instead of redirection/diversion. That's the difference. Sure you can create a vacuum in water, You need to check the definition of vacuum if you believe this. "Vacuum in water" is an oxymoron. just like in air. The only difference is that water doesn't change it's volume (as much, but it does a small amoutn) when the pressure changes. The *liquid* volume does not change, that's a basic property of liquids. Their volume is temperature dependent, not pressure dependent. If you reduce the pressure, dissolved gases will evolve (that *is* cavitation) but you now have bubbles suspended in a liquid, i.e. foam. There's still a vacuum though. Don't think so. And you can certainly create a vacuum in water without cavitation. Cavitation only occurs if the pressure of the water drops below it's vapor pressure. Yes, and you would create a vacuum without doing this exactly how? Fluid is not elastic. Move it from one point too quickly (what you'd *have* to do to create a local low pressure area) and you will liberate dissolve gas (even gaseous water) due to the low pressure and/or high temperature created by the shear. Water doesn't stretch. There's a whole art/science of creating props that work without cavitation for use with submarines. Quite so. They do not, however, generate 'pockets of vacuum' in doing so. Keith Hughes |
push vs pull vis a vis rudders
steeeeeeeeeeeeeeeeeeeeeeeve!!
That's ok. Fluid on the pressure side of the prop is nowhere near laminar either and will in fact be totally non-uniform around the rudder. you are mixing mixie pixie dust with polymorphism. the words _sound_ alike, but you described a totally different issue from the one you addressed. |
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