I once read (I forget where) as a rule of thumb, a sailboat''s auxilliary engine should be about 1 HP for every 500#s of displacement.
Elsewhere I''ve read, two blades should be sufficient, but three blades will provide a smoother application of power, as well as more thrust when backing down. They also cause more prop-walk. The size and pitch of the prop are also critical, and need to be matched to achieve max hull speed while the engine''s running at its ideal cruising RPM without over-stressing it.
~ Happy trails and sails to you ~ _/) ~