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Sam.Spoons View Drop Down
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    Posted: 12 May 19 at 10:23pm
Sorry, clearly cross purposes, I was referring to the vertical wrinkles you mentioned. I agree that wrinkles (as opposed to 'creases' though that was the term Sean Cox used) do fall within the boundary layer and don't affect performance.

I believe creases/wrinkles are common when a sail is tensioned beyond (or below) its design limits and the limits of adjustment are exceeded. Sailcloth with more stretch will accommodate a wider range of adjustment/shape but will hold it's shape less well under load and will have sub standard high wind performance. A low stretch cloth will make a sail which keeps it's shape better under load but will have less range of adjustment and be 'stiffer' and harder to read in light winds. Under load the aim is to allow gust response but keep the CoE reasonably constant so the sail spills in the gusts without unduly moving the CoE. Floppy head designs in windsurfing which set on downhaul alone are a perfect example but they tend to be slow in lighter winds. Dinghies have and significantly more sophisticated rig controls so can cope with a wider range of wind speeds with a single sail so the dinghy equivalent is a bendy mast and lots of cunningham.
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SoggyBadger View Drop Down
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Posts: 552 		Post Options Post Options   Quote SoggyBadger Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 19 at 7:04pm
Originally posted by Sam.Spoons


Surely the flow across the sail is mostly horizontal (that's what my teltails tell me) so a vertical creases is across the flow?


And your tell-tails are telling the truth. I'm rather confused about where you think this vertical crease comes from though? I've not seen one of those which was causes by anything other than too much Cunningham or luff tension, neither of which is related to the sail's panel layout.
Best wishes from deep in the woods

SB

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Oatsandbeans View Drop Down
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 19 at 4:05pm
Exactly!
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davidyacht View Drop Down
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Posts: 1345 		Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 19 at 3:16pm
Which is why I have a dacron sail for light winds tucked away ...
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Oatsandbeans View Drop Down
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Posts: 382 		Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 12 May 19 at 9:42am
It is interesting Davidyacht. That is definitley very sensible to have a compromise and use a rig that is good at everything.
, but if someone gave me the choice of two rigs -one a perfect compromise that would be competitive in all conditions or another that was a complete "rocket" in the conditions that we most race in and a bit tricky in some conditions ( that posibly may require me to adjust my technique -and learn how to deal with it) i would go for the " rocket" every time.
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Sam.Spoons View Drop Down
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 10 May 19 at 9:47pm
Originally posted by SoggyBadger

Originally posted by Sam.Spoons

Yes, I fear that is the case.

I still believe that a change of shape relies on stretch to keep it smooth, if the sail had no stretch changing it's shape must induce creases in one direction or another. I can't think of a way to explain but I can't see how it can be otherwise.


It rather depends on how the panels are laid out. On a cross-cut sail the cloth will naturally fall into a smooth shape even using laminates. Radial cut sails generally have enough panels in them that the curve closely approximates a smooth curve. The vertical wrinkles you often get at the luff of a laminated main don't matter as they're going with the air flow not across it.

Surely the flow across the sail is mostly horizontal (that's what my teltails tell me) so a vertical creases is across the flow?

It's no problem getting a sail to have a smooth shape under a specific tension on a specific mast like bending a piece of flat metal sheet into a shallow bowl shape. But the metal will only bend in one direction at a time, to get a 3D curve you must stretch the metal in some places (done with seam shaping on a conventional sail and by heat moulding on a 3DL). In the case of metal it then retains that shape but we need sails to change shape and then return so, to achieve that, stretch must remain within the elastic limit. 

I'm sorry I can't seem to explain this clearly but, to me, it's obvious..........


Edited by Sam.Spoons - 10 May 19 at 9:56pm
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SoggyBadger View Drop Down
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Posts: 552 		Post Options Post Options   Quote SoggyBadger Quote  Post ReplyReply Direct Link To This Post Posted: 10 May 19 at 9:25pm
Originally posted by Sam.Spoons

Yes, I fear that is the case.

I still believe that a change of shape relies on stretch to keep it smooth, if the sail had no stretch changing it's shape must induce creases in one direction or another. I can't think of a way to explain but I can't see how it can be otherwise.


It rather depends on how the panels are laid out. On a cross-cut sail the cloth will naturally fall into a smooth shape even using laminates. Radial cut sails generally have enough panels in them that the curve closely approximates a smooth curve. The vertical wrinkles you often get at the luff of a laminated main don't matter as they're going with the air flow not across it.
Best wishes from deep in the woods

SB

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Sam.Spoons View Drop Down
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Posts: 3401 		Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 10 May 19 at 9:59am
Clap Clap Clap

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davidyacht View Drop Down
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Posts: 1345 		Post Options Post Options   Quote davidyacht Quote  Post ReplyReply Direct Link To This Post Posted: 10 May 19 at 7:06am
The quest of the best sail is the ultimate compromise ... the ability to have gust response, stability, light weight, fullness to punch through waves, flat for low drag, a plethora of different materials, different mast bends, price, fashion and the personality of the sailmaker have been a constant drain on my wallet for forty years
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iGRF View Drop Down
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Posts: 6499 		Post Options Post Options   Quote iGRF Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 9:43pm
I think your getting too tied up in the detail and forgetting what the holy grail of sailmaking was all about.

The idea of the perfect shape that held, resisting the wind force to shift the flow point beyond the correct design parameter is always the goal, yet with the ability for some on the fly correction or auto correction for gust response without losing drive and to do that a sail built with cloth that doesn't arbitrarily shift or stretch about is obviously the best outcome.

The best example I ever came across was Cuben Fibre which was a multi directional random scatter fibre laminate, I've still got it thirteen years on and it works well, think they went on to create a waft and weave look because it wasn't generally adopted, it was quite expensive, but light, stiff, low stretch, it had it all, got a picture somewhere maybe I'll tag it on later.



Edited by iGRF - 09 May 19 at 9:44pm
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