When selecting an outboard motor, you will need to measure the motor to make sure it will fit your boat's transom. With outboard motors, the most important measurement is the shaft length. Boats are generally rated for long or short shaft motors, so if you're looking at an outboard, you need to know how to measure. An outboard with a shaft length that is too short for the boat will cause the propeller to lift out of the water when the boat is on plane.
Measure the outboard when it is on a transom or mounting frame, if possible.
Place your measuring tape next to the outboard.
Measure the area from the top of the mounting bracket that holds the outboard on the transom, to the bottom of the cavitation plate just over the propeller.
Write down the measurement. If the measurement is approximately 15 inches, it's a short shaft. If the measurement is 20 inches, it's a long shaft.
Most outboard motors won't be exactly 15 or 20 inches. Some may be a little longer or shorter. If you're selecting an outboard motor, make sure to err on the long side.