RE: Board/sail combo for gusty flat water sailing
Your wind conditions of 15-25 on flat water with a current is exactly the same as I sail in. I'm typically 76-80 kg and have found no better board than a Hypersonic 111. From all reports the H111 was far superior than the 105. Anyway what I can say is that with lots of fine tuning you could certainly have a 1 board solution and go from 8-25 knots. An alternative to this board would be a 122 Isonic. In 15-25 I would suggest a good race or freerace sail on either board.
Heres what i typically Use
8.5 / H111 / 48cm fin / 8-15 knots ish
8.5 / H111 / 41cm fin / 13-18 knots ish
6.5 / H111 / 34cm fin / 15-25 knots
6.5 / H111 / 31cm fin / 20-25 knots
Offcourse if you require a little more bottom end, go 51cm and or 9.0m, on the other hand if you have some solid 20 -25 knot days, grab a 5.8. But these 2 sail sizes seem to cover most of it. Just change fins to increase your range. The main thing to realise with gusty conditions is that usually nothing feels right, but of all the boards i have sailed the H111 is as good as it gets.