Initially I reduced prebend to increase draft in the middle of my sail and have the leech stand up better there as well. This allowed me to stand double trap much sooner when at 150 kg. I placed more power in the rig (that helped me get on the trap much sooner) and that propelled the boat alot faster.

So initially my prebend was TOO curved for the luff round in my mainsail, thus in effect depowering the middle of my mainsail TOO much for a 150 kg crewweight. At 150 kg I could harness alot more grunt then I initially had.

If I were to decrease me crew weight from 150 kg to say 130 kg I would increase the prebend a little again, making the middle of my mainsail a little flatter. With some additional downhaul tension I can then flatten my top and induce some extra flattening of the middle section as well. When the prebend is increased just by the right amount I should end up at close to the proper curve for a 130 kg weight.

But to make things more complex. If I would also derotate my mast a little more when going from 150 kg to 130 kg, reducing mainsail draft that way, then I should reduced my prebend a little less. This because the derotation would make my top become fuller which I will have to downhaul out again by extra downhaul tension. This tension has a side effect on the middle of the sail making it flatten still more. So in order to prevent overdoing it I would need to make it a little too full at zero downhaul tension.

This is the rough guide to it. As you can also play with spreader rake, spreader arm length and diamond wires tension etc. It can get complicited as there are some interdepenceis, but this also means that you have a wide range of adjustments.

I'm still keeping it relatively simple as I would like to find simple adjustments that I can make between races on the beach. Under these circumstances I very much would like to avoid adjusting spreader arm rake and length. So I'm trying to find a good base setting for these and then make do with the thigns that I can easily adjust with the sails hosted and the next racing starting in 10 minutes.

Wouter