Almost 90% of sail area that produce lift is in front of foil-mast, so sail COE is in front of foil CLR,so how the hell board dont turn downwind?
Where is CLR ,on foil-mast or maybe somewhere infront?

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fpw9082 said..
Almost 90% of sail area that produce lift is in front of foil-mast, so sail COE is in front of foil CLR,so how the hell board dont turn downwind?
Where is CLR ,on foil-mast or maybe somewhere infront?

This is Dietrich Hanke's, Ultra Fins, take on the lateral forces. Its a simplification. I can't tell you the short comings of this diagram. But there are posters on this site that can. I use it because something is better than nothing. Hope this helps!

The diagram for longitudinal forces.

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lakeeffect said..

fpw9082 said..
Almost 90% of sail area that produce lift is in front of foil-mast, so sail COE is in front of foil CLR,so how the hell board dont turn downwind?
Where is CLR ,on foil-mast or maybe somewhere infront?

This is Dietrich Hanke's, Ultra Fins, take on the lateral forces. Its a simplification. I can't tell you the short comings of this diagram. But there are posters on this site that can. I use it because something is better than nothing. Hope this helps!

The diagram for longitudinal forces.

I mean on yaw in realtion to COE vs CLR...

Yeah it always made me wonder aswell, especially once you start riding 3.0's and 2.5's and the clew doesnt even reach the mast hahahah. I asked the same question to a friend of mine who's a nautical designer of racing yaghts. He said he didnt know, but it had to be balanced otherwise it doesnt work.

Roll angle resulting in a lift component pushing against the sails sideways lift component from the front wing is one part of the equation. (When you roll the board towards you the frontwing will have to start pushing upwind) Even though we had this discussion in another thread and I took a little different stance on the subject I do believe there has to be something happening there.

It becomes funny when riding 2.5's and 3/4th of your sail is in front of the front wing (and it still works). My theory is that the stab pulls down, resulting in "moment" (Force * lever) which is working somewhere in front of the front wing and thus working against the sideways component of the sail.

Lastly (another personal thought on the subject), the transference of the sails power to the board is of importance. For sailing boats these pictures always make a lot of sense because the mast is fixed in place with stays and the sail pulls more or less on the same place. On a windsurfer (foil) you can determine more or less yourself where the force of the sail is going to be transferred to the board, which is through your body in any case (independent of how your harnesslines are set / force is distributed over the legs, your frontleg is basically the stay, and your backleg the sheetblock, mostly anyway...). Your body is on top of the foil.

I think a lot of the magic (things we dont get) is in the points of application of the different forces, not so much in the forces themselves.

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WhiteofHeart said..
Yeah it always made me wonder aswell, especially once you start riding 3.0's and 2.5's and the clew doesnt even reach the mast hahahah. I asked the same question to a friend of mine who's a nautical designer of racing yaghts. He said he didnt know, but it had to be balanced otherwise it doesnt work.

It is funny thing that even windsurfing desingers dont have explain..(but this is first thing you have learn in wsurf-school,"board go in straight line only if COE and CLR is in same line")
It seems in windsurifng nobody knows anything but evertyhing works in practice.This is miracle.
Our sport evolution is based on trial and error method..:) :)

(i have my half-explain,when board is heel to windward than front wing handled lots side force,so CLR is just in front of back feet,,BUT there is lots of situation where board is flat and still going in stragiht line so this is still unclear...)

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WhiteofHeart said..
He said he didnt know, but it had to be balanced otherwise it doesnt work.

Yes that's true. Balancing moments is an exact science,( unlike climate science) You'd have to do the diagram from above and consider moments about the CLR of the mast. ( You can consider moments about any vertical axis and get the same answer). So looking at your picture with the blue sail, if you looked down you're saying the sail has a clockwise moment about the CLR of the foil mast? Agreed, looks like it. The foil looks like it's leaning to windward so there's a component of foil lift giving a moment in the anticlockwise direction around the foil mast. The drag of the apparent wind on the sailor also contributes an anticlockwise moment, but windage on the nose of the board, clockwise. The tail of the foil is also at an angle so it is generating a bit of anticlockwise moment. ( It's set up for downward lift remember). Can't think of any more hidden moments we haven't thought of? If that's all there is and the sailor is going in a straight line, they'll balance.

It's a good question, sure doesn't look like they balance.

Of course with the mast leaning windward, as it looks like it does in your picture? Agreed? Then you have to be careful to make it clear if you're taking moments about a vertical axis that passes through the mast at some point or whether you take it along the inclined axis of the mast. Either way it'll all still balance. But if you incline the axis with the mast the mass of the rig and board now has an anticlockwise moment due to gravity.

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fpw9082 said..

Why are you balance forces for pitch?

No i was thinking from above as in your diagram. Your diagram is on a starboard tack, I was looking at your photo with the blue sail on port tack. Your red arrow becomes a clockwise moment on port tack. If you take moments about the foil mast you can leave your black arrow out when doing the balance.

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Ian K said..

fpw9082 said..

Why are you balance forces for pitch?

No i was thinking from above as in your diagram. Your diagram is on a starboard tack, I was looking at your photo with the blue sail on port tack. Your red arrow is a clockwise moment on port tack. If you take moments about the foil mast you can leave your black arrow out when doing the balance.

Sorry I read it too fast...
So on the end where do you think where is CLR when foiling and when board is FLAT?
(if board is heel to windward than front wing start produce side force as well so CLR moves more forward between sailor feet,but lets find CLR when board is flat becuse this situation is unclear..)

WoH said, "Roll angle resulting in a lift component pushing against the sails sideways lift component from the front wing is one part of the equation. (When you roll the board towards you the front wing will have to start pushing upwind) Even though we had this discussion in another thread and I took a little different stance on the subject I do believe there has to be something happening there."

I like that thought. I'm sure you could balance the heeling moment of the sail by rolling the board and foil to the windward. Jim Drake estimated the point of effort on a sail as 30% back from the luff and 40% down from the head. The following diagram is from a paper he did in retirement I believe for Starboard. I assume that in the diagram the sail is tilted back. It's obvious that the board in that condition would head up wind. Remember this is a simplification.

I think the question you should be asking is: Is the board ever dead flat when the sail is loaded and you are going in a straight line? I think you'll find the answer is no.

We'll have to find a picture of a foil being sailed flat to base it on. Maybe there's a drone shot somewhere? But I'm with you, it doesn't look like it should balance. But I've no doubt that if we do it properly it will.

CLR2 in the OP is correct. The front wing is a fulcrum around which everything rotates. The stab pushes down to lift everything located at CLR2.

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CJW said..
I think the question you should be asking is: Is the board ever dead flat when the sail is loaded and you are going in a straight line? I think you'll find the answer is no.

Yes maybe your answer is key for my question.
In this video board seems to me very flat

This is still a bit of a mystery to me and I went to school for yacht design! Here are some of my thoughts for what they are worth.

One factor not fully mentioned yet is what I call the "tail feather effect". The foil (or fin) is at the back of the bus but the driving force is well in front of it. This makes a powerful balance to keep things going STRAIGHT. When you foot steer the board onto the course you want the board wants to go straight. This helps the overall balance but is only a part of it.

I started windfoiling on a narrow tail board and struggled to get upwind reliably. I made a quantum leap in upwind ability when I realized I had to CONSTANTLY keep foot steering the board into the wind with heel pressure. But my old and weak feet would tire quickly. My current set up is a wide tail design and it goes upwind considerably better and is way easier on my feet, same foil and sail. On the wide board most of the time spent sailing to windward is with my weight on my toes driving the board away from the wind.

Boards are steered by applying more weight or force on one side or the other. On all points of sail across the wind the sailor's weight is off centerline so it is ALWAYS creating a turning moment to windward regardless of roll angle. This is the main force that is balancing out the sails side force when the board is sailed flat. We fine tune the balance with foot steering but make the bulk of the turning moment up with our weight off centerline. That force increases the further the sailor is off centerline so less work is required of the feet.

To put it in boat jargon, moving your weight outboard creates weather helm. This is one of the reasons why race boards are so wide. Adding windward roll to the equation creates additional weather helm and lift to windward.

I love this forum! Writing this has helped me wrap my head around all this! Thank you to all the others posting!

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fpw9082 said..

CJW said..
I think the question you should be asking is: Is the board ever dead flat when the sail is loaded and you are going in a straight line? I think you'll find the answer is no.

Yes maybe your answer is key for my question.
In this video board seems to me very flat

When you look watch videos, focus on the riders toes. You can see in this opening image of the Sam Ross video that he is on his heels because his toes are off the deck. He talks a lot about back heel locked down, ie turning the board into the wind with foot steering.

I ask this question on email to Dietrich Hanke (aeronautical engineer -founder of -mauiultra fins)
this is his words.......

"The primary resistance is from the front wing, the fuselage and the rear wing. They all are in line with the fuselage. A small part comes from the part of the mast which is wetted and which act as a fin to compensate the side force. That means that the center line of drag is a little bit above the fuselage center line. A position of CLR makes no sense important is only the line of resistance and the distance to the board creating a moment. The moment is always the same independant of the x-position of the CLR. The position of the COE of the sail has no influence on the hydrofoil calculation. Have a look on a kite only the body weight and position is of importance.

every body rotates about the center of gravity, not the COR. The COR is nonsense. The forces relative to the center of gravity produce a moment which forces the body to rotate. Are the moments balanced the board goes straight. In case of non planing the sail force acting at the COE before the cg let the board going downward, the moment is left turning. Is the COE behind the cg the board is going upwind, the moment is right turning. In case of planing the board will be controlled by body movement that means by weight force movements or by rotating th board about the longitudinal axis and not by shifting the sail forward or aft. In case of foiling going up or downward the board must be rotated about the longitudinal axis. That's the same when an aircraft makes a turn, it rotates the wings."

(so he say that clr/coe is irrelevant for board steering(yaw),so all sailing book in the world are wrong??
does someone of you understand what he wants to say??

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fpw9082 said..
(so he say that clr/coe is irrelevant for board steering(yaw),so all sailing book in the world are wrong??
does someone of you understand what he wants to say??

I think his kite example is of importance, which goes in the same direction as my "the points of application of the forces might be of more importance than we know"

A kitefoiler practically never has the COE of the kite in line with the CLR of the foil (the window in which the kite moves is huge, as the lines are 20+m long), however, the kiter transfers the power through the legs, directly to the foil in such a way that the foil pushes the rider upwind. I think something similar is also happening with windsurfing to some degree, even though the sail is fixed to the board at the mastbase.

I dont know if you ever wakeboarded behind a boat (with a foil), but as you cant the board and start pushing against the boat (your "COE") you start going "upwind" of the boat and going past it! While through the entire arc the COE moves relative to the CLR (in truth the other way round, but ok.). I think there is a similar mechanic also at work with the windfoil. Where the transferrence of the power from the sail through your legs / board "changes" something. Your harnessline position down to the mm is of huge importance for the upwind angle you can get, 5mm = 10 degrees in the end? Those 5mm exaclty determine wether the preassure is on your front o your back hand, and therefore your front or back leg, and that is directly related to this mechanic!

Engineers, we love you.

Dietrcih text:
"every body rotates about the center of gravity, not the COR. The COR is nonsense. The forces relative to the center of gravity produce a moment which forces the body to rotate. Are the moments balanced the board goes straight.In case of non planing the sail force acting at the COE before the cg let the board going downward, the moment is left turning. Is the COE behind the cg the board is going upwind, the moment is right turning. "

Can some one explain his words,that board will rotate about center of gravity???
For me COE in relation to c.g. has nothing to do with board turning.

We tilt the rig so COE is nearly between our feet, favoring front foot.
COE of foil is near COE of front wing, not near the foil mast.

I ask Dietrich why then all sailing book show picture of COE above CLR for boat goes in straight line,does it mean they are all are wrong.
classic picture from sailing books:



his answer is:

"That is a popular explanation, not scientific. A boat goes straight when the sum of forces and the sum of moments are zero. Indoor example it is said if the the sail force position CE and the side force position CLR ( resistance is the wrong word) is in one line then there the moment is zero. But the requirement that the forces must be zero is not mentioned. You have to look in a scientific book of physics of sailing not in a popular one. Otherwise it would be beneficial to study the basics of mechanics.
Further windsurfing and yachting are different because the sail is connected via a pivot with the board."

(I still dont understand nothing...)

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LeeD said..
We tilt the rig so COE is nearly between our feet, favoring front foot.
COE of foil is near COE of front wing, not near the foil mast.

For upwind sailing the sail has to be upright, not tilted. COE of the sail is well in front of the front wing of the foil, especially if sails become small (<4m). You are completely missing the point of discussion once again Lee.

Of course I'm missing the point.
This discussion is for academics and engineers, not normal people.
And what goes upwind must eventually go downwind so YOUR scenario is reversed.

Nobody sails....foils...with an upright sail.

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fpw9082 said..
Dietrcih text:
"every body rotates about the center of gravity, not the COR. The COR is nonsense. The forces relative to the center of gravity produce a moment which forces the body to rotate. Are the moments balanced the board goes straight.In case of non planing the sail force acting at the COE before the cg let the board going downward, the moment is left turning. Is the COE behind the cg the board is going upwind, the moment is right turning. "

Can some one explain his words,that board will rotate about center of gravity???
For me COE in relation to c.g. has nothing to do with board turning.

A body rotates around its C of G when all external forces are removed. But to get it to rotate in the first place you need to apply external forces. It generally only rotates about its C of G once it's released. Think of winding up and releasing a discus. (But you can't ignore wind resistance in a looping windsurfer, they won't rotate around the exact C of G )

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LeeD said..
Nobody sails....foils...with an upright sail.

I've got a dozen photos of me doing exactly that. Not me (he's way better than I'll ever hope to be)

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Ian K said..
We'll have to find a picture of a foil being sailed flat to base it on. Maybe there's a drone shot somewhere? But I'm with you, it doesn't look like it should balance. But I've no doubt that if we do it properly it will.

Interesting how we balance the forces - subconscious leg micro-movements?

Tilted slightly into the wind by the look of it

Infinity 65 wing (840 sq cm)

Mast is raked windward on both pics, more on racer with redsail. Close to 10 degrees is good.

Mast is raked windward on both pics, more on racer with redsail. Close to 10 degrees is good.

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azymuth said..

Interesting how we balance the forces - subconscious leg micro-movements?

Tilted slightly into the wind by the look of it

Infinity 65 wing (840 sq cm)

Taking me a while to get those subconscious micro movements dialled in. Can you feel the height of the foil through your feet or do you always need to keep a visual reference?