Rolf,
I've done some tow in measurements several years ago to evaluate the benfits of fitting a sub deck to hulls.
I used a loose guage on the forestay to ensure the loading was the same and I was comparing apples to apples.

On a very light weight timber platform the hulls towed in 30mm.
A glass platform was around half that and a timber platform with a sub deck in the hulls was half that again.

Unfortunately I don't remember the loose guage setting but if I find my notes on this subject I'll let you know.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!