well boblee, i have 21 inch sides on a 13 x 8 air ranger. i'm running a lycoming 540 and i have pretty good water clearance. however, if you were going to run a SBC cast iron block, you will probably have to go higher if you want to have any confidence. but, if you're running an A/C engine, the minimum height of the sides will be dependent on the width of the boat.....