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

[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.

 

[hutchphd]

Attemps to create a helicopter by watching a hummingbird were not very fruitful. I think the motion of rudder propulsion is more like that of a tuna than a duck but may be completely different from either because of the suppleness of the natural systems. I do sort of understand an Evinrude.

 

[DaveC426913]

I think the motion of rudder propulsion is more like that of a tuna than a duck

Yes. A stiff rudder cannot minimize its cross section like a duck's foot.

 

[pbuk]

Nor can it flex longitudinally like a tuna.

 

[DaveC426913]

Tuna is one of the stiffer swimmers.

 

[hutchphd]

Nor can it flex longitudinally like a tuna.

I disagree. The combination of boathull plus rudder does flex. I think a rudder by itself would work less well.

 

[sophiecentaur]

Yes. A stiff rudder cannot minimize its cross section like a duck's foot.

But its effective cross section will depend on the speed relative to the water.

I disagree. The combination of boathull plus rudder does flex. I think a rudder by itself would work less well.

Yes. A rudder at the back of a large rectangular raft would behave differently.

 

[Jurgen M]

A stiff rudder cannot minimize its cross section like a duck's foot.

For sure rudder can not do this but it would be better if can do it.
This is just one more confirmation that rudder produce drag during forward stroke.

 

[erobz]

To me it seems like the rudder and the hull make the crude motion of a swimming fish.

 

[DaveC426913]

For sure rudder can not do this but it would be better if can do it.
This is just one more confirmation that rudder produce drag during forward stroke.

All things with mass produce drag. Generalizations are never, ever useful.

The issue is whether it produces more thrust than drag.

 

[Jurgen M]

The issue is whether it produces more thrust than drag.

How do you mean "whether it", so you think that rudder maybe produce net thrust in forward stroke?

How would you describe rudder during forward stroke, in terms of pressures and net force direction?

 

[pbuk]

How do you mean "whether it", so you doubt that rudder maybe produce net thrust in forward stroke?

He meant whether it produces more force than drag over the whole cycle. Please stop revisiting over and over again the idea that a flat surface moving forwards through the water can produce a net force in the forward direction, it cannot.

 

[Jurgen M]

Please stop revisiting over and over again the idea that a flat surface moving forwards through the water can produce a net force in the forward direction, it cannot.

This not my idea, this completely unlogic idea starts members from this forum.
For sure it can not produce thrust.

 

[berkeman]

This thread has more than run its course. Thanks for your contributions everyone. Thread is closed.