Just picked up a 2010 Isonic 86 and will be using KA Koncepts in the 5.0 to 6.4 range but i was wondering if any one has used a 7.5m on this board?
Russ had his first go on the same board with a 7.5 Koncept, have a look threw his post and a few comments he and others said.
gpsteamchallenge.com.au/sailor_session/show?date=2011-05-22&team=27
I use my 7.5 Koncept on the CA50speed and it feels nice. Shouldn't be any problem on the isonic.
Have had an 86 fo a couple of years now ,best all round board I have ever owened. its sweetest with my 6.7 pryde racing,but i use it offen with a 7.8 works particularly well of the breeze (used that combo for the LOC this year)the 7.5 should be fine.
Jet said he has used the 7.5 Overdrive on his Isonic 76 on the lighter days. Worked fine with 30 vector slalom. He weighs under 70 kg.
Actually he uses the 6.7 Overdrive for all other days.
It would work ok off the wind for speed but not so good across or upwind.
Great board by the way still is of the best boards ive owned.
Great board, your gonna love it 
I have a 2009 I Sonic 86 myself and use my 7.3m KA Koncept on it all the time, and also the 2008 7.5m KA Koncept i had previously and the sail suits the board perfectly when you pair it with a 36cm fin
I find the 5.0m feels a little small for the board and i prefer to use this sail size on my I Sonic Speed Specials, for me the best sails for the board are the 5.7m (30/32cm fin), 6.4m (32/34cm fin) & 7.3m KA Koncept's (34/36cm fin)
you can use a large sail on a small board it just feels different/crap.
you will definately not sail as high into the wind and the board will plane much lower over the water.
you will also not necessarily plane that much quicker as weight of larger rig sinks the small board.
i go the other way,too small a sail on a larger board.
my f2 x-speed is 110 litres,i like my 5.6m best on this board,it planes real high out of the water. my 6.2m also works well on this board,sails a little lower in water,my 7.7m on this board it sails way lower and very hard to get upwind!
it depends on what sort of feel for sailing you enjoy,a larger board and smaller sail
makes the sail feel extra light and manouverable. very easy to do slow gybes and still maintain balance. gybing a large sail on small board is much more difficult.
have seen some freaks on river using 7m and larger sails on narrow speed boards but not for me.
Red it will be fine for flatish water speed sailing. I use a 7.5 Koncept on the LM Medium which is 80 litres and narrower than the Isonic , especially in the tail. It works sweet at the PiT and Invy, or at Rye between the sand bars.
I also love the iSonic 86 - a ripper of a board. I use mine mainly with a 6.2 and a 7.0m, both feel great. As others have said, flatter water and off-the-wind, the bigger sails will get you going and the board will probably have a higher top end once you get going than a larger board.
I had an i86 and it was a very nice board. Found it best with a 6.7, did try a 7 but not really in the best conditions. Guess just try it and see 
