Main sail design seems a dark art with the sail makers using terms of seam shape and luff curve. Now some say they are luff curve fans and others seam shape ( we are talking crosscut sails here ).
Now I can get my head around seam shape which will give a series of slightly
bulged
panels which when seamed together will give a fixed amount of shape to the sail. Now this will obviously give some depth to the sail at the point where the designer thinks appropriate.
Now add in luff curve, and this is what I cannot get my head around. Anybody offer a good description of what the luff curve does and how depending on how the mast is set ( diamond tension ) will effect that crosscut sail.
Now add in downhaul ?
Luff curve is basically the feature that makes your downhaul effective. Take a curved edge of a piece of fabric and fix it to a straight mast, the mast is going to force belly/shape into that fabric. The more you tension the downhaul, the more bend is induced into the mast. As the mast bends, it more closely matches the curve of the fabric (luff curve) and so shape/belly is reduced. That is the basic concept, anyway.
If you want to see an extreme example of luff curve, look at any modern windsurfing sail, they have a huge amount of luff curve. Loosen the downhaul and the amount of draft gets huge. Tighten the downhaul/bend the mast and the sail becomes lean and fast.
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Go look at Sailcut. That is a good example of
luff curve defines amount of depth, broadseaming defines position of depth
Increase mast bend, and the extra cloth in the luff curve is sucked out of the volume the sail makes. Straighten the mast and the sail gets extra volume.
You have the same effect in jibs if you have a S curve in the luff.
If you play with sailcut you can easily see that the developed panels you cut and their luff curve is wildly different from the curve you specified. This extra material in the panels is the cloth added to give the correct 3D share of the sail.
Still wondering? <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />
Yup still wondering.
If you have a luff curve of say 50mm and a perfect fitting sail to the mast at 50mm. If you now bend the mast to 100mm, the leach will now be less tensioned and the sail will twist away allowing the rear of the sail to flatten at the top.
The luff however will see very little change and should hold its depth and shape ?
This is correct but the numbers dont add up as the effect of bending the mst from 50 to 100mm have a significant effect on the draft. You essentially elongated the chord and shrunk the surface of your foil. This is what we aim for with our pre-bent masts. When a gust hits and powers up the rig, the mast bends, sail flattens (changes
gear
) and you dont have to do much at all to enjoy the power of the gust <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />
Then you sheet in to tension the leech and start all over.
Consider what happens when you go from 50mm to 300mm curve.
OK then, my present sail is I believe too flat, its not at all powerful at low speed, once the boat is up to say 6 - 8 knots, the sail looks good and seems to be doing what I would expect.
So how do I get a crosscut sail to become more powerful ( yes I will loose some top end speed )
You re-cut the luff to get more curve. Remove cloth from the top and bottom with a taper. Start carefully and think twice before cutting <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />
30mm here and 50mm there is enought to have impact.
Could I also move the spreaders further foward and reduce the diamond tension a bit to straighten the mast to get the same effect.
Playing with the pre-bend will give you a great feel for what can be acheived by re-cutting the luff. Think of the relationship between the luff curve and mast shape as a million tiny out hauls.