Wend down to the boat (an F31 trimaran) yesterday, on a lovely sunny day, so rare here in the winter. And as usual, the bright light revealed something I had not noticed before: the mylar is starting to go in my main. So thoughts turn to a new main.
My question for the theoretically minded and aerodynamically informed: What is the latest thinking on plan form for mains? the F31 has a rotating mast, with no backstay, and the stock sail plan calls for a pretty substantial square top (38"/965mm). The plan for my existing sail varies from stock in that I specified 4"/100mm less on the foot and 4"/100mm more on the head. I have no empirical comparative evidence but this seems to work ok-- weather helm about right, twist seems controllable with gobs of mainsheet.
The plan form of the existing sail is sort of a clipped ellipse. From what I see in the pages of Seahorse, the vestiges of elliptical plan form seem to be disappearing. I see super straight leeches, very little draft, and really big heads, with the top of the main absolutely flush with the top of the mast.
So for my next main, I am thinking of modifications to both sail and spars (rotating carbon mast, alu boom)
1. add another 100 mm to the head, and straighten the leech even more.
2. new carbon boom and gooseneck mounted about 12"/300mm lower on the mast for more luff length.
3. New composite mast head that adds a foot to hoist, but more importantly, is faired into the head of the main. With the change in boom location, I would gain about 2'/600mm of luff length on the existing 40.55 luff length.
4. Head board designed to take maximum advantage of the new mast head so that the head of the sail is flush with the tip of the mast.
5. put the clew 12"/100mm higher for better head clearance.
6. Less draft in the main, for lower drag and better LD ratio up wind.
I am particularly interested in DDW's take on the plan form issues. My thought is to make the total sail area only slightly larger, but to increase the luff length, increase the head size, make the leech straighter, and make the sail flatter. The goal is to maximize upwind performance. don't care about the reaches, or downwind. Thoughts?