jsl
Well-Known Member
Is there a formula for predicting stretch caused by normal halyard tension? I want to decide whether the jib I've got (41' when laid out on the floor) is the right height for my foil (41'9").
Luff stretch: how much to expect?
- Thread starter jsl
- Start date
KenMcCulloch
Well-Known Member
I would put the sail up and see if you can tension the luff adequately to give a good shape, without running out of space between the head of the sail and the top of the stay. 9" stretch on a sail that size doesn't seem like very much but a lot depends on the materials used and the cut of the sail.
wooslehunter
Well-Known Member
A sailmaker told me 5% when I bought a new sail a couple of years ago.
charles_reed
Well-Known Member
Depends on the luff rope material - but based on a terylene rope I'd reckon 5" would be as much as you'd expect, less for other materials.
jsl
Well-Known Member
My goodness! That's a couple of feet!
jsl
Well-Known Member
That's about 1%, which sounds sensible. (There's no conventional luff-rope, because the sail is free in the foil.) My worry is that if I just heave it up to the top of the foil I will damage the top fitting. Instructions say that there should be about 2" between upper swivel and top of foil.
PuffTheMagicDragon
Well-Known Member
[Sticking my neck out and risking the onslaught of flames}
Luff tension used to be very important before the use of foils as one had to eliminate, as much as was possible, any sagging between the hanks.
With a foil, the luff is supported throughout its entire length and such sagging is not physically possible. The only tension that is required is merely to pull out most of the tiny creases along the luff; any remaining will usually disappear once the sail fills.
The instructions that came with my Facnor gear actually advise against putting in too much tension.
I would hoist the genoa with minimum tension on the halliard and go out for a sail. Watch out for the belly of the sail when beating; if it appears too far aft, then apply some more tension to the luff until the deepest part has moved to around one third of the chord from the luff. You don't need much tension.
Luff tension used to be very important before the use of foils as one had to eliminate, as much as was possible, any sagging between the hanks.
With a foil, the luff is supported throughout its entire length and such sagging is not physically possible. The only tension that is required is merely to pull out most of the tiny creases along the luff; any remaining will usually disappear once the sail fills.
The instructions that came with my Facnor gear actually advise against putting in too much tension.
I would hoist the genoa with minimum tension on the halliard and go out for a sail. Watch out for the belly of the sail when beating; if it appears too far aft, then apply some more tension to the luff until the deepest part has moved to around one third of the chord from the luff. You don't need much tension.
charles_reed
Well-Known Member
Well
there is a luff rope, it's the bit (usually 5mm diameter) which goes inside the foil groove.
As has already been pointed out, with a foil you need considerably less tension than hanks to forestay.
So even with this tiny luff rope you still only get about 1% stretch.
If he used Vectran you'll have no measurable stretch.
After all it's a fairly modest sail.
there is a luff rope, it's the bit (usually 5mm diameter) which goes inside the foil groove.
As has already been pointed out, with a foil you need considerably less tension than hanks to forestay.
So even with this tiny luff rope you still only get about 1% stretch.
If he used Vectran you'll have no measurable stretch.
After all it's a fairly modest sail.
matelot
Well-Known Member
Re: Well
The luff is made smaller than the sail cloth behind it so that you have to put tension on the luff rope to get sail shape whether or not you have roller reefing. Then you alter tension to alter sail shape and the position of max draft.
Sounds as if your sail is second hand . In that case you dont know how much the rope has taken a permanent stretch already. All I can suggest is to haul it up the roller, tension it as much as you dare and then see if there are horizontal creases behind the luff. If there are horizontals, then its too long. If there are vertical creases then its too taut.
The luff is made smaller than the sail cloth behind it so that you have to put tension on the luff rope to get sail shape whether or not you have roller reefing. Then you alter tension to alter sail shape and the position of max draft.
Sounds as if your sail is second hand . In that case you dont know how much the rope has taken a permanent stretch already. All I can suggest is to haul it up the roller, tension it as much as you dare and then see if there are horizontal creases behind the luff. If there are horizontals, then its too long. If there are vertical creases then its too taut.
William_H
Well-Known Member
Re: Well
Firstly I don't imagine you will get into trouble with only 9 inches of allowance for luff stretch.
I do disagree though that a luff in a foil does not need a lot of tension. I think it is just that using hanks makes lack of tension obvious.
Any lack of tension puts cloth excess into the sail near the luff and so contributes to excess camber. I would put just as much tension on a jib in a foil as a jib on hanks.
I occasionally sail on a 40ft Farr where the luff is in a foil but only to permit easy jib changes (non rolling) certainly the luff tension is treated the same as if it were on hanks.
Perhaps this is why boats with jib in a roller foil perform to windward so poorly.
To the OP try the jib out. It is not so expensive to get a jib cut down if the luff is too long. olewill
Firstly I don't imagine you will get into trouble with only 9 inches of allowance for luff stretch.
I do disagree though that a luff in a foil does not need a lot of tension. I think it is just that using hanks makes lack of tension obvious.
Any lack of tension puts cloth excess into the sail near the luff and so contributes to excess camber. I would put just as much tension on a jib in a foil as a jib on hanks.
I occasionally sail on a 40ft Farr where the luff is in a foil but only to permit easy jib changes (non rolling) certainly the luff tension is treated the same as if it were on hanks.
Perhaps this is why boats with jib in a roller foil perform to windward so poorly.
To the OP try the jib out. It is not so expensive to get a jib cut down if the luff is too long. olewill