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

[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.