RE: isonic 111 - trouble with back foot
I agree with Geo that probably you are using very small fins for your height/weight. I´m sailing my new IS111 mainly with a Code Red 7.7 and I found that the 36 cm fin is okay while sailing well powered (+ 14 knots) I am weighing 89 KG. (1,85 tall)
Since your weight is around 100 KG you would need more fin surface in order to obtain enough lift. (38/40 for the 7.8)
As you are using the boom in the highest position you are obtaining a high proportion of the lift throught the vertical component of force produced by the sail (wich is probablly too much leaned windward) this means that you are losing forward performance.
I guess that if you try to use the boom in a lower position, with long harness lines and a bigger fin your sailing performance will improve, not only with the 7.8 but with all your sails.
then you can trimm the straps and the mastfoot positions.
Here you can see the opinion of kevin Pritchard as regards boom height and harness lines (he is a tall guy too)
I hope this will help you, sorry for mi poor english :|.
Isonic 2009 122/101/86 , F2 Mauiproject W 90 lts
Slalom: Code red 8.8 NP RSR 7.8/6.7 evo/6.2 evo NP RS6 5.4
Wave: NP zone 4.4 North Ice 4.7/5.3
The rider: Age 48 (yeah, I know, so old, but still on form)
Height 1,84 m
Weight 87 kg