I have a 17' drift boat and use a 8 horse on it. I checked on getting a lisence and they told me if it is powerd buy any motor it has to be lisenced. no size just do you put a motor on it? I know dudes that don't and use a eletric trolling motor and have never lisenced it and not had any truble and probably won't but if you have my luck you better get it.

dcrzfitter