B Why does a stiff boat rudder produce forward thrust when pumped left to right?

[jbriggs444]

When there is no wind , boat is at rest and sail is in centerline position and you start push boom/sail forward(analogy in my case move rudder from 2 to 3),boat will start to move back,because pressure field is now reversed..

What is the point of analyzing the first part of 1/4 of a cycle in isolation? For the 2 to 3 stroke there should be a steady rotation phase and a decelleration at the end of the stroke phase. You seem to want to analyze the steady rotation phase and ignore the decelleration.

You say that the "pressure field is now reversed". By this you mean that the pressure is high on the front of the rudder and low behind during the steady rotation phase?

Can you see that this will change during the decelleration phase?

 

[Jurgen M]

You say that the "pressure field is now reversed". By this you mean that the pressure is high on the front of the rudder and low behind during the steady rotation phase?

Can you see that this will change during the decelleration phase?

I use nylon as rudder, to see where is high and low pressure side during rotation.
We can see in which side is nylon angled during 2 to 3 position,high pressure is at the front and low at the back,that mean resultant force is point backawards, prependicular to rudder surface(in case of stiff rudder)

It seems when rudder stops at postion 3, nylon angled to the front, that mean pressure field is reversed,so high is back low is at front,so resultant force switch to forward.
But time from 2 to 3 is longer than just this "second" when rudder stops at 3, so I think net resultant force from 2 to 3 is backwards. But who knows, maybe is resultant force at position 3 enough greater in magnitute than from 2 to 3 so net force is forward...

Interest to me is that almost all water that goes backward is done by low pressure side, it is very hard to see any backward movement of water at rudder high pressure side, indeed lots of water near the front edge of rudder(pivot point) goes forward and "leak" to low pressure side!
Obviulsy centifugal force is here too weak for low speed rudder swings to win against pressure differential.
Water is "pulled" backward by low pressure side not "pushed" by high pressure side,like was my first intuition.

Slow down speed and increase resolution to see better what is going on.
.

 

[sophiecentaur]

We’re rapidly approaching the old chestnut “how do planes fly?”

 

[sophiecentaur]

We’re rapidly approaching the old chestnut “how do planes fly?”

Frankly, I would think it's quite sufficient to say that the force you use during the 'push' phase is a lot higher than the force you use during the 'recovery phase. The distance over which the force acts is the same (in the boat frame) so more work is done pushing water backwards than is done pushing it forward.

I'm sure someone could bring Bernoulli into it but . . . .

If a rudder were produced with some varying geometry over the cycle then you could go even faster. However, you wouldn't be allowed to race with one of those any more than you are allowed to use your emergency paddle.

 

[Jurgen M]

I'm sure someone could bring Bernoulli into it but . . . .

I think it is very complex to calculate this, when boat start moving we have induce flow which increase pressure at low pressure side when rudder rotate from 1 to 2 and increase pressure at high pressure side when rudder rotate from 2 to 3.
Also rudder accelarate and decelerate, which aditionaly complicates calculation.

 

[Jurgen M]

You say that the "pressure field is now reversed". By this you mean that the pressure is high on the front of the rudder and low behind during the steady rotation phase?

Can you see that this will change during the decelleration phase?

If you move paddles from position 0 to 1, boat will start go backward,not forward.

 

[jbriggs444]

If you move paddles from position 0 to 1, boat will start go backward,not forward.

But you still have not clarified what you mean by "the pressure field is reversed".

I am not sure what point you are trying to make.

 

[Jurgen M]

But you still have not clarified what you mean by "the pressure field is reversed".

I am not sure what point you are trying to make.

My main question is " does forward rudder stroke (when rudder moves from position 2 towards 3 or 2 towards 1 ) produce thrust or reverse-thrust?

Yes when rudder make backward stroke( 1 towards 2 or 3 towards 2) high pressure is at back and low pressure is at the front side of rudder.

When rudder make forward stroke(moves from position 2 towards 3 or 2 towards 1 ) high pressure is at front and low pressure is at the back side of rudder.

So I tell pressure field is reversed during forward and backwards strokes in relation to boat forward direction.

 

[pbuk]

My main question is " does forward rudder stroke (when rudder moves from position 2 towards 3 or 2 towards 1 ) produce thrust or reverse-thrust?

It does not produce forward thrust. It will produce drag which is a force opposite to the direction of motion relative to the overall direction of motion of the water. The amount of drag will vary from insignificant to quite a lot depending on a number of variables; you appear to want to call this drag "reverse thrust" but this would not be a term I would use within the normal range of these variables.

 

[Jurgen M]

It does not produce forward thrust. It will produce drag which is a force opposite to the direction of motion relative to the overall direction of motion of the water. The amount of drag will vary from insignificant to quite a lot depending on a number of variables; you appear to want to call this drag "reverse thrust" but this would not be a term I would use within the normal range of these variables.

@jbriggs444 doesn't agree with you, he tell it will produce thrust even at forward stroke...

 

[jbriggs444]

@jbriggs444 doesn't agree with you, he tell it will produce thrust even at forward stroke...

Please stop putting words into my mouth. I keep trying to point out that the forward and reverse half-strokes do not produce thrusts that are the reverse of each other.

If one is trying to explain an asymmetric result, it may be useful to concentrate on asymmetries, rather than on symmetries.

 

[jbriggs444]

So I tell pressure field is reversed during forward and backwards strokes in relation to boat forward direction.

Reversed in direction, maybe. But not unchanged in magnitude.

When I hear the word "reversed", I tend to think about taking a pressure field during the forward stroke and inverting the sign on all of the values to obtain the pressure field on the return stroke. That would not be a correct result for the situation at hand.

 

[Jurgen M]

Please stop putting words into my mouth.

You state this in bold:
"Centrifugal force would be rearward only."

and this after my text

Yes centrifugal force push water reward all the time.

"Then you have an explanation for the forward thrust"

So you confirm forward thrust when rudder move from 2 to 3..

Why now you change your opinion?

 

[jbriggs444]

You state this in bold:
"Centrifugal force would be rearward only."

and this after my text

"Then you have an explanation for the forward thrust"

So you confirm forward thrust when rudder move from 2 to 3..

Why now you change your opinion?

I have not. That quoted passage says nothing about the thrust during the individual half strokes. Only about the net for the pairs of half-strokes.

If one can explain a net rearward thrust on the water than one has an explanation for a net forward thrust on the boat.

 

[Jurgen M]

I have not. That quoted passage says nothing about the thrust during the individual half strokes. Only about the net for the pairs of half-strokes.

Your response is to my post where I strictly talking about rudder move from 2 towards 3(forward stroke).

OK , so your opinion is that forward stroke(2 towards 3) produce reverse-thrust/drag?
If yes, isn't that contradictory with Newton 3law, because water is pushed backward even in forward stroke,that implies forward stroke must produce thrust...

So in forward stroke, if we look from pressures view it must be drag, but if we look from Newton 3law view, it must be thrust.
But these two views must give same result.

 

[Jurgen M]

It does not produce forward thrust. It will produce drag which is a force opposite to the direction of motion relative to the overall direction of motion of the water.

So in forward stroke water is pushed forward and reaction is backward ?

 

[jbriggs444]

Your response is to my post where I strictly talking about rudder move from 2 towards 3(forward stroke).

OK , so your opinion is that forward stroke(2 towards 3) produce reverse-thrust/drag?

You call that a "forward stroke". I would call it a half-stroke. A full stroke would be from the limit on the left, past the centerline and on toward the limit on the right. No matter. Call it a "forward stroke" from centerline (2) to the right hand limit (3).

I would expect that portion of the stroke to act to reduce the boat's velocity, yes. But without a CFD model, I would not want to guarantee it.

If yes, isn't that contradictory with Newton 3law, because water is pushed backward even in forward stroke,that implies forward stroke must produce thrust...

Wait, what? Are you putting words into my mouth again? I've not stated that water is pushed rearward during your "forward stroke" from 2 to 3.

So in forward stroke, if we look from pressures view it must be drag, but if we look from Newton 3law view, it must be thrust.
But these two views must give same result.

What are you talking about? You've not derived a pressure field. You've not derived a flow field. But you are making pronouncements about them.

 

[Jurgen M]

Please stop putting words into my mouth.

You state this:

Wait, what? Are you putting words into my mouth again? I've not stated that water is pushed rearward during your "forward stroke" from 2 to 3.

Look at my video,you can see that lots of water has backward component from 2 towards 3- "half stroke",especially when rudder stops at position 3.

 

[jbriggs444]

You state this:

Look at my video,you can see that lots of water has backward component from 2 towards 3- "half stroke",especially when rudder stops at position 3.

You state this:

Look at my video,you can see that lots of water has backward component from 2 towards 3- "half stroke",especially when rudder stops at position 3.

I will take your word for it. So back to your complaint. You claim that the pressure point of view has high pressure on the forward side of the rudder and hence you expect a rearward flow.

But you've not computed a pressure field. So you do not really know that the forward side has high pressure. And you are looking at velocity rather than acceleration. So your expectation that flow will track with pressure gradient is wrong-headed. Water has mass.

 

[Jurgen M]

I will take your word for it. So back to your complaint. You claim that the pressure point of view has high pressure on the forward side of the rudder and hence you expect a rearward flow.

But you've not computed a pressure field.

I don't need computed something that I can see with my eyes.
You can see in which side is nylon at my rudder tense/angled,so high pressure is 100% at the front during stady state from 2 towards 3, also you can expect that,because rudder is rotating in this direction "hitting" water infront of it..

But when rudder stops in position 3, water inertia now hit back side and make here high pressure,now lots of water going out parallel to rudder back surface.This "jet" of water has side and backward component.
That I see in slow motion.

 

[Jurgen M]

You claim that the pressure point of view has high pressure on the forward side of the rudder

Imagine aircraft when fly, any " forward half stroke" movement of rudder will cause strong drag and side force which yaw the plane, it is very obvius that high pressure is at the front side of rudder.

 

[Lnewqban]

Imagine aircraft when fly, any " forward half stroke" movement of rudder will cause strong drag and side force which yaw the plane, it is very obvius that high pressure is at the front side of rudder.

Being so short, what stops the boat from yawing as well?
What stops water to naturally flow horizontally from high to low pressure areas?

 

[jbriggs444]

Imagine aircraft when fly, any " forward half stroke" movement of rudder will cause strong drag and side force which yaw the plane, it is very obvius that high pressure is at the front side of rudder.

We are not in an airplane being propelled with engines. We are in a boat being propelled by sculling. "Obvious" is not a valid argument.

 

[DaveC426913]

Being so short, what stops the boat from yawing as well?

The keel. That's it's primary function.

 

[Jurgen M]

Being so short, what stops the boat from yawing as well?
What stops water to naturally flow horizontally from high to low pressure areas?

View attachment 305584

The keel just minimize yawing, when you pumping with rudder, boat is yawing left to right all the time..

 

[Vanadium 50]

I don't need computed something that I can see with my eyes.

You can see pressure with your eyes?

Whatever your job is, quit it now and become an industrial consultant specializing in chemical plants. You will make millions.

 

[Lnewqban]

The keel just minimize yawing, when you pumping with rudder, boat is yawing left to right all the time..

Then, there is more happening pressure-wise that just around the rudder.
Besides, the sail is forcing an anti-clockwise yaw (looking at the boat from above), while you are seating on the left side and using your right hand on the rudder.
The hull also rocks some, so water is being pumped around somehow.

If you have noticed, there is no much forward movement while you are pumping the rudder from a full stop.
Then, water begins to flow more and more around the boat.

I don’t know for sure, but I believe that all the above makes water flow aft and the boat to “wave” its way forward.
A more rudimentary movement than the one of the fish shown in post #32 above.

 

[sophiecentaur]

If you move paddles from position 0 to 1, boat will start go backward,not forward.

This is so blindingly obvious that I have to conclude that people are getting their backwards's and forwards's mixed up. But the net propulsive force will be small in this case because the majority of the time, the net forces will be lateral. Note; a pair of oars is much easier to operate for all but the most clueless beginners and I don't think bringing sculls into the argument helps.

Fact is that the (original) system works so the has to be more force pushing water backwards when you waggle the tiller the correct way. A lot of the more recent posts seem to be getting nowhere, yet the proper explanation / description is to be found among this thread.

As with a lot of boating operations, back and forward and left and right are often confused.

 

[Jurgen M]

We are not in an airplane being propelled with engines. We are in a boat being propelled by sculling. "Obvious" is not a valid argument.

What is difference, any movement of rudder from centerline position will cause high pressure at the front side of rudder, reslutant force has side and drag component,which slow down the boat/aircraft.

Dont agree?

 

[DaveC426913]

What is difference, any movement of rudder from centerline position will cause high pressure at the front side of rudder, reslutant force has side and drag component,which slow down the boat/aircraft.

This may be a source of confusion. While there are similar components, this scenario is not comparable to an airplane.

As we just established, a boat can certainly experience an amount of yaw when being sculled via rudder. That's not comparable to a plane (which would nosedive).

Thus, a rudder deflection does not necessarily only induce drag - as one might think if one assumes movement has to be straight along the centre line.

Also, the propulsion does not necessarily have to be along the centre line of the boat to have its gross movement be straight(ish). That's what the keel is there to do. (There's no real aerodynamic equivalent of a keel).

We should drop the comparison to an airplane.

 

[jbriggs444]

What is difference, any movement of rudder from centerline position will cause high pressure at the front side of rudder, reslutant force has side and drag component,which slow down the boat/aircraft.

Dont agree?

Air has mass too. It is not just velocity that matters. Acceleration enters in. It would be nice to see you acknowledge this fact.

At the end of a stroke (of sail or of rudder) as the foil is brought to a halt, I would expect the pressure gradient to change directions due to the resulting fluid acceleration.

So I neither agree nor disagree. I claim that the question has not been asked with sufficient precision.

 

[Jurgen M]

This is so blindingly obvious that I have to conclude that people are getting their backwards's and forwards's mixed up.

No I just think they are kidding with me or not interested in topic at all.

Fact is that the (original) system works so the has to be more force pushing water backwards when you waggle the tiller the correct way. A lot of the more recent posts seem to be getting nowhere, yet the proper explanation / description is to be found among this thread.

After so many posts we almost menage to agree that forward half stroke produce drag, now question is why pumping rudder produce net thrust if forward half strokes produce drag?

So backward half strokes somehow produce more thrust than forward half strokes drag.
Maybe some people from fluid dynamics background can help

 

[DaveC426913]

high pressure at the front side of rudder

Also, define "front side of the rudder" without assuming "front" is the nose of the boat.

See the white arrow on the left? That may apply to an airplane, but it is not fixed for a boat that's yawing left and right.

(refer back to post 80)

 

[jbriggs444]

No I just think they are kidding with me or not interested in topic at all.

After so many posts we almost menage to agree that forward half stroke produce drag, now question is why pumping rudder produce net thrust if forward half strokes produce drag?

So backward half strokes somehow produce more thrust than forward half strokes drag.
Maybe some people from fluid dynamics background can help

You keep wanting to characterize both water and air as massless fluids where acceleration produces no thrust and all is symmetric between forward and return strokes.

 

[Jurgen M]

Air has mass too. It is not just velocity that matters. Acceleration enters in. It would be nice to see you acknowledge this fact.
At the end of a stroke (of sail or of rudder) as the foil is brought to a halt, I would expect the pressure gradient to change directions due to the resulting fluid acceleration.

Yes pressures change sides(low pressure at the back side of rudder become high pressure) when rudder coming to the end,that can be seen in slow motion from shape/tension of nylon in my video.

But if I compare rudder forward half stroke with paddle move from position 0 toward 1(which 100% move boat backward), then I can't find a way how would forward half stroke produce thrust.

 

[jbriggs444]

Yes pressures change sides(low pressure at the back side of rudder become high pressure) when rudder coming to the end,that can be seen in slow motion from shape/tension of nylon in my video.

But if I compare rudder forward half stroke with paddle move from position 0 toward 1(which 100% move boat backward), then I can't find a way how would forward half stroke produce thrust.

Why would it have to? If you have more forward thrust during the one half stroke than drag during the other half stroke, you have a net forward thrust, yes? So what you are searching for is an asymmetry -- a way in which the pressure gradients during the two halves do not have equal and opposite profiles.

I have difficulty with your terminology. I think that your "forward stroke" is as the sail/rudder moves to the sides. The other stroke -- perhaps the "return stroke" is as the sail/rudder moves back toward the centerline. I also have difficulty with your numbering. It seems to change between 2 and 3 and 0 and 1 without much rhyme or reason.

 

[Jurgen M]

Also, define "front side of the rudder" without assuming "front" is the nose of the boat.

View attachment 305593
See the white arrow on the left? That may apply to an airplane, but it is not fixed for a boat that's yawing left and right.

(refer back to post 80)

I think it is logicaly ,that front is toward nose of boat...

Boat yawing angle is very little compare to rudder angle when pumping.

Why would it have to? If you have more forward thrust during the one half stroke than drag during the other half stroke, you have a net forward thrust, yes?

I have difficulty with your terminology. I think that your "forward stroke" is as the sail/rudder moves to the sides. The other stroke -- perhaps the "return stroke" is as the sail/rudder moves back toward the centerline. I also have difficulty with your numbering. It seems to change between 2 and 3 and 0 and 1 without much rhyme or reason.

Yes forward stroke("half stroke") is when rudder moves from centerline to the sides.

Backward stroke("half stroke") or " return stroke "is when rudder moves from sides to the centerline.

I use 0 and 1 for example with boat and two paddles, to separate with orignal rudder-case with 1,2,3 position

 

[DaveC426913]

I think it is logicaly ,that front is toward nose of boat...

Boat yawing angle is very little compare to rudder angle when pumping.

But the devil is in the details. The diagram you posted in post 79 treats front as exactly - and always exactly - frontward. With a turning boat and a thrust that is not along the centre line, the posted diagram is just not informative.

I reassert that you should not make comparisons to aerodynamic scenarios.

 

[Jurgen M]

If you have more forward thrust during the one half stroke than drag during the other half stroke, you have a net forward thrust, yes? So what you are searching for is an asymmetry -- a way in which the pressure gradients during the two halves do not have equal and opposite profiles.

There is only two solution for net thrust:

1.
Backward strokes produce more thrust then forward strokes drag.
Then must be some asymetry in pressures in thes two types of strokes,but we must find how/why this asymetry is created

or

2.
backward and forward strokes produce thrust.
This is unlikely happend, becuasue I can't see how would forward stroke produce thrust, especially when boat moving and induced/head flow " hitting" at front part of rudder every time when rudder moves to sides.

 

[jbriggs444]

There is only two solution for net thrust:

1.
Backward strokes produce more thrust then forward strokes drag.
Then must be some asymetry in pressures in thes two types of strokes,but we must find how/why this asymetry is created

or

2.
backward and forward strokes produce thrust.
This is unlikely happend, becuasue I can't see how would forward stroke produce thrust, especially when boat moving and induced/head flow " hitting" at front part of rudder every time when rudder moves to sides.

2 is indeed a tough sell. With the right parameters, it might be achieved, but I will not attempt to make a plausibility argument here.

1 is easy. Any asymmetry will do. And we have an obvious one.

At the end of the forward stroke, as the foil is brought to rest, this slows the entrained fluid mass. This results in a forward thrust.

At the beginning of the backward stroke, as the foil is sped back up toward the center, this accelerates the entrained fluid mass. This results in a forward thrust.

The symmetry that was expected has the pressure gradient equal and opposite throughout the corresponding portions of the two strokes. But "forward" is not equal and opposite to "forward".

 

[Jurgen M]

2 is indeed a tough sell. With the right parameters, it might be achieved, but I will not attempt to make a plausibility argument here.

1 is easy. Any asymmetry will do. And we have an obvious one.

At the end of the forward stroke, as the foil is brought to rest, this slows the entrained fluid mass. This results in a forward thrust.

At the beginning of the backward stroke, as the foil is sped back up toward the center, this accelerates the entrained fluid mass. This results in a forward thrust.

The symmetry that was expected has the pressure gradient equal and opposite throughout the corresponding portions of the two strokes. But "forward" is not equal and opposite to "forward".

Something like this?c=centerline,rudder constant speed
M=middle,rudder constant speed
D=point where rudder start deccelerate
A=point where rudder stops accelerate
S=point where rudder complety stops

H=high pressure
L=low pressure
red arrow= resultant force perpendicular to rudder surface

 

[hutchphd]

I know for certain that use of a a paddle creates a vortex that persists at the end ofthe stroke. I will bet they are important here.

 

[jbriggs444]

Something like this?

Yes, that depiction matches what I have in mind.

 

[pbuk]

Something like this?

No it is not like that at all. Let's start with the diagram on the right with the rudder stationary at position S.

1. The rudder is accelerated rapidly backwards, creating a high pressure on its rear face and pushing the boat forwards.
2. Somewhere around M the force on the tiller is reduced and the rotational speed of the rudder slows.
3. This allows pressure on each side of the rudder to nearly equalise, reducing yaw and allowing the flow to become laminar reducing drag.
4. This slow movement of the rudder is continued through C until it reaches S on the other side when the cycle is repeated.

Please don't complicate things by talking about pumping the sail (particularly not strong wind pumping as shown in the video), the factors at work in pumping are completely different.

 

[hutchphd]

https://www.sciencedaily.com/releases/2022/02/220222121224.htm

 

[pbuk]

https://www.sciencedaily.com/releases/2022/02/220222121224.htm

A boat is not a fish, however it is possible to create something of that effect by moving your body weight to heel (rotate about the longitudinal axis) the boat from side to side in time with the rudder movements. But we are already confused enough about the basic principles involved without adding to it with lower-order refinements.

 

[hutchphd]

I understand your point but am not convinced that it is not a primary effect. Truly I don't know.

 

[pbuk]

I understand your point but am not convinced that it is not a primary effect. Truly I don't know.

The article summary talks about "precise control of body fluctuations" and "a wavelike motion at all points of the body"; here we have a piece of plywood with a single hinge so it can't be the same thing.

 

[hutchphd]

Certainly not with the finesse of fish. But rowing a boat generates all kinds of eddies if you look closely, particularly at the surface deformations. Insufficient data: no use guessing.

 

[pbuk]

But rowing a boat generates all kinds of eddies if you look closely, particularly at the surface deformations. Insufficient data: no use guessing.

We are not rowing we are doing something usually called "tiller waggling" or sometimes "rudder sculling"; I have 50 years of data and am not guessing.