So im looking at a couple of boards. Me 100kg. Wanting to know what type n size fins is the go for Falcon 125 n Isonic 125s? Sails 9.5 down to 6.5. Ive got 57cmVmax, 53cmS10, JP 44Slalom, BPrace 42.5. Water conditions relatively flat at GB. Whats the go ?

57 too big, 53 when marginal, 42.5 well powered. My best guess is something around 46. On my rs5 125 I use a 44 carbon fin for 8.7 and 42 canefire carbon for 7.9 and 7.1, but I'm lighter than you at 76 kgs.

For a board 85cm wide I prefer 50 cm wide.

For marginal conditions go even bigger.

Others I know use smaller like 46 or 44 on an 85cm wide board but then they use the board in 15 - 20 knots.

For 10-16 knots I prefer a 50cm fin.

Also prefer a very wide chord fin in wide boards.

The wider the fin the more stable the board will be in a windward / leeward rail direction.

The 50 cm carbon Z fins are the best. There's a 50 slalom Z fin model and a 50 F for formula model Z fin which is a bit wider. The 50 Z fin F model was the best ever till I lost it.

Cutting down a 70 cm formula fin to around 47cm can also work very well on 85 cm wide boards.

G 10 fins in this size are rubbish.

Ok, so ive purchased 2013 Fanatic Falcon 122. 230cm x 76cm. Rigged 8m n took it for a spin in 3kts. Surprisingly it floated my 100kg ok n uphauled it easily. Putt putt putt. Gimme some wind!!!
Any more suggestions on fins much appreciated. Thanks Crew.

I'd say both these boards are too small for anything larger than 8.5 for your size.

As for fins, both these should have recommended fin for matching sail size written on the side (near back footstraps)? Your 44-42 should be OK for powered conditions. Maybe add a 46 to the necklace.

Only my 2 bobs worth.

76 cm is a lot different from 85cm wide.

I used a 46 carbon Lockwood on a Patrik 77cm wide and it was on the big side but planed very early. I tried a 40 G10 Volt and the board had no lift,no upwind performance and it felt dead.

Theres a 44 cm carbon Lockwood for sale on seabreeze going cheap.

For your weight with a 8m I'd say 46 cm would be a good all rounder.