Physics of sailing/windsurfing systems

[John Mcrain]

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..

 

[DrStupid]

Does windsurfing has pitching moment and if yes how it is transfer to the board

It is transferred to the surfer. The surfer needs to lean back for compensation.

 

[A.T.]

Does windsurfing has pitching moment and if yes how it is transfer to the board

The pitching moment you have shown for the boat is the external moment on the whole vessel. To analyse the moments on individual parts, you have to define precisely how you cut the system into parts.

 

[John Mcrain]

The pitching moment you have shown for the boat is the external moment on the whole vessel. To analyse the moments on individual parts, you have to define precisely how you cut the system into parts.

When I read windsurf tunning tips,they say that if you put boom down or use sail that has center of effort higher,that will put nose down.

So I allways wonder how can "sail pitching moment" press nose down if sail is connect to board with universal joint?

In sailing boat it is clear ,but I can't understand physics of wsurf system when talk about board trim(press nose up or down..)..

 

[anorlunda]

So I allways wonder how can "sail pitching moment" press nose down if sail is connect to board with universal joint?

It sounds like you thing the nose is pressed down only by a twisting force at the base of the mast.

The sail is attached to the boat at three places. 1) bow, 2) mast step, 3) stern. The stern connection is via the main sheet.

Suppose the main sheet pulls up, and the mast step pushes down. Doesn't that press down the bow? The universal joint doesn't change that at all.

 

[John Mcrain]

It sounds like you thing the nose is pressed down only by a twisting force at the base of the mast.

The sail is attached to the boat at three places. 1) bow, 2) mast step, 3) stern. The stern connection is via the main sheet.

Suppose the main sheet pulls up, and the mast step pushes down. Doesn't that press down the bow? The universal joint doesn't change that at all.

I am talking about windsurf not sailing boat.Wsurf has only two points that connect to board,sailor feet and universal joint...Niether of these are fixed connection..

 

[anorlunda]

I am talking about windsurf not sailing boat.Wsurf has only two points that connect to board,sailor feet and universal joint...Niether of these are fixed connection..

The same principles apply. Mechanical engineers use a general called the Free Body Diagram. You can use that on a windsurfer also. The key is to consider all the forces, the mast, the surfer, buoyancy, gravity, and drag on the wetted portions of the hull.

Read that article on Free Body Diagram and try to apply it to a windsurfer's hull. Post your analysis here, and we can help you with it.

Remember that the language of science and engineering is math. If you're looking for purely verbal explanations, a science or engineering forum is not the right place.

 

[A.T.]

So I allways wonder how can "sail pitching moment" press nose down if sail is connect to board with universal joint?

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]

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.

I already said two connection,feet and unversal joint..

How do you mean that two joint in combination can transfer moment in wsurf case?
I am not talking about catapult case ,where sail force is much greather what sailor can hold,so sail,sailor and board rotate into crash,...

 

[John Mcrain]

The same principles apply. Mechanical engineers use a general called the Free Body Diagram. You can use that on a windsurfer also. The key is to consider all the forces, the mast, the surfer, buoyancy, gravity, and drag on the wetted portions of the hull.

Read that article on Free Body Diagram and try to apply it to a windsurfer's hull. Post your analysis here, and we can help you with it.

Remember that the language of science and engineering is math. If you're looking for purely verbal explanations, a science or engineering forum is not the right place.

How do you mean same principle apply,so I can neglect joints and calculate like they are fixed connection?

 

[russ_watters]

How do you mean same principle apply,so I can neglect joints and calculate like they are fixed connection?

No, he said draw a fbd and what's going on will become clear.

 

[jbriggs444]

How do you mean same principle apply,so I can neglect joints and calculate like they are fixed connection?

Yes.

In a more general case you could consider a simple structure: A triangular truss segment. Three pieces of wood, each 1 foot long connected with hinges (or ball joints) at their ends. Despite the fact that all of the joints are free to rotate, the structure as a whole is rigid.

One can consider the wind surfing situation in this light. There are three pieces (board, mast, sailor) and three joints (base of mast, hands on mast, feet on board). The structure is nominally rigid because of this.

Of course, the sailor still has some control to exert. He can push or pull with his hands, move his body forward and aft, walk his feet fore and aft on the boat or walk his hands up and down on the mast. But it is probably best to concentrate on the static situation and not worry about dynamics or control.

 

[John Mcrain]

In a more general case you could consider a simple structure: A triangular truss segment. Three pieces of wood, each 1 foot long connected with hinges (or ball joints) at their ends. Despite the fact that all of the joints are free to rotate, the structure as a whole is rigid.

One can consider the wind surfing situation in this light. There are three pieces (board, mast, sailor) and three joints (base of mast, hands on mast, feet on board). The structure is nominally rigid because of this.

Of course, the sailor still has some control to exert. He can push or pull with his hands, move his body forward and aft, walk his feet fore and aft on the boat or walk his hands up and down on the mast.

I think this example is wrong,because from this description sailor must be in tension to hold sail power.Sailor hold sail power with his weight through harness lines ,his body is not in tension..

Sail power is control with changing sail angle of atack with boom,sheet out=push back hand out , sheet in=pull back hand towards you..
You can see at this picture,board nose is lift up,this ussusaly happened when you sheet out(reduced angle of attack of sail=reduced sail power).But once you again sheet in(increase sail power) nose of board will be pushed down,and board will be in control..

So my question is how sail power can push nose of board down,if sail is connect to board with universal joint and sailor stay at back of board in both footstraps all the time?
I can't understand concept of pitching moment at board in windsurf system..

 

[jbriggs444]

I think this example is wrong,because from this description sailor must be in tension to hold sail power.Sailor hold sail power with his weight through harness lines ,his body is not in tension..

The model is not wrong. It is just not complete. In particular, it does not include any external forces on the sailor, the mast or the board.

For instance, the presence of the force of gravity means that the sailors legs can be in compression while his arms are in tension. [I do not consider a quibble about whether it is arms or harness that attaches sailor to the mast to be very relevant -- though with two (or three!) attachment points, you have at least one more truss element and a way to exert torque]

Please identify the relevant forces and bodies and create a free body diagram.

 

[John Mcrain]

Please identify the relevant forces and bodies and create a free body diagram.

front view:
forget about arms, use only harness line that stay horizontal as picture show..

Questions :
1) Why this diagram treats wsurf as fixed connection,one rigid thing?

2)If drive force increase or center of effort in sail move up,will "board lift" force move to the right(towards nose)?If yes,what cause to put nose down if joints(feet and sail-joint) can't transfer moment to the board?(I use "board lift" position as indicator of what board nose is doing(we call this "board trim" or in sailing/aviation "pitching moment"),because board lift depend on wetted area,for example: if board lift is moved forward ,that mean wetted area increase so nose must be more down,"board is glued to the water" which increase control but reduce speed because of increased wetted area...etc)

 

[anorlunda]

You can simplify by looking only on forces acting on the hull. That diagram "balances of forces and motion" is missing some critical forces on the hull.

1. The force of the mast via universal joint or fixed joint.
2. The force of gravity on the weight of the board. That acts through the center of gravity of the board.
3. Buoyancy. It depends on portion of the board below the surface, which depends on the board angle. I think that is what the diagram calls board lift. Buoyancy acts through the center of buoyancy which is not the same place as the center of gravity.
4. If there is a significant bow wave, then there is an additional force lifting the forward end of the wetted surface. (probably not significant for a windsurfer).
5. Don't forget that the weight of the person pressing down on the board varies as he pulls on the boom, and the location of that force varies as the person moves. If you are trying to calculate nose up/down you should probably model the left foot and right foot separately.

 

[John Mcrain]

You can simplify by looking only on forces acting on the hull. That diagram "balances of forces and motion" is missing some critical forces on the hull.

1. The force of the mast via universal joint or fixed joint

This is internal forces,why do you think that internal forces can change net moment of some system?
When you do free body diagram ,internal forces are irrelevant

If you sit in car and push car window with hands ,car will not move forward or rotate,internal arrangement is irrelevant ..

 

[anorlunda]

This is internal forces

Do you mean the force of the mast pushing on the board? If the FBD describes only the board, that is not an internal force, it is external.

 

[DaveE]

I am talking about windsurf not sailing boat.Wsurf has only two points that connect to board,sailor feet and universal joint...Niether of these are fixed connection..

But each of these points is fixed in a certain direction. Think of a train on tracks; free to move in one direction, not in the other. For example, The U-joint is free to rotate in any direction but not to translate across the board surface.

Perhaps this will help you think about this problem. Consider a windsurfer moving in equilibrium, all parameters unchanging. Now imagine that at any of these points (sailor hands, feet, U-joint) things are allowed to slip; i.e the sailor let's go or suddenly the board is very slippery. What moves, and in which direction? This may help with insight into the forces at play.

 

[John Mcrain]

Do you mean the force of the mast pushing on the board? If the FBD describes only the board, that is not an internal force, it is external.

Yes but that was your comment for picture "balances and forces" which don't descirbe only board

 

[John Mcrain]

I can't believe that this topic is hard even at physics forum.

 

[russ_watters]

I can't believe that this topic is hard even at physics forum.

Hard for whom?

BTW, your OP talked about the pitching moment, but now you are looking at both pitch and roll. Is that your intent? Which are you having trouble with -- or both?

 

[John Mcrain]

Hard for whom?

BTW, your OP talked about the pitching moment, but now you are looking at both pitch and roll. Is that your intent? Which are you having trouble with -- or both?

Where do you see that I am talking about roll?

I am talking only on board nose up/down movement ,that is pitch..

Do you understand what is my question?

 

[russ_watters]

Where do you see that I am talking about roll?

The first picture in post #15 looks like a front view.

If you're only talking about pitch, then fair enough. From the questions in that post:

1) Why this diagram treats wsurf as fixed connection, one rigid thing?

The moments around the center of lift or pivot/anchor sum to zero, so it doesn't move. That doesn't mean it couldn't if they didn't sum to zero. If you prove or stipulate that they don't move, then you don't need to solve for those forces to figure out what happens to the board.

For that matter, you also know if the whole thing is stable, the torques about the center of buoyancy/lift sum to zero as well.

2)If drive force increase or center of effort in sail move up, will "board lift" force move to the right(towards nose)?If yes,what cause to put nose down if joints(feet and sail-joint) can't transfer moment to the board?

It looks to me like if the drive force increases or the center of lift moves up, the rider has to lean back further, pitching the nose up/moving the center of lift back.

 

[John Mcrain]

The first picture in post #15 looks like a front view.
It looks to me like if the drive force increases or the center of lift moves up, the rider has to lean back further, pitching the nose up/moving the center of lift back.

I just draw front view so that members see what is going on and I deliberately draw harness lines horizontal,so they can't say that rider transfer his weight to mast foot/universal joint..

Your description is not what going on at wsurf.
When rider sheet in(increase sail power),nose goes down and when sheet out nose lift up.

You can see this in video at 1:44,rider is planning in both footstraps which are at back of board,he open sail too much,we call it "sheet out" (decrease sail power) and you can see how nose lift up.



My question is why when rider sheet in, nose goes down,how can I explain physics behind this?

(In fixed connection-sailboat is easy,I just say pitching moment increase which rotate nose down but in wsurf I don't know how to explain this movement..)

 

[russ_watters]

Your description is not what going on at wsurf.
When rider sheet in(increase sail power),nose goes down and when sheet out nose lift up.

You can see this in video at 1:44,rider is planning in both footstraps which are at back of board, he open sail too much, we call it "sheet out" (decrease sail power) and you can see how nose lift up.

My question is why when rider sheet in, nose goes down, how can I explain physics behind this?

(In fixed connection-sailboat is easy, I just say pitching moment increase which rotate nose down but in wsurf I don't know how to explain this movement..)

Is this your take or do you have a source that is telling you these things? What's "wsurf"? Because what you are describing doesn't look to me like what I'm seeing in the video. In the video, at ~1:44, the rider is coming ashore. He's sheeting out to slow down, while at the same time he appears to be leaning back to deliberately pitch up (maybe keep the board from grounding or to add drag, or he's holding up the sail while dismounting?). [edit] the board is also planing, so the lift/drag situation changes dramatically as it slows down.

In the diagram (labeled fig 8) everything is balanced. If you want to examine what happens when something changes, mark up the diagram to describe what is happening. Be specific, because right now you are jumping around. You've now listed 3 separate changes that could be analyzed (larger drive force, larger sail/higher center of force and sheeting out). Pick one and analyze it!

 

[John Mcrain]

He's sheeting out to slow down, while at the same time he appears to be leaning back to deliberately pitch up

Nose pitch up allways happen when you sheet out, this is consequnece of reduced "mast foot pressure" as wsurf comunity love to say..
But I don't love to use this term,because mast foot pressure is internal force and internal force is irrelevant for system.So I try explain to me everything with external forces ,but as you can see I have trouble.

Who can explain real physics behind this fine tunning I give him million dollars! :)
Specialy part how boom height can affect board trim"pitch"...

 

[russ_watters]

Nose pitch up allways happen when you sheet out, this is consequnece of reduced "mast foot pressure" as wsurf comunity love to say..

Source? Who is "wsurf"?

Who can explain real physics behind this fine tunning I give him million dollars! :)
Specialy part how boom height can affect board trim"pitch"...

You should draw it on the diagram. What happens to the geometry when you change the boom height?

 

[John Mcrain]

Source? Who is "wsurf"?

You should draw it on the diagram. What happens to the geometry when you change the boom height?

Once of tons links about mast foot pressure:
http://www.guycribb.com/userfiles/documents/downforce.pdf
(author use harness lines / hands angle as indicator of mast foot prressure,so he state if harness lines/hand point downward,you transfer some of your weight to the mast foot pressing the board down)
But I have three counterqeustion for that theory
1)harness lines/hands angle is internal force,how they can affect pitch of system?
2)also you can put harness lines /hands 100% horizontal and still put nose down every time you sheet in,so what now press nose down??
3)when you put boom up ,harnesslines/hands have more downward angle so from this theory mast foot pressure increase and nose must go down,but this is not case,because when you put boom up nose start to fly as video,practice confirm

I can't figure out what happened with geometry when change boom height.
I guess from my logic that rider arms has fixed length and some tunning tips links, low boom make sail more upright(less rake ,less tilt to windward),high boom put sail more above you(more rake ,more tilt to windward)...

 

[A.T.]

I can't figure out what happened with geometry when change boom height.

The problem here is that you have an active human with many joints as part of the system, so biomechanics is part of it. Any change of the vessels geometry affects the pose the human will take, and where his center of mass will end up.

 

[John Mcrain]

The problem here is that you have an active human with many joints as part of the system, so biomechanics is part of it. Any change of the vessels geometry affects the pose the human will take, and where his center of mass will end up.

Yes that is right.
What do you think about " mast foot pressure theory",does it failed like I state with my three counterquestions in post 29?

 

[A.T.]

What do you think about " mast foot pressure theory",

I don't see much theory there, just things that were empirically found to work. To verify how they work exactly, you would need motion analysis and force measurements, which is difficult to do on water.

 

[John Mcrain]

I don't see much theory there, just things that were empirically found to work. To verify how they work exactly, you would need motion analysis and force measurements, which is difficult to do on water.

But mast foot pressure is internal force,internal force can not change net moment of system..?
So you don't have explanation how sail power press nose down?

 

[willem2]

But mast foot pressure is internal force,internal force can not change net moment of system..?
So you don't have explanation how sail power press nose down?

It's hard to see what the problem is here.
If you look at the complete surfer/plank/system, there's a couple that would press the nose down, from the wind force and the drag, and there will have to be a couple from gravity + buoyant force in the opposite direction.

If you look at only the plank, there can't be a large net torque on it from the sail and the surfer for two reasons.

1. there is indeed no way for such a torque to be exerted on the plank. (unless you stand before the mast in a tailwind and push against the sail).

2. The only way to counteract such a torque, would be for the nose to go deeper in the water and for the buoyant force to act further forward. This is undesirable. You'd flip forward at higher speeds.

So the surfer will have to stand or lean backwards to counteract the torque from the sail.

 

[A.T.]

But mast foot pressure is internal force,internal force can not change net moment of system..?
So you don't have explanation how sail power press nose down?

As I told you in post #3 already, you have to decide if you are analyzing the moments on the whole system or just on the board.