Measure from the cavitation plate to the point on the engine that rests
on the top of the transom of the boat. That length is typically an inch
or two longer than the "official length" (15", 20", 25", 30")

Good luck and good boating,
Jim

RB wrote:

How do I tell what shaft length my o/b motor is? Does the serial # tell me
that info? If I need to measure, what takeoff points do I use for top and
bottom?