Gill, with regards to opening up the main with the kite up, I can't really explain the aerodynamic forces, however, over the weekend, we experimented with opening up the main and easing the spin off about an arm's length while bearing off downwind (once the hull was already flying and there was a good bit of apparent wind) and we found that we could bear off and go deeper for a longer time before having to head back up to generate more apparent wind.
I actually found that by easing the main at the right time helped to power up the rig and keep our hull flying downwind. Hope this helps.
Exactly what I would suggest doing.
Seconded!! <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />
This is the kind of information, while generaly correct, can get you in trouble or at least make you slower if not used properly.
Keep in mind that if you want to sail fast down wind you are trying to maintain a constant apparent wind. In which case the main sail trim will not need to vary.
The biggest mistake I see with new spin drivers, is that they head up until the hull rises, then bear off til it comes back down. This technique almost always results in over stearing and huge fluctuations in your speed. On the 16 if your hull comes up without a noticable increase in speed your weight distribution and or trim are not correct. On the 16 you need to move your weight around a lot. The acceleration is what you are trying to achieve, not the lifting of a hull.
Also be very aware that buy easing the sheet or traveler in conjunction with bearing off in a gust, you open up the sail and present a big flat area of sail dirrectly in front of the wind 8.5 meters up in the air. This is the perfect recipe for stuffing the bows in every big gust.
