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

[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.

 

[hutchphd]

Yes I understand the setup, I just don't necessarilly believe the analysis is complete.. Not a big deal.

 

[256bits]

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.

That sounds reasonable with the non-constant rotational velocity of the rudder.

 

[Jurgen M]

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.

I didnt include in my description, purposely reduced rudder rotation during strokes,like we have learn in sailing school.(Because I was expcted that we will come with anylasis to this conclusion.
I want that theory confirm what we are doing in practice is correct)

But fur sure ,you must reduced rotation speed during forwards half strokes and increase rotation speed during backward half strokes.

"Hard on rudder from sides to center, easy from center to the sides", our coach yelled at us.

Rudder is very unefficient in producing thrust, it is not like fish tail.Fish tail and flippers produce thrust in both half-strokes,becuase of flexibility.
That is why I noted few times, that rudder is stiff.

 

[sophiecentaur]

That sounds reasonable with the non-constant rotational velocity of the rudder.

Totally reasonable. Followed by:

"Hard on rudder from sides to center, easy from center to the sides", our coach yelled at us.

and that statement, along with a conservation law, explains the whole thing to some extent. Whatever mechanism happens to be at work under the water, the evidence involves the forces on the tiller and the distance moved by the face of the rudder.
For a deeper explanation, we have to look at the way the water moves and even the effect of the keel on that motion. If we were not dealing with water (and talking about trying the same trick on a sandy beach) the force / speed relationship could be different. There would be more 'grip' at low speeds and less grip as the rudder breaks free from the sand. I could even believe the effect would be totally the reverse.

 

[Jurgen M]

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

Yes it is so obvius why duck minimize surface area of foot during forward stroke.