Of all the specification mistakes buyers make, shaft length is the most common — and the most annoying to fix, because it usually means exchanging the motor. The rule is simple: the anti-ventilation plate (the flat fin just above the propeller) should sit roughly level with, or just below, the bottom of the hull when the motor is trimmed down.
Measure your transom height vertically from the top edge where the motor mounts to the bottom of the hull. Around 15 inches is a short shaft, around 20 inches is a long shaft, and around 25 inches is extra-long. Sailboats and some work boats use extra-long; most inflatables and small aluminium boats use short; most fibreglass runabouts use long.
What goes wrong with the wrong length
Too short and the propeller ventilates — it sucks air in turns and chop, revs flare, and thrust disappears. Too long and you drag extra lower unit through the water, losing speed and fuel economy, and risk hitting the bottom in shallows.
Every listing on our site states the shaft length in its specification. If the motor you want is listed in the wrong length for your boat, email us — we often have the same model in another shaft length, or can tell you when one is coming in.