Alternatively, you can accept that a 30hp engine with a hi thrust prop, sited deep enough to avoid cavitation, will be fine with an 8m Catalac (empiracal test!)

The real problem is getting the prop deep enough. On my 9m Catalac, the skeg of the outboard was the deepest part of the boat. I also fitted wings on the cavitation plate - the sort that are designed to lift a power boat onto the plane. They have little real effect on raising the boat out of the water, but they do two things that make progress over deep seas possible. They move the pivot point of the boat further aft, thus reducing the tendency to hobbihorse, and they reduce cavitation.

In the 14 years with my outboard configured this way, I never had any cavitation from my prop.