If mounting directly to a boat transom: Measure from the top of the transom down to the bottom of the hull or keel (lowest point). Measure the area of the transom directly where the engine will be mounted. See diagram below.
If using an auxiliary mounting bracket: With the bracket in the down position, measure from the top of the bracket’s mounting surface down to the bottom of the hull or keel (lowest point where engine will be mounted). Tip: For most auxiliary applications, it is possible to use a longer shaft length than what is called for, provided there is sufficient clearance when the engine is in the up position. This can be especially useful in heavy seas.
If measuring an existing outboard: Measure from the inside top of mounting bracket down to anti-cavitation plate above propeller (this is the largest plate/fin directly above the propeller). See diagram below.