ASA and PSIA Instructor
Join Date: Apr 2000
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I don't think I've ever seen anyting bigger than a 10 HP hanging off a stern. In addition to the weight at the end of the boat, you'd need to be concerned about the ability of the mounting system to handle additional power..
As to your desire for extra push for weather relief, I'd think the limiting factor may be more the question of keeping the prop in the water than just HP. Even assuming you start with a long shaft motor, where you install it on the stern will affect its effectiveness.
Does your current prop pop out in a seaway?