Outboard engines come in a range of sizes to meet the needs of different boats and sailing conditions. Starting at 15 inches in length, short shaft outboard engines suit smaller craft, such as canoes and inflatables. Pontoon and most other sailboats require a long shaft engine, while larger vessels, such as barges in need of high thrusting power, require an extra long shaft outboard.
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Lay the outboard engine on the ground.
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Identify the outboard engine's shaft length. This runs on the side opposite the tiller. It extends from the top of the mounting clamp bracket to the cavitation plate.
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Using a measuring tape, measure the outboard engine's shaft length. Record this in inches.
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Identify your boat's transom, which is at the stern. It comprises the panels at the back of the boat that remain out of the water during operation.
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Use a yardstick to measure down on a vertical plane from the top centre of the transom to the boat's hull. This will give you an approximate shaft length that is within an inch. Transoms are rarely vertical, so do not measure a sloped or angled transom. Just draw an imaginary vertical line down from the transom.
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Use the approximate shaft length to determine what type of outboard engine you'll need. If it's between 15 and 17 inches, choose a short shaft outboard engine. Boats measuring 18 to 21 inches require a long shaft, and vessels up to 27 inches can use an extra long shaft.