For small boats, engine power is related to boat displacement. A sailboat outboard should be a small as possible (low engine weight and correct proppellor pitch) The last one is mostly forgotten.
The formula is roughly, 4 hp per 2200lb
Then go to the internet to find a
propellor calculator to determine your
propellor size.
Petrol engines are mostly lighter than diesel (Tohatsu)
Good luck!!