I'm building a 20 ft Sharpie designed by Bjorn Thomasson (see link below). I spoke to Bjorn (well emailed back and forth) and he recommended an inboard electric motor in one of the cockpit lockers with an off center shaft going straight back from the locker.
My question is that there is a wide variety of 'electric motors' out there that will run off of one or two 12v car batteries. Since I can waterproof the motor compartment I don't see the need for a marine specific motor. Thinking of a 1hp motor with a reverse and variable speed controller. This sharpie design is supposed to slip through the water with ease and I'm not going to use the motor except in an emergency when, say, my wife needs to get me back to shore and not fool with sails. Oh, I plan on never being on blue water, mostly just reservoirs in central Indiana and possibly a cruise up Lake Michigan.