Excerpted from an earlier discussion
Here ➥ Engine Size
Corporation suggests 2 hp for every 1,000 lb of displacement
for coastal cruising, and 2.5 hp per 1,000 lb of displacement for offshore
In their book "The Gougeon Brothers on Boat Construction" the authors suggest a good rule-of-thumb is 1 hp for every 500 lb of displacement.
Some people suggest 1 hp for every ft. of boat waterline length.
Some people suggest 3 - 5 hp (continuous rating) per long ton (to calculate the boat's weight in long tons divide the displacement by 2,240).