RE: which iS for 65Kg guy wanting to plane from 9 kts on?
I would say get an iS122 with a 8.5-9.0 sail to plane as of 9 knots for a 65 kg body weight.
I'm 65 kg as well and I can plane on iS133 (or HS105) as of :
- 7 knots with a 54 cm fin and NP RX2 10.6 sail;
- 10 knots with a 48 cm fin and a NP RS2 8.2 sail.
Larger board do require bigger sail and bigger fin to overcome drag. For a 65 kg light weight sailor, I've witnessed on several time that it's not really necessary to go up to iS155 or FW to plane as of 9 knots IMHO. This is a different story for a medium or heavy weight of course...!