How to Choose the Right Outboard Shaft Length

Put the boat on its trailer or stands with the hull as close to level, side to side, as you can. A tilted boat throws off every measurement. Make sure it's secure and can't roll.

Measure where the motor will actually clamp or bolt: usually the center of the transom notch or motor well. On some boats the motor mounts in a cutout that's lower than the rest of the transom. Measure there, not at the highest point.

Tohatsu's method: measure from the top of the transom (where the motor bracket sits) straight down to the bottom of the keel (the lowest point of the hull at that spot). Hold the tape square to the hull bottom, not slanted along the angled transom face. A straightedge laid along the hull bottom and sticking out past the transom gives you something to measure to.

Typical match-ups from Tohatsu: about 15 to 17 inches needs a short shaft; about 20 to 22 inches needs a long shaft; some boats need a 25-inch shaft. Treat these as a starting point, not the final word.

Tohatsu measures its shafts from the bottom of the engine mounting bracket (where it rests on the transom) to the anti-ventilation plate. For a kicker, Boating Magazine's install steps measure from the plate to the upper inside edge of the clamp. Mercury describes its kicker shaft length a bit differently, from the top inside of the mounting bracket to the center of the prop. The lesson: brands measure from slightly different points, so compare like with like and ask the dealer which reference their number uses.

If a motor is already on the boat, put it straight up and down (shaft vertical, not trimmed in or out). Hold a straightedge flat against the hull bottom, near the keel, extending back over the gearcase. Boating Magazine describes technicians using exactly this check during a repower:

• Plate even with the straightedge: the usual target.
• Plate well below it: the motor is mounted low (or the shaft is too long). Expect extra drag and spray.
• Plate well above it: the motor is high (or the shaft too short). Watch for ventilation and weak water pressure.

Mercury's kicker advice gives a useful limit: choose a shaft that puts the plate no more than 1 inch below the hull bottom.

A V-hull's bottom rises as you move away from the keel. Boating Magazine points out that a kicker mounted off to one side needs a shaft that matches the transom height at that spot, not the maximum height at the center. Mercury says the same for its kickers. The same logic explains different shafts on twin and triple rigs.

If you already own the motor, the boat will tell you:

• Too short or mounted too high: prop blows out or "slips" in turns and when trimmed up, RPM jumps without more speed, weak telltale or overheating alarms (intakes sucking air), poor bite in chop.
• Too long or mounted too low: sluggish top speed and higher fuel burn from drag, a rooster tail or sheet of spray off the gearcase, the boat feels like it's plowing, the lower unit hits bottom in shallow water sooner, and spray can come up over the transom.

Our prop ventilation and cavitation guide covers other causes of blowouts (damaged prop, hull hooks, through-hull fittings in the water flow) so you don't blame shaft length for everything.

Most outboard brackets have several vertical bolt-hole positions, letting a tech raise or lower the motor in steps. Boating Magazine's repower crew used the straightedge result to pick which hole to use. Raising a motor one hole at a time is a common way to trade a little drag for speed on a performance boat; going too far brings back ventilation and cooling problems. Moving the motor means unbolting and re-sealing through-bolts in the transom, so it's usually a shop job.

A jack plate (manual or hydraulic) mounts between the transom and the motor and lets you change height without re-drilling. Owners of bass boats, flats boats and shallow-water skiffs use them to run higher for speed or to get through skinny water. A plate that sets the motor back also moves the prop into cleaner water, which can allow a higher mount. See our hydraulic jack plate guide for care and limits. A jack plate can't fix a shaft that's far too short.

At the chosen height, tilt the motor fully up and steer lock to lock. Make sure the cowl, cables, hoses and the gearcase clear the transom, splashwell, swim platform and any kicker. On a trailer, confirm the skeg clears the road at full trailer tilt (or use a transom support as the maker recommends).

Run the boat with a typical load. Note wide-open-throttle RPM (it should land in the range in your owner's manual), top speed, how the boat handles in turns and whether the telltale and water pressure stay strong. If the motor ventilates at normal trim, it's probably too high; if it's slow, wet and draggy, it may be too low.

Log the transom height, the motor's shaft length, which bracket holes are used (or jack plate setting) and your sea trial numbers in BoatMend. It saves time when you change props, sell the motor or repower.