I use 2 fins, one is a cut down select 46 race fin to 15cm and the original cemicircula GO fin cut down to 15cm. In this mode I have taught people up to 60Kg and 4m sail.
For advanced maneuvers up to 108Kg and 5.6 sail.
If you plan to be semi planning, you have to go back to the original fin.
The small fin setup is for when you have the daggerboard down all the time and not planning. The board is very maneuverable and only needs a small movement of the rig to turn.
You can try this with your other Starts. If the people are having trouble with the board turning to much I just put the large fin back on and change back to the small fin when they are learning maneuvers.
I hope this helps as everyone who sails this set up has realy enjoyed themselves.
thanks a lot for your explanation
I think I'll try tomorrow myself, probably an M with a cut down fin