Luff curve

Now that I’m all pumped up with Claudio’s sail block I’m ready to move on to the next (and equally important) part of sail making. Does anyone have a reliable method of cutting the luff curve on paneled sails. I have read the Sails Etc notes and they say to add 50% to 100% depending on the amount of camber cut into the sail. I’m wondering if there is a way to hang the sail and mark where the luff would be if you had a straight mast? Then you could add the extra to match the curve of the real mast. In the past I have not had much luck getting the luff curve right. I end up re-cutting or loosening the ties in the middle of the sail(which is less than ideal).



The reason you may be getting a hollow in your luff, may be because you are cutting the luff with the sail stretched out on a flat table. I have found a way to cut a perfect curve for the luff is tape down the sail at the head and the tack. Then lift the clew off the table about 150mm. Lay a batten along the luff. Clamp down the ends, pull the centre out around 15mm, clamp and cut with a very sharp knife or blade. This should give you a slight luff round that will fit most masts. If you have any doubts may I suggest that you use paper. Of course the measurements are dependent on the height and shape of your sail. The key is lifting the clew off the table.

I’ve been doing that. Do you put any tension on the sail when you lift the clew? By that I mean do you try to lift the whole leach off the table? Do you leave the foot loose or tight?

Jon posted over in the sailmaking thread about luff curve having an effect on camber. I agree, it does, but I remember reading somewhere that luff curve on a flat sail is for mast shape and camber, and luff curve on a panelled(broadseamed) sail is for mast shape alone. Can anyone elaborate on that? It seems too simplified for me.

I’m not sure to have understood but there are several ways to change the draft and draft position of a sail :

  1. making a sail with a certains form during production.
  2. defining the amount of luff that will intervene when the backstay is pulled. The mast bending will produce 2 effects : a) flatten the sail and reduce the draft and b) shifting the draft toward the leech. This because the bending will increase the distance between luff and leech stretching the sail.
  3. the Cunningham when tensioned will pull the luff and will tends to bring back the draft toward the mast.
  4. the last control is operated by the outhaul.
    This is what I was using during real scale racing.

Luff curve is for both sail shape and also to allow for mast flex. The sail can be flattened a little for heavy or light conditions by bending the mast

If insufficient curve has been built in to the sail for a particular mast setup, it might look OK on land, but when powered upon the water, the mast will bend too much pulling the sail material near the mast forward. This flattens the forward section of the sail, and if continued will cause big wrinkles from this area towards the tack. This can be compensated somewhat by tightening Cunninham. The sail will also twist off considerably.

Generally in bigger boat sailing, the forward mainsail section will be flattened to allow close headsail sheeting up wind, and filled out for downwind.


In response to your enquiry re

  1. tension on the luff: Just pull the luff out so that it will not slide around.
  2. The clew: Pull the foot out so the sail sits at what you consider to be optimum draft. And the height of the clew off the table is dictated by the approximate twist you would have when the sail is set. In effect you set the sail up on the table as you would on the boat then you cut the luff curve.

Hope this is of help