If you are ready to replace the outboard motor on your boat, you will need to know whether it is a standard or short shaft motor. The shaft length is the most critical dimension of your outboard motor when replacing it. Installing a motor with a shaft that is too short can cause the propeller to be lifted out of the water when the boat is moving. Contrary to what many think, the shaft length is actually measured starting from the transom, or where the motor frame is mounted to the boat.
Place your boat where you can easily measure the motor, such as on a trailer.
Place the end of the measuring tape on the transom of your boat, or where the motor mounts to your boat frame. If you are using a mounting frame and do not have the motor mounted directly to your boat, start the measurement from there instead.
Measure the distance from the mounting frame area down to the cavitation plate. This will be a straight, horizontal protrusion just above the propeller.
Determine your total shaft length. Shafts that are approximately 15 inches long are considered short shafts. Shafts that are approximately 20 inches long are considered long, or standard, shafts.
The shaft lengths will not be exact when measured. When selecting a motor, err on the long side of the shaft. Most boats can use a longer shaft than what is called for, as long as there is proper clearance to lift the motor out of the water when it is in the up position.