The 'eyeball' rule for 'displacement hulls' is 1 hp per 500 to 700 pounds of boat weight.
Use 700 for the minimum value - for minimum performance in adverse conditions.
Use 500 for maximum performance
eg.: 7000 lb. boat divided by 700 = 10 hp. for 'light' and relatively flat conditions
eg.: 7000/500 = 14 hp to 'power through' directly into very heavy chop, etc.