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

[anorlunda]

I really don't want to enter this discussion. But I just want to point out that the physics of this topic are the same as the sculling method of propulsion.

 

[Lnewqban]

Consider that when the rudder is deflected, the pivot point moves sideways, which makes the hull of the boat deflect some in opposite direction to the rudder.
That creates areas of higher and lower pressure on both sides of the boat, which eventually start moving water from high to low pressure zones.
Not the natural and fluid movement of a fish, but not as effective neither.

Please, see:
https://thefishingwire.com/the-physics-of-fish-movement/

 

[sophiecentaur]

For me it is simple (and perhaps helped by the fact that I was doing this less than 18 hours ago, after the end of the race of course) - pull back hard towards the centreline to push water backwards and create forward thrust, then ease the rudder away from the centreline without stalling to set up for the next thrust whilst creating as little drag as possible.

I agree. I have done the same thing. There is a technique that involves non uniform movement of the tiller. You push harder when the tiller is to one side and pushing forward. It's not just waving a piece of wood under water.

It also works much better with a simple rudder hung on the transom and not with a nearly balanced rudder.

 

[Jurgen M]

Since the motion is rearward and is real, there must be a reaction force somewhere.

But where is this reaction force,I can't find it?

 

[Jurgen M]

I agree. I have done the same thing. There is a technique that involves non uniform movement of the tiller. You push harder when the tiller is to one side and pushing forward. It's not just waving a piece of wood under water.

It also works much better with a simple rudder hung on the transom and not with a nearly balanced rudder.

What do you say, is rudder produce thrust or drag when move from position 2 to 3?

 

[Jurgen M]

For me it is simple (and perhaps helped by the fact that I was doing this less than 18 hours ago, after the end of the race of course) - pull back hard towards the centreline to push water backwards and create forward thrust, then ease the rudder away from the centreline without stalling to set up for the next thrust whilst creating as little drag as possible.

People who don't sail optimist ,dont understend that swing of rudder from position 2 to 3 slows down boat...

 

[jbriggs444]

But where is this reaction force,I can't find it?

Look harder.

 

[Jurgen M]

Look harder.

If you know answer why don't you share with us?

 

[jbriggs444]

If you know answer why don't you share with us?

What happens at the end of the stroke from 2 to 3 when the rudder stops and the tangential motion of the slug of water is halted but its radial motion is not?

An analysis that determines that the force from 2 to 3 is equal and opposite to the force from 3 to 2 misses an important asymmetry -- the water's acceleration is in the same direction for both strokes.

 

[sophiecentaur]

What do you say, is rudder produce thrust or drag when move from position 2 to 3?

You push the tiller forwards and the rudder goes backwards. The reaction force from the water pushes you forwards. You then ease up , once the tiller is amidships and there is less reaction force ( in the direction you don’t want) as the tiller ends up pointing sideways. You then push the tiller forwards again, pruducing a forwards force on the boat. Etc. etc.
Remember the water is a fluid and the forces depend on the velocity of the rudder pushing against it.

 

[jbriggs444]

Remember the water is a fluid and the forces depend on the velocity of the rudder pushing against it.

Water has mass. It is not an ideal massless viscous fluid. The forces also depend on the acceleration imparted to the water.

 

[Jurgen M]

What happens at the end of the stroke from 2 to 3 when the rudder stops and the tangential motion of the slug of water is halted but its radial motion is not?

You want to say that on the end of stroke 3, high pressure is at back part of rudder so resultant force point forward,like picture below?

 

[jbriggs444]

You want to say that on the end of stroke 3, high pressure is at back part of rudder so resultant force point forward,like picture below?

Yes.

 

[Jurgen M]

Yes.

But during 2 to 3, rudder produce drag and when stops in position 3 produce thrust?

 

[Baluncore]

Maybe 2D analysis is insufficient. A 3D analysis would throw water outwards and backwards when the rudder is moved, with the water being replaced from below the rudder post. That is a part turn of a centrifugal pump. The reaction to moving water along that path is forward.

 

[Swamp Thing]

I agree. I have done the same thing. There is a technique that involves non uniform movement of the tiller. You push harder when the tiller is to one side and pushing forward. It's not just waving a piece of wood under water.

It also works much better with a simple rudder hung on the transom and not with a nearly balanced rudder.

After you heave mightily at the handle (tiller?), do you get a definite sense that the rudder (with the water) is now coasting along with relatively less effort on your part, until you heave in the opposite direction?

 

[pbuk]

People who don't sail optimist ,dont understend that swing of rudder from position 2 to 3 slows down boat...

What makes you think that I have not sailed an Optimist? In fact an Oppy is an easy boat to rudder scull due to the low aspect rudder. There is a whole section of the international umpires class notes on the RRS for Oppies on sculling.

 

[pbuk]

After you heave mightily at the handle (tiller?), do you get a definite sense that the rudder (with the water) is now coasting along with relatively less effort on your part, until you heave in the opposite direction?

Yes, that is what I happens when you ensure more laminar flow by not stalling the rudder as you ease it to the other side.

 

[Jurgen M]

What makes you think that I have not sailed an Optimist? In fact an Oppy is an easy boat to rudder scull due to the low aspect rudder. There is a whole section of the international umpires class notes on the RRS for Oppies on sculling.

I didnt say that.

When you pull "pumping" sail toward centerline(in my case move rudder position 1 toward 2) boat accelerate forward,because you increase static pressure on the windward side of sail and decrease pressure at the leeward side of sail.

Try this,
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..
Can you try this on laser or optimist?

 

[sophiecentaur]

After you heave mightily at the handle (tiller?), do you get a definite sense that the rudder (with the water) is now coasting along with relatively less effort on your part, until you heave in the opposite direction?

Very much so. It's the same with sculling / rowing. You don't actually need to take the oar out of the water for the return stroke in order to make forward progress. (Not the fastest method but it's still valid.)

 

[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...