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rodney View Drop Down
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Post Options Post Options   Quote rodney Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 8:01am
When it comes to Finn sails my preference is always radial regardless of conditions.  I have both but the cross cut sail stays in storage and is, probably, soon for sale.  Having said that I've been using radial Finns sails for a very long time so it may be just that I am comfortable with them?
Rodney Cobb
Suntouched Sailboats Limited
http://www.suntouched.co.uk
rodney@suntouched.co.uk
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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 12:01pm
Originally posted by mozzy

Is that not an urban myth? At the top end there seems to be a constant drive for low stretch and they can still change sail shape through mast curve and clew tension. The only downside to low stretch seems to be durability.  Just because it's zero stretch doesn't mean it's a ridged 3D shape. 

Are sail-makers not pedalling that line that a bit of stretch aids performance because it sells more sails? Whereas the truth is the stretchy material is cheaper, especially in low volume, easier to work with and they can't be dealing with the inevitable warranty returns after people crease their ultra brittle fibre sails? 

I can't see that (well maybe they are trying to big up cheaper materials), but a non stretch sail would only set one way and any changes would introduce creases. In fact, most laminate sails do have creases much of the time which I attribute to the fact that they don't stretch much. Sean Cox (Demon Sails designer) always said that small creases didn't affect performance as they were within the boundary layer, I suspect he is well qualified to comment as his day job was designing fighter plane wings.

My Hyde replica sail looks awful down wind in the light stuff but seems to go well enough. Once you get some pressure the mast bends and it looks fine. The Blaze M7 mast is pretty stiff though so that doesn't help. 


Edited by Sam.Spoons - 09 May 19 at 12:01pm
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 1:28pm
Originally posted by Sam.Spoons

a non stretch sail would only set one way and any changes would introduce creases. 

Well, you say that, but I don't believe it's true. Low stretch is tensile strength with low plastic deformation under tension.    

A sail that was very stiff would only hold one shape and would either crease (plastic deformation) or break (brittle) if you tried to change it's shape. 

Dyneema rope is very low stretch, lower than wire for the weight. But it's much less stiff. 

Dyneema / kevlar / carbon /polyester can all be used in laminate to reduce stretch, they won't mean the sail is stiff though. They can be made stiff if woven correctly in a resin however. In fact, almost all the stiffness in laminate sails is from the laminate, not the low stretch fibre. 

Sails that are very stiff are hard to set because they take quite a bit of force to deform. So you can't see your luff collapsing. They are also difficult to set in to different shapes without quite a bit of force from the wind, so again not good in the light. If you had a sail that was extremely stiff it would effectively be a hard shell and you wouldn't be able to tack let alone change it's shape. 

But low stretch does not equal stiff. 
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Oatsandbeans View Drop Down
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 1:44pm
Sorry but low stretch does equal stiff- my day job is testing materials!
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mozzy View Drop Down
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 1:55pm
Well illuminate us!

How can dyneema rope have the same tensile strength as steel rod (or wire), but then bend around a pulley without deforming? Why will a steel rod the same tensile strength as Dyneema stand up on end due to stiffness but dyneema rope won't? 
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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 2:29pm
If you could make a steel wire with the same number and diameter of fibres as dyneema rope it would be very flexible. If you take a metre of 2.5mm 7x19 wire you'll struggle to stand it on end, but it'll bend around your halyard sheave quite easily. 
Spice 346 "Flat Broke"
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Oatsandbeans View Drop Down
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 3:51pm
Yes. The reason that high stiffness fibres can bend round sheaves is that they are made from filaments typically 10 microns in diammeter.
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Sam.Spoons View Drop Down
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Post Options Post Options   Quote Sam.Spoons Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 3:58pm
My experience of this came from braided copper electrical cables.
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Post Options Post Options   Quote mozzy Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 9:15pm
I still feel the stiffness of the cloth is more related to the thickness or weight than the stretch. 

I can't imagine a low stretch sail creasing up when you pull depth out. 
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Oatsandbeans View Drop Down
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Post Options Post Options   Quote Oatsandbeans Quote  Post ReplyReply Direct Link To This Post Posted: 09 May 19 at 9:30pm
You may be getting confused with the stiffness of the cloth to bending , you can guage this with bening it in your fingers which is very thickness dependant, and the stiffness to tensiles loads( which is difficult to assess unless you have the kit), it is its resistance to temsile loads whcih is what the sail makers are interested in. Interestingly the latest sails ( North 3 di) appear to have extremely high in plane stiffness. They don't need to be pulled about ( and can't be) like a more flexible sail but they hold their design shape despite changes in the wind loading due to their amazing stiffness properties ( I am not an expert on sailcloth but it looks like that to me)
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