I Physics of sailing/windsurfing systems

[pbuk]

yes..

Well you need to get rid of that problem. How do you think that the rig knows that it has a flexible joint at the foot when it is held in place? Do you think that it is impossible for a rudder to turn a boat because it is attached by a flexible joint?

 

[pbuk]

feet are fixed above right weight scale,just like in wsurf,where feet can not move ,they are allways in footstraps
line is fixed in horizontal position as I said

sail force can very ,so sailor must lean back more and change line attached point to balance sail torque...

again as long as lines stay horizontal ,hydrodnamic lift will be allways under sailor feet..

In order to keep the harness line horizontal you would have to be 12 feet tall.

But if you put lines at downward angle,you can now press left weight scale,so hydrodynamic lift is shift somewhere forward,nose is " press down"

The only thing that changes the hydrodynamic lift is the amount of the board that is in the water when in displacement mode (which you never are of course) or the speed and angle of attack when in planing mode.

 

[John Mcrain]

No, the left scale will read > 0kg due to the torque from the wind force [edit] and the mass of the board and the rig. But this diagram is useless because (i) unless this sailor is on a dead run they will fall into leeward and (ii) the board is supported by a buoyant force not a pair of scales.

Board and rig has very little weight compare to sailor,so I use approximation as they are massless..I thought that was clear.. You are wrong, there is no any torque from the wind force, because joint can't transfer torque to board.
Line must be at angle to press left weight scale.

I really don't understand how you don't see that without any calcualtion,this must be crystal clear

 

[jbriggs444]

sail force can very ,so sailor must lean back more and change line attached point to balance sail torque...

So now the attachment point of line to mast is in a greased Teflon track so that it is free to slide up and down the mast as the sailor leans back. That is an interesting model. Let us put some numbers on it.

Let us make the simplifying assumption that the surfer's center of mass is at the same level as his hands (a distance h from his feet). We can do a force balance on the sailor and see that for a lean angle of $\theta$ (measured from vertical = 0) that the torque from the line must match the torque from gravity: $h t \cos \theta = h mg \sin \theta$.

The torque on the mast is also equal to $h t \cos \theta$. So the maximum torque on the mast is mgh which is approached in the limit as the sailor reaches the horizontal.

If the force from the wind exceeds this, the mast falls over. The only useful effect of the surfer's lean is that it puts his center of gravity farther back. And there is a limit on how far he can move his center of gravity with his feet stuck in the cleats.

 

[pbuk]

because joint can't transfer torque to board.

So what stops the mast from rotating forwards?

 

[John Mcrain]

So now the attachment point of line to sail is in a greased Teflon track so that it is free to slide up and down the mast as the sailor leans back. That is an interesting model.

Yes it can be like this if you want.
Or you can stop, lower the boom(because line is conected to boom) try this new geometry in second run and see what is board doing at water..

For sure sail force is limited by sailor max "righting moment",when sail torque is greather than that,sailor and sail is catapulted...

 

[pbuk]

48 hours ago you posted this:

I don't understand physics of this..

Why have you now decided that you understand it so well that you are going to tell everyone else that they are wrong?

 

[jbriggs444]

Yes it can be like this if you want.
Or you can stop, lower the boom(because line is conected to boom) try this new geometry in second run and see what is board doing at water..

For sure sail force is limited by sailor max "righting moment",when sail torque is greather than that,sailor and sail is catapulted...

Not what I want; what you have demanded. You want the line to be horizontal. So I am making the line horizontal.

Using this model, the scale readings are both unchanged until the moment when the maximum wind force is exceeded. That is because the increased down pitch from the wind torque is matched by the increased up pitch from the surfer's center of mass moving back.

If you change your mind about the model and use a line with a fixed attachment point on the mast, the situation changes. There is a vertical down force from line on mast and then from mast on board. The scale readings change as wind force increases because the mast force increases.

 

[John Mcrain]

48 hours ago you posted this:

Why have you now decided that you understand it so well that you are going to tell everyone else that they are wrong?

No I just tell you ,that you are wrong only for my let's call it "horizontal line case".
@jbriggs444 just confrim that I am correct for this specific case,read his posts and explanation..

 

[pbuk]

I suggest you take a look at this page http://joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html and come back with any questions.

 

[jbriggs444]

No I just tell you ,that you are wrong only for my let's call it "horizontal line case".
@jbriggs444 just confrim that I am correct for this specific case,read his posts and explanation..

Let me be clear. I am focused on the part of the problem where we take a toy model and use it to generate predictions. The part where we decide which toy model matches the real world situation best, I make no claims. The frustrating bit for me has been trying to figure out which exact toy model is being considered.

 

[John Mcrain]

That is because the increased down pitch from the wind torque is matched by the increased up pitch from the surfer's center of mass moving back.

You just explain what I am talking about last 50 posts specifily for "horizontal line case".

 

[John Mcrain]

Not what I want; what you have demanded. You want the line to be horizontal. So I am making the line horizontal.

Yes your idea of sliding teflon track is very good.
In reality you can stop and just low the boom and try new settings in second run..

(As I said before,in reality line allways has some downward angle,and you don't change boom height when sailing so line attachment point remain the same during sailing..
But I deliberately use horizontal line case to prove that you can't press nose down with this settings..)

 

[jbriggs444]

Yes your idea of sliding teflon track is very good.
In reality you can stop and just low the boom and try new settings in second run..

(As I said before,in reality line allways has some downward angle,and you don't change boom height when sailing so line attachment point remain the same during sailing..
But I deliberately use horizontal line case to prove that you can't press nose down with this settings..)

I am not sure that I understand what you are saying. That the toy model is never correct, but that it proves the thing that you want it to prove so that it must be right anyway?

 

[John Mcrain]

I am not sure that I understand what you are saying. That the toy model is never correct, but that it proves the thing that you want it to prove so that it must be right anyway?

As you see ,my english is not so good,so I have very hard time to explain so complex system with words.
I didnt understand your question.Can you simplfy?

 

[John Mcrain]

One more thing about lines angle

If feet and mast lay in same line ,than for given sailor lean out angle ,all three angles (blue rope) give same sail force.It doesn metter where is attachment point at mast

If feet is out from center line,than for given lean out angle, sail force change with different lines angle.
lowest angle give small sail force,higher you choose angle sail force is higer..
also the more distance is from feet to center line,grather sail force will be..

all of this I can prove with math

 

[pbuk]

If feet and mast lay in same line ,than for given sailor lean out angle ,all three angles (blue rope) give same sail force.It doesn metter where is attachment point at mast

This is wrong, but perhaps this time it is due to a problem of language. All three angles give the same righting moment, but not the same force. Also in this case the mast step joint is free to rotate laterally and so none of the righting moment can be transferred to the board; unless the righting moment is exactly equal to the heeling moment provided by the force of the wind on the sail then the rig and sailor will fall into the water.

If feet is out from center line,than for given lean out angle, sail force change with different lines angle.
lowest angle give small sail force,higher you choose angle sail force is higer..

The total righting moment is the same wherever the wishbone is mounted but now it has two components: the rig righting moment and the [component forcing the windward rail into the water - I can't remember what this is called right now]. And yes, the higher up the rig the wishbone is mounted the greater proportion of the righting moment goes into the rig.

also the more distance is from feet to center line,grather sail force will be..

No, this just increases the [component forcing the windward rail into the water].

all of this I can prove with math

I don't think attempting that would be a good use of anybody's time. It would be more profitable to get out on the water and do some sailing.

 

[A.T.]

When you connect three beams via universal joints to a triangle, you have a rigid shape. A force applied to one corner will create a torque on the opposite beam. Do you have a problem with two universal joints transferring a torque here?

yes..

You don't understand why a triangular truss is rigid?

 

[John Mcrain]

You don't understand why a triangular truss is rigid?

Yes I understand that but that has nothing to do with wsurf system.
If sailor-sail-board is rigid system like you suggest,they will allways "rotate" togeather..

You can see at video that sail and sailor are " moving" separatley from board.Board is almost flat and sail and sailor moving all the time...


If you really make wsurf system rigid,so "welded" sail to board, borad will be moving in pitch and roll direction all the time ,it will be completely out of control..

 

[pbuk]

Yes of course a real windsurfer is moving all the time, but you have not taken this into account either - you have only considered the simplified situation assuming that the position is completely stable.

Lets look at an different (but still dynamic) system: a tiller on a monohull dinghy attached to the rudder with a universal joint like this (best seen from about 1 minute in):


See how the sailor is constantly moving the rudder by applying a torque about its pivot point? The only way the sailor can apply this torque is through the universal joint attaching the tiller.

This clearly demonstrates that you can apply a torque through a UJ so you need to stop believing that you can't.

Once you have stopped focussing on this you can start looking at a better free body diagram like the ones on the site I linked a few posts ago.

 

[A.T.]

When you connect three beams via universal joints to a triangle, you have a rigid shape. A force applied to one corner will create a torque on the opposite beam. Do you have a problem with two universal joints transferring a torque here?

Yes I understand that .but that has nothing to do with wsurf system.

It has a lot more to with the wsurf system than the single joint you keep showing:

Because my brain can not understand how universal joint can transfer torque to the board?

View attachment 279075

Do you understand that multiple universal joints attached at different locations can transfer a torque?

 

[John Mcrain]

Lets look at an different (but still dynamic) system: a tiller on a monohull dinghy attached to the rudder with a universal joint like this (best seen from about 1 minute in):


See how the sailor is constantly moving the rudder by applying a torque about its pivot point? The only way the sailor can apply this torque is through the universal joint attaching the tiller.

This clearly demonstrates that you can apply a torque through a UJ so you need to stop believing that you can't.

This is wrong discription of physics.

Universal joint transfer side force from tiller-extension to tiller.Conseqeunce of side force at tiller is rotating rudder around his pivot point.
So universal joint at the end of tiller does not trasfer torque to tiller,it transfer side force, and consequence of this side force is rotating rudder..

I never said that universal joint can not transfer forces...

Does any member agree with me about this what I just explain in this post?

 

[russ_watters]

You are wrong, there is no any torque from the wind force, because joint can't transfer torque to board.
Line must be at angle to press left weight scale.

I really don't understand how you don't see that without any calcualtion,this must be crystal clear
[separate post]
Does any member agree with me about this what I just explain in this post?

@John Mcrain you need to start doing better, because at this point we are starting to doubt your sincerity. You are stating false things and misrepresenting what people are telling you. The key issue is so simple it is difficult for us to believe that you are really having this much trouble with it. So I need you to be crystal clear in your understanding of the following:

1. In scenario 1, the mast has a force/torque applied by force F and falls over. The board has no torque applied due to fore F.
2. In scenario 2, the mast does not fall over. The entire structure acts like one rigid object and force F pushes forward and applies a clockwise torque to the entire system. The entire object rotates clockwise unless there are other forces.

So:
3. A single universal joint cannot transfer a torque around itself.
4. A connected series of universal joints becomes a rigid object. Torques can be transferred via internal forces. E.G., in scenario 2, the torque is not transferred through the universal joint connecting the mast to the board, it is transferred by linear forces through the brace and its connection points.

Do you understand and agree with these statements?

 

[John Mcrain]

@John Mcrain you need to start doing better, because at this point we are starting to doubt your sincerity. You are stating false things and misrepresenting what people are telling you. The key issue is so simple it is difficult for us to believe that you are really having this much trouble with it. So I need you to be crystal clear in your understanding of the following:

View attachment 279132

1. In scenario 1, the mast has a force/torque applied by force F and falls over. The board has no torque applied due to fore F.
2. In scenario 2, the mast does not fall over. The entire structure acts like one rigid object and force F pushes forward and applies a clockwise torque to the entire system. The entire object rotates clockwise unless there are other forces.

So:
3. A single universal joint cannot transfer a torque around itself.
4. A connected series of universal joints becomes a rigid object. Torques can be transferred via internal forces. E.G., in scenario 2, the torque is not transferred through the universal joint connecting the mast to the board, it is transferred by linear forces through the brace and its connection points.

Do you understand and agree with these statements?

Yes I understand evertything you said and 100% agree with all what you said.

Now you tell me one think,do you understand that sailor do not hold sail like on picture below?

Also quote my words that you don't agree with me ,what I said wrong...?

 

[russ_watters]

Yes I understand evertything you said and 100% agree with all what you said.

Ok, good.

Now you tell me one think, do you understand that sailor do not hold sail like on picture below?

The internal geometry is irrelevant to the points I was making, so I don't understand the point of that question. E.G.; he could do that. Whether he does or doesn't is a new scenario that hasn't been described.

Also quote my words that you don't agree with me ,what I said wrong...?

I did. And you repeated it several times.
[edit] Here it is again:

here is no any torque from the wind force, because joint can't transfer torque to board.

That's wrong. There *is* a torque on the board from the wind force because a series of universal joints *can* (and does) transfer torque to the board.

 

[sophiecentaur]

Universal Joint: Is there a bit of cross-purpose going on here? The use of this term in this thread is not a common one. A so-called Universal Joint is normally used for actually transferring a torque between a shaft and an object (or another shaft).This applies in car transmissions and some socket spanner sets. In that context it is not truly 'universal' as a ball and socket would be. Otoh the rubber 'finger' and ball that's used on a sailboard transmits no torque; the sailor doesn't grip the mast like a screwdriver and when the sail changes direction, the joint will move round and allow the sail angle to be set in any direction the sailor sets by her body / arm / feet positions. There is no torque applied to that UJ because it will move however you want it to, with no reaction.
The only explicit torque that can be applied here is by the sailor's feet (as a pair) push / pulling laterally on the surface of the deck.

 

[John Mcrain]

The internal geometry is irrelevant to the points I was making, so I don't understand the point of that question.

once again
Do you understand that left weight scale will show 0kg for every scenario where blue line and mast force is 100% horizontal,feet must stay above right scale,and system must be in equlibrium?

 

[russ_watters]

Universal Joint: Is there a bit of cross-purpose going on here? The use of this term in this thread is not a common one. A so-called Universal Joint is normally used for actually transferring a torque between a shaft and an object (or another shaft).This applies in car transmissions and some socket spanner sets. In that context it is not truly 'universal' as a ball and socket would be.

You're right that the actual joint here is a ball-and-socket, which is free to rotate in all three axes, un-constrained. I don't think that's in dispute here. I don't think calling it a universal joint is a point of confusion.

 

[russ_watters]

once again
Do you understand that left weight scale will show 0kg for every scenario where blue line and mast force is 100% horizontal, feet must stay above right scale, and system must be in equlibrium?

View attachment 279138

Certainly not. You must be adding assumptions/constraints you aren't saying (or you're just wrong). The bad grammar doesn't help either. I'll not speculate about an under-defined scenario. Draw all the forces and label all the lengths, and solve for your scenario.

You're jumping around so much here that it is difficult to know what your ultimate goal is. That makes it look like your ultimate goal is a never-ending argument.

 

[russ_watters]

To put a finer point on it, this was your original question:

Sailing boat has pitching moment caused by sail drive force and hydrodnamic drag froce in the water,distance between them is lever arm.
It cause nose of boat to push down..

Does windsurfing has pitching moment and if yes how it is transfer to the board ,if sail is connected to board with universal joint and sailor feet also act as joint. But joint can not transfer moment..?
I don't understand physics of this..

Do you now understand that a series of members connected with non-rigid joints forms a rigid object that enables the sail force to apply a torque to the board (entire system)? You replied to my post #113 in a way that suggests this misunderstanding has been cleared-up and you now understand, but your other recent posts imply you are still holding on to the misunderstanding.