I found an interesting site: sizing the engines on a power boat.
While acknowledging that there are many models, some of which may be close, they have a simple diagram which I liked:
They have put their boat Platypus on it as an example. Note that the diagram and scales are for “high speed boats capable of planning”.
It is pretty good for the Ligure 50 as for 36,000lb, 135hp motors (note 2x) would give an estimate of about 11kn, which is about right.
135hp motors – 11kn
170hp motors – 15kn
250hp motors – 18kn
320hp motors – 20kn
Of course all of these are simply close guesses. A particular boat really needs some good analysis like Snodog did.
Blue Nomads 