Physics of sailing/windsurfing systems

[A.T.]

board has footstraps at tail and when you planning, both feet are in footstraps all the time..

What is your point here? Did you read my post #43?

But he has two feet, and can transfer the weight between them. So the effective center of pressure of the feet combined is not fixed.

This is just like having a foot that can move forward and backward along the board.

 

[jbriggs444]

External forces are not fixed,when sailor hold sail at 4 degrees AoA(relativly open sail-sheet out) sail force is way smaller compare when he sheet in and make sail at 18 degrees AoA..

So I am going from low sail power to high sail power ,and question is why when I sheet in(increase sail power) nose goes down?

Why would it not? You've increased an external downward pitching torque. Surely one would expect the craft to nose down unless/until countered by another external torque. The details of the internal bracing do not change that.

If the craft is stable (not a certainty - it could be maintained in an unstable equilibrium by active controls -- e.g. a unicycle), one would expect a pitch down change to result in a restoring upward pitching torque.

 

[John Mcrain]

What is your point here? Did you read my post #43?

This is just like having a foot that can move forward and backward along the board.

Yes I am...

So only way to press nose down is thorugh sailor feet weight distribution?

What about sail "pitching moment"?

 

[A.T.]

So only way to press nose down is thorugh sailor feet weight distribution?

Who said that?

 

[John Mcrain]

Why would it not? You've increased an external downward pitching torque.

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

If mast and board is connected with fixed conection(like all sailboat) than it is very easy to understand boat pitching caused by sail force..

If push at mast B,beam A will not rotate...let say joint allows 360 degress rotation

 

[pbuk]

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

Because when the mast is held in place by the sailor the mast step is not acting as a universal joint. If the sailor let's go of the wishbone then the mast will rotate about the universal joint, in exactly the same way as the mast on a sailing boat will fall if any of the stays under tension breaks (or the gate collapses for an unstayed mast).

 

[John Mcrain]

Because when the mast is held in place by the sailor the mast step is not acting as a universal joint. If the sailor let's go of the wishbone then the mast will rotate about the universal joint, in exactly the same way as the mast on a sailing boat will fall if any of the stays under tension breaks (or the gate collapses for an unstayed mast).

So I can treat mast like is welded to the board and use only external forces for calculating pitch torque?

 

[pbuk]

Why are you picking on the pitching force: do you think that there is no heeling force either?

 

[John Mcrain]

Why are you picking on the pitching force: do you think that there is no heeling force either?

Because pitch torque put nose down...and this is theme of this topic..

 

[pbuk]

OK, let's leave heel out of this.

So I can treat mast like is welded to the board and use only external forces for calculating pitch torque?

Yes, as long as the board/sailor/rig system is entirely rigid (which it is not in real life of course, the sailor is continuously pumping the rig).

 

[A.T.]

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

This was explained in post #8 already:

That's not the only connection between the sail and the board. The surfer also connects them. Two universal joints in combination can transfer moments.

 

[John Mcrain]

OK, let's leave heel out of this.

Yes, as long as the board/sailor/rig system is entirely rigid (which it is not in real life of course, the sailor is continuously pumping the rig).

Yes but in windsurf ,internal angles of lines and geometry determine how much will be magnitude of sail force,which at the end determine how much will be pitch torque..
So I can't neglect internal "geometry"..

 

[A.T.]

So I can treat mast like is welded to the board and use only external forces for calculating pitch torque?

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?

 

[pbuk]

Yes but in windsurf ,internal angles of lines and geometry determine how much will be magnitude of sail force,which at the end determine how much will be pitch torque..
So I can't neglect internal "geometry"..

No, the flow of the wind over the sail determines the magnitude and direction of the sail force. You must ignore internal geometry.

 

[John Mcrain]

No, the flow of the wind over the sail determines the magnitude and direction of the sail force. You must ignore internal geometry.

Yes wind increase sail force,but useing optimal "internal geometry" allow sailor to hold more sail power,increase his "righting moment".
For example if you put boom at 1m height,your righting moment is very small..

harness line angle only don't change sailor righting moment if sail feet is at centerline,in line with mast.
But sailor feet can be even 0.5m out of center line ,which increase righting moment

 

[John Mcrain]

Do you have a problem with two universal joints transferring a torque here?

yes..

 

[John Mcrain]

You must ignore internal geometry.

Do you see as long as harness line(blue rope) is 100% horizontal,you can not press left weight scale?
Scale will read 0kg..

 

[jbriggs444]

Do you see as long as harness line(blue rope) is 100% horizontal,you can not press left weight scale?

View attachment 279086

Again, what are you holding fixed and what are you allowing to vary? If you hold the line angle fixed at directly horizontal, that constrains the relationship that can exist between center of mass, foot position and tension when the sail force is allowed to vary and the mast is required not to fall over.

 

[pbuk]

Do you see as long as harness line(blue rope) is 100% horizontal,you can not press left weight scale?
Scale will read 0kg..

View attachment 279086

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.

 

[John Mcrain]

Again, what are you holding fixed and what are you allowing to vary? If you hold the line angle fixed at directly horizontal, that constrains the relationship that can exist between center of mass, foot position and tension when the sail force is allowed to vary and the mast is required not to fall over.

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..

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"

 

[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?