get away with? really? As in most of the time, that motor will work and not cavitate, but when it gets rough, and I am heading through a cut and and am not able to keep the prop in the water, and might get pushed into the rocks if the engine doesn't start to get some momentum.... if you only sail on a lake you might get away with a 15, (or an electric for that matter), but if you are in Fotuna CA, and plan to sail in the ocean. 20 or 25" shaft is what a wise person would get. honestly, I have had a kicker (15") on a 21' skiff, and had pushed it so low the cowling was below the water. had to since it was coming out of the water on the down side of the wave. I would go with a lower hp, before I went with a shorter shaft. best of luck!