where is pressure point at slalom sails located?
Percantage height/width..
Above or below boom?

Same sail, same board, same wind-strength, but different rider-weight -> different pressure point. ( It is called "centre of effort". )

I assume fpw9082 is referring to the location of the centre of effort of the sail.

No idea why though.

The whole CoE vs CLR question is quite different related to a windsurfer than to a boat. A boat has a fixed mast location hence the coe / clr relationship is far more constant.

A windsurfer rakes the sail (and mast) backwards and forwards relative to the board and thus the coe to clr relationship is less dependent on where the CoE is in the sail.

A windurfer's body is also a far greater proportion of the weight of the whole than any boat and also moves around a lot more to balance the forces, thus the position of the sailor is a variable not really too relevant to the same query in how a boat sails.


But, in an answer to the original query of where the CoE is in a slalom sail then the answer is; there isn't a single defined point.

As a generalization, about 1/3 way from the mast, about boom height.

But depends on how high you rig your boom
And how the sail is rigged
And what sail it is
And how you define "slalom sail"
And at what point in time you want the CoE to represent

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mathew said..
Same sail, same board, same wind-strength, but different rider-weight -> different pressure point. ( It is called "centre of effort". )

when you rake sail, pressure point move down in relation to ground, but does pressure point also move down at the sail(in relation to sail)?

Where is aprox,about boom height or higher?

That would depend on the sail, mast bend, the amount of downhaul and perhaps outhaul. I believe, the more twist the lower the C o E. And I can't see how sail rake would affect it. Very roughly around boom height perhaps a bit above, I doubt it would ever be below

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decrepit said..
That would depend on the sail, mast bend, the amount of downhaul and perhaps outhaul. I believe, the more twist the lower the C o E. And I can't see how sail rake would affect it. Very roughly around boom height perhaps a bit above, I doubt it would ever be below

...and apparent wind, so by definition speed. COE on windsurfing sails is almost impossible to be specific about. They are rigged in a very rigid yet simple manner (far less adjustments than a yacht or dingy sail -no boom vang, no stay tension, no mast pre bend no spreader pressure no kicking strap, no leach line, no mast stiffness adjustment, no boom bend adjustment etc etc.) On the contrary they are a far more dynamic system, so hence the variables.

If you rig a windsurf sail it will have a CoE location.

That CoE won't move if you "move" the sail. Rake it forwards or backwards or put it on the next beach or wear it on a hat, and it won't move the CoE relative to the sail.

But load the sail with wind, then the sail flexes and its shape changes and so the CoE position changes slightly (unless fpw9082 is actually xyz or MWsails or whatever he was last and trying to trick us into admitting a rigid wing is the future because the CoE is constant for any load).

A "stable" feeling sail will not have much change for various wind strengths (loads), and a fully battened, fully cammed slalom sail will be about as stable CoE as they come. Certainly more so than a 3 batten soft wave sail.

But, I am curious to the point of the question. Because I am not sure that any discussion about CoE can exclude CLR - unless the question is about stability and a sail's feel in the hand, in which case surely location of CoE is not as interesting as stability of CoE.

And the whole issue of CoE and CLR in sailing vessels has not much relevance to windsurfers when the CoE can be (and has to be) constantly moved relative to the vessel and thus relative to the CLR. Where the CoE is in the sail is like a one side equation. What is 2+ =. There is something missing.


As to where the typical CoE "region" is on a windsurf sail, then I still reckon 1/3 way back from the luff, about boom height.

As to where exactly and the answer is "depends..on so many variables"

ok so qnswer is 1/3 from mast at boom height aprox..
Never below boom?


If harness lines stay so close together, that mean coe move very little

Does even sail designers know where each sail has coe or this is just spexualtion?

You design the sail to have a specific CoE when rigged in a certain way, you have no control over how other people rig it. It could end up pretty much anywhere.

most modern sail CoE won't move much at all if rigged some where near right.

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Ben1973 said..
You design the sail to have a specific CoE when rigged in a certain way, you have no control over how other people rig it. It could end up pretty much anywhere.

most modern sail CoE won't move much at all if rigged some where near right.

you think sail designers know where coe is,in 5cm precision?

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fpw9082 said..

Ben1973 said..
You design the sail to have a specific CoE when rigged in a certain way, you have no control over how other people rig it. It could end up pretty much anywhere.

most modern sail CoE won't move much at all if rigged some where near right.

you think sail designers know where coe is,in 5cm precision?

As others have stated, it's going to vary according to rider weight, batten tension, batten stiffness, rig tension (outhaul, downhaul). Once those things are determined, you can try to determine the more precise CoE. We do it all the time when we set our harness lines for balance between the front and back hand. Slalom sails have a more rigid structure so it will vary less than other sails with changes in wind strength (which affects batten shape, etc.)

An experienced sail designer will know fairly well where the CoE will end up based on dozens if not hundreds of iterations in their career. They rarely start from scratch.

When you sail with soft wave sails and have lines close together, does pull switch from front to back hand or pull at both hands are constant?

Does sail deformation caused by wind power cahnge coe or lines close together can cover this coe movements?

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fpw9082 said..
...or this is just spexualtion?

Macquarie dictionary just announced their words of the year. I'm surprised this one missed the cut

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fpw9082 said..
When you sail with soft wave sails and have lines close together, does pull switch from front to back hand or pull at both hands are constant?

Does sail deformation caused by wind power change coe or lines close together can cover this coe movements?

4 Q's

Answers:
Yes
No

Yes
No

Like talking to a blind person about colors.

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fpw9082 said..

Ben1973 said..
You design the sail to have a specific CoE when rigged in a certain way, you have no control over how other people rig it. It could end up pretty much anywhere.

most modern sail CoE won't move much at all if rigged some where near right.

you think sail designers know where coe is,in 5cm precision?

Yes, if the software they use is half decent (assuming they know how to use the software)

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fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that. Which begs the question: Why would you want to know that??? If it is just so you can put your harness lines in exactly the right spot with the fixing ends very close together on the boom without trial and error................

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sailquik said..

fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that.

A number of the sailmakers (eg Duotone) mark the centre point for locating harness lines, which should line up with the COE. My experience with Duotone is that it is within 10cm depending on wind conditions and how the sail is rigged.

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John340 said..

sailquik said..

fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that.

A number of the sailmakers (eg Duotone) mark the centre point for locating harness lines, which should line up with the COE. My experience with Duotone is that it is within 10cm depending on wind conditions and how the sail is rigged.

Which they would know in the fore/aft direction from actual on water testing.

Very unlikely directly from their design program.

A 1-2 cm change in downhaul or outhaul, or a change in the mast curve or stiffness, or a change in the wind strength etc. can easily move the CoE more than 5cm vertical and horizontal.

And for camless freeride, and especially wave sails, it is likely far more than that.

Changing brands I have had to move harness lines up to about 10cm for same size race sail. Changing size of the same brand about 2cm difference for each size. If you get your lines right for any given sail or day the rest doesn't matter.
If you are having that much trouble finding the coe buy a boom for each sail and never move the lines once they are set.

A few notes here:

1) With any typical aerofoil shape the lift tends to be 1/3 back from the front so that gives you your basic horizontal measurement.
On a windsurf sail that horizontal measurement should be in the middle of where you set the harness lines, usually a 1/3 back between mast and boom end.

2) if you sail with a slack outhaul then an increase in wind will often mean the centre of pull shifts back within this horizontal measurement - meaning you may need to shoot the harness lines back to keep the even feel on each hand.

3) The vertical measurement - or how high the pull is up the sail - will depend on the sail plan. So if there is a lot of sail area below the boom compared to the area above the boom then the centre of effort (or centre of pull) will be lower. Conversely, a high aspect sail, where there is a lot of area above the boom compared to the sail area below the boom, generally has a higher centre of effort.

4) Point 3) is only part of the story because a sail reacts to different wind strengths with the head of the sail twisting open in gusts and this twist can be increased if you rig the sail with more down haul which generally flattens the sail at the head. As the sail twists open it releases wind and so the drive can then shift down wards in terms of our vertical measurement. if however the sail is set with too little down haul then the head cannot open to spill wind efficiently. So this means the centre of pull can shift upwards in a gust with a tight leech sail.

5) None of this really matters, because you work with what you've got. if you are overpowered then flatten the sail a bit more with downhaul to dump more power from the head and hopefully stabilise the centre of pull. If instead you are underpowered then ease the downhaul a touch to get a higher pull and more power from the sail.

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Basher said..
A few notes here:
..........

Spot on!

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John340 said..

sailquik said..

fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that.

A number of the sailmakers (eg Duotone) mark the centre point for locating harness lines, which should line up with the COE. My experience with Duotone is that it is within 10cm depending on wind conditions and how the sail is rigged.

this marked place is not sail coe..this is just place that lying on line which connect coe and mast base..

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fpw9082 said..

John340 said..

sailquik said..

fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that.

A number of the sailmakers (eg Duotone) mark the centre point for locating harness lines, which should line up with the COE. My experience with Duotone is that it is within 10cm depending on wind conditions and how the sail is rigged.

this marked place is not sail coe..this is just place that lying on line which connect coe and mast base..

i have an idea for you.

go for a sail, take some measurements, report back here with your findings on where CoE is on your sail.

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fpw9082 said..

John340 said..

sailquik said..

fpw9082 said..
you think sail designers know where coe is,in 5cm precision?

CoE with 5cm precision?? I very much doubt it.

Do they know when they have got it pretty much where they want it when sailing in particular circumstances?

Absolutely!

I doubt they could put an X on the spot though, or even have any interest in trying to do that.

A number of the sailmakers (eg Duotone) mark the centre point for locating harness lines, which should line up with the COE. My experience with Duotone is that it is within 10cm depending on wind conditions and how the sail is rigged.

this marked place is not sail coe..this is just place that lying on line which connect coe and mast base..

Why would you ask a question, plead that you dont know much, then re-answer it with a stupid answer. Ok Mr Smarty pants, what is that mark if its not the COE.

that bit of the boom that well placed harness lines split that means you can sail with both hands off the boom?

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Reflex Films said..
that bit of the boom that well placed harness lines split that means you can sail with both hands off the boom?

Sounds like Slalom sail PLEASURE point going on there.

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fpw9082 said..

this marked place is not sail coe..this is just place that lying on line which connect coe and mast base..

And, that's the one that matters. Draft forward or back of the harness lines you have to deal with and most of us have our harness lines pretty wired to the spot. It matters because our harness lines are the hinge.

Vertically it matters much less because there's no hinge. If it's higher or lower 5-10 cm, the avg windsurfer would be hard pressed to know the difference. And, as soon as you think it does, it's beyond the sailmaker's control anyway. Because of surface friction with the water, the wind gradient is going to vary with sea state. If you are in dead flat water, you're going to have more wind lower in the sail than normal and if you are punching through the white water or break, it may be dead the first meter or more up the sail. Or take the extreme example of a wave sail used for foiling where it's going to be a whole lot more in the wind than it is going through white water on an onshore wind.

And then it goes back to what was said most eloquently above:

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Basher said..
5) None of this really matters, because you work with what you've got...