The mid section length mentioned is an average value and there will be some wiggling around those numbers by roughly an inch +/- from vendor to vendor. But a 15 is roughly 15 and a 20 is roughly 20. It's important to have your antiventilation plate (horizontal plate just above the top of the propeller) near even with the bottom of the hull.....like put a straight edge (ruler-yardstick) on the hull and extend it to where the engine lower unit is positioned),with the engine sitting vertical on the transom (rear board that the engine mounts on). So, as the others have said, measure your current engine, if satisfied with it's performance, considering it's hp, and get one about the same length.
