Agree with most comments above but I strongly doubt you will be able to plane as of 8 knots with an 8.7 sail at 73 kg. I guess the NP chart is kindda off the mark on this case because I cannot do it for sure in real life.
Please, find below what I'm using for my 183 cm/65 kg body weight on iSonic 117 wide carbon 2012 :
- Reflex II 11.0 m2 + 55/54 cm fin : planing as of 7 knots of wind
- Reflex III 8.6 m2 + 48/44 cm fin : planing as of 10 knots of wind
- Code Red 7.7 m2 + 44/40 cm fin : planing as of 11 knots of wind
- Raf Jet 6.9 m2 (no cams) + 44/40 cm fin : planing as of 12 knots of wind
- RS1 6.6 m2 + 40 cm fin : planing as of 14 knots of wind.
Do not dismiss the 3 knots difference between 7 and 10 knots of wind as not significant. On the contrary, this is a huge difference in fact. The lighter the wind, the more sail surface you'll need to start and sustain the planing with your big board. BTW, the planing difference between an 8.5 and a 9.5 sail is about 1 knot earlier. That is small in absolute term but it is plenty enough to be able to plane with a 9.5 sail whereas you will shlogg with an 8.5 sail if the wind is just too light for you.
Since the iSonic 137 is really a big board for your medium weight, I would strongly suggest you to use the H2 9.3 m2 sail (or something larger) if you really want to maximize its low range of usage. For the record, I've tried my 11 m2 sail + 55 cm fin on an iSonic 127 carbon 2011 and it was flying upwind and downwind without any problem as of 7 knots of wind.
However, the main concern that I have for you is I fear that your sail of 7.2 m2 is quite a bit too small on such a big board for your relatively light weight (iSonic 137 is mainly designed for 95+ kg rider for PWA slalom) and I doubt it will be an efficient combo when the wind is strong with some serious chop. You can try with the 7.2 sail and a small fin, but do not expect some miracle control and handling...
Last edited by Jean-Marc; 9th November 2012 at 01:43 PM.