How to Mount or Re-Tighten an Outboard on the Transom

Find the capacity plate near the helm. Your motor's horsepower must not exceed the maximum listed. Federal rules (33 CFR Part 183, Subpart B) require this plate, with a maximum horsepower rating, on monohull boats under 20 feet except sailboats, canoes, kayaks and inflatables. Bigger boats may have no plate; ask the builder for the rating. Also compare the new motor's weight with the old one: a modern four-stroke can weigh much more than the two-stroke it replaces, which loads the transom harder and can make the stern sit lower. The plate's weight capacity includes the motor. Next, measure the transom height from the top of the transom to the bottom of the hull at the center (the keel). This tells you which shaft length you need (short, long or extra long). A shaft that is too short leaves the prop and water intake too high; too long and the motor drags.

Before you tighten anything, inspect the transom inside and out:

• Cracks in the gelcoat that run outward from the bolt holes or the motor bracket
• Rust stains or black streaks weeping from bolt holes (water getting into the core)
• Soft spots when you press with your thumb or tap with a plastic hammer (a dull thud instead of a sharp knock)
• On aluminum boats: cracked welds, loose rivets or bent transom braces near the motor

Then do the flex test: with the boat on the trailer, have a helper push the tilted-down motor up and down firmly from the lower unit while you watch the top of the transom inside. If the transom visibly moves or you hear cracking, stop. A transom that flexes is often rotten inside, and tightening the bolts will just crush it more (see the transom and stringer rot guide).

The motor must sit on the boat's centerline, or the boat may steer oddly. Many transoms have a small center mark. If not, find the center of the keel at the stern and draw a line straight up the transom, or measure from both sides of the transom and mark the middle. When you reinstall a motor that was already there, put it back in its old position and double-check that it is centered.

With the motor clamped on, vertical (not trimmed out) and centered, lay a straightedge along the bottom of the hull so it sticks out under the motor. For most boats, the anti-ventilation plate (the flat plate just above the propeller) should be level with the bottom of the hull or slightly below it. Yamaha's guide for small motors says level or up to about an inch below.

• Too low: extra drag, spray and slower speed.
• Too high: the prop can suck air (ventilate) and the water intakes can lose cooling water.

Performance hulls and jack plates follow different rules; use the boat maker's or dealer's recommendation. A jack plate (a bracket that moves the motor up and back) is a common owner question: it lets you fine-tune height, but it also moves the motor's weight farther aft, adds leverage on the transom and must be rated for your motor's weight and horsepower. Treat it as a dealer install. Bolted brackets have several hole positions so you can raise or lower the motor one step at a time.

On clamp-on motors, turn both clamp screws in evenly so they press with about the same force. Tighten them firmly by hand. On many small motors the handles can be turned until they meet in the middle, where you can padlock them together: that keeps them from working loose and discourages thieves. Check clamp screws before every trip; vibration loosens them.

Run a stainless cable or chain from the motor's restraint point (many small motors have one) to a solid fitting inside the boat. If the clamps ever loosen, the cable keeps the motor from going to the bottom. Leave enough slack to steer and tilt.

On a bolted motor, look at each bolt from inside the boat and outside:

• The nut should be tight with washers under both the head and the nut.
• There should be no gap between the motor bracket and the transom.
• Bolts should be stainless, not plain steel, and not bent or badly rusted.

To re-tighten, hold the bolt head with one wrench and turn the nut with the other, to the torque in your installation manual. Tighten in a crisscross pattern. Snug is the goal: over-tightening crushes the transom core and actually makes the motor looser over time.

A backing plate (also called a transom plate or washer plate) is a metal or composite bar that sits under the nuts inside the boat and spreads the bolt load across a much bigger area than washers alone. Boating Magazine's repower project added one even on a sound transom and called it good practice. If your boat has only small washers inside, adding plates the next time the bolts come out is a cheap upgrade.

Only drill if the motor bracket does not line up with existing holes, and ideally have a dealer do it. If you drill:

1. Mark the holes from the bracket with the motor centered and at the correct height.
2. Remove the motor and drill straight and square to the transom, the size the installation manual calls for.
3. Vacuum or blow out the shavings.
4. Look inside the hole: wet, dark or crumbly core means rot (see the transom rot guide). Many pros seal exposed wood core with epoxy before bolting through.

Fill each hole and coat the bolt shank with marine bedding sealant rated for below the waterline, so water cannot travel along the bolt into the transom core. Use the bolts that came with the motor or stainless equivalents, with flat washers and lock nuts or lock washers. Re-mount the motor, snug the clamp screws first (not fully), insert the bolts and check centering and height one more time. Then tighten the bolts to spec without crushing the transom, and finally tighten the clamp screws. Wipe off squeezed-out sealant.

The Boating Magazine repower showed a factory detail worth knowing: the techs ran the upper bolts from inside out so the ends would not hit the steering, and the lower bolts from outside in so the threads stay out of the water. Follow your installation manual.

Turn the motor lock to lock and tilt it all the way up and down. Fuel line, cables, steering and wiring should move freely with no pinching. Make sure the tilt lock works and the motor clears the boat and any swim platform.

On the first run, check that the motor runs straight, the prop does not ventilate in turns, and the telltale is strong. With the boat floating at rest and loaded, make sure waves cannot easily reach the engine's lower cowl. After a few hours of running, re-check the clamp screws and bolts, and check again periodically through the season. Yamaha recommends this re-check after the first few hours.