Hi Tammo
I see you are re-building a 555 Ace-cat. I'm not sure how much of it you are re-building but just some informal advice. I had a original Ace-cat and it was very heavy and sluggish, It had thick meranti stringers and a 20mm marine play deck sandwichiced in what looked like 20 layers of glass. It took minimum twin 90's to get that boat to move.
I recently went for a ride on a 555Acecat that had been re-built from hull up, all with Nida-core no wood,the guy bought the hull for R16K cus the owner thought it was rubbish, I am more convinced than ever that the hull design on those boats are not the problem, just the weight. This re-born ace had twin 60 Yammies on the back and rode beautifully, felt as good as my 23 butt I had. the owner reckons two things improved the boat, 1: He shaved about 400kg's of weight of the hull and 2: he also put the tanks under the deck and this in turn lowered the centre of gravity.
Butt-cat and Ace-craft have proven one point for sure, if its a mono, beware of making it too light and if its a Cat build it strong and light.