by 
HungryGuy
ANSWERS: 2
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Boils down to the boats intended purpose, hull type, prop and weight of the boat or how much it will displace. Also you need to consider if it's 2 stroke, 4 stroke, gas or diesel. I don't think there can be a "calculation" for that. There are too many variables. For example,a ski boat needs to get out of the hole fast, a cruising boat with the same weight , but different hull type will require a different engine to get out of the hole in the same time. There may be a calcualtion to use for figuring out how much energy is required to move a certain mass of water,but then we come back to hull type, weight and everything else. If you already own the boat and have operated it with the old engine, wouldn't you be qualified to answer your question? You know how it handled given the previous performance. When I re engined my boat (1975 Carver Santa Cruz) from a V8 233 up to a 325, the guy I had do it simply asked my boating habits. He then off his head, matched an engine, prop and lower leg. He didn't look anything up. Guess you need experience is all. Knowing what works and what won't. Hehehe...just like building a PC.
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In the case of a yacht, a lot of 'horsepower' is not needed. What is needed is ' torque' or pushing power. This can be a smallish [ 5 h.p.] motor turning a large propellor. A small SEAGULL motor with a five blade prop, can move a 30 foot vessel. But it will never get up enough speed to 'plane' or skim over the water. But it will move it. On the other hand, racing speed boats require a 'planing' type hull with a large horsepower motor, driving a fairly small prop. at great revs. to get the speed.
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