Wouter,

I think you are underestimating the force these foils will have when they kick up.

I had a foil (approx 350 wide x 200 long) on the bottom of a pivoting centreboard about 15 years ago & the first time I pivoted the board down was fine & the attachment seemed to work really well. But when it came time to pull the centreboard up the foil simply ripped off the bottom & caused a bit of damage to the centrboard & pin. The boat actually lunged quiet badly when this happened.

So I can see your transoms taking a beating no matter how well the design.

Just my 2 cents worth (or 0.01 euro).

Marcus