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

[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 happened, 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 happened, 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".