Once you know the maximum power you can fit on your boat, you’ll have a good idea of which outboard sizes fit the bill. You don’t have to put the most power you can back there, but if you put less, you will go slower and may have trouble getting a fast boat up on plane. If you’re looking to repower a boat, it’s best to consult with an experienced mechanic since the magic involves the boat shape, hull design, weight aboard, engine selection, and even prop size and pitch. You’ll also need to know the shaft length of the motors you’re considering. here