How to Add or Replace a Swim Platform on Your Boat

Tap the inside and outside of the transom with a plastic mallet and listen for dull spots, look for cracks and stress lines around existing fittings, and push on the top of the outboard or around the drive to see if anything flexes (the transom and stringer rot guide explains how). Find out what it's made of: BoatUS notes transoms are often wood-cored, so any new hole needs sealing. Measure the thickness where you'll drill, too; it decides the backing in step 9. Watch out for molded bumps and curves near a sterndrive or at the transom corners: owners in forums often find these are a thin fiberglass shell, sometimes filled with foam, not the full-thickness cored transom. Look (or put a phone camera) behind the spot, and put brackets on a flat, solid section instead.

Buy a kit designed for your setup: outboard or sterndrive, side or full-width, and your transom angle (BoatUS's kit mounted at a standard 10-degree transom angle). Read the maker's instructions before ordering, since they list the space you need. The maker of the kit BoatUS installed said outboard owners often discover their transom isn't tall enough to fit both the lower strut and the upper handrail on a single-handrail model (about 22 inches apart on the BoatUS boat). In that case a model with two handrails can work, as long as the inner rail still clears the engine.

Measure the flat area on each side of the engine, the height of the transom above the waterline, and where the rub rail, stern eyes, drain plug, trim tabs, transducer, livewell fittings and exhaust outlets are. On a full-width platform, Discover Boating's installation sets the platform top below the curved edge of the rub rail and supports it on the stern eyes while fitting it. Write the numbers down and compare them with the kit's dimensions before you drill anything.

This is where add-on platforms go wrong:

• Outboards: Boating Magazine's method is to turn the wheel so the motor swings as far as possible toward the platform side, mark a line on the transom at the motor's full swing, and keep the whole platform outside that line. Then tilt the motor all the way up at full turn and check again.
• Outboards on a jack plate or setback bracket: the motor moves up, down and aft, so the swing changes. Boating Magazine's jack plate install checks full steering range with the plate at its lowest position and then tilts the engine up to look for interference; do your platform check the same way, at both the lowest and highest jack plate positions, and remember the lower unit sits farther aft than on a plain transom.
• Sterndrives: check the drive trimmed fully up and at full trailer tilt, turned to both sides. The platform must clear the drive, the trim rams and the steering at every position, and leave room to service or remove the drive later.
• Both: the ladder, when lowered, must be well away from the propeller.

On outboard boats, BoatUS mounted its platform to port, a common choice that keeps swimmers from having to climb over the steering cables and rigging, which usually run on the starboard side inside the transom. Also look at where the engine and generator exhaust comes out and keep the platform and ladder as far from it as you can. Make sure there's a handhold within reach of someone standing on the platform.

For a small side platform, Boating Magazine positions it parallel to the waterline and about 3 to 4 inches above it, with room to through-bolt both the upper and lower mounting points. For a full-width platform, Discover Boating's instructions level it side to side, keep it parallel to the waterline with a slight downward slope toward the back, and support the outer end on a 2x4 while marking. Your kit's instructions win if they say otherwise.

Hold the assembled platform in place with a helper, mark the holes, and then go inside the boat and look and feel behind each mark with a light and mirror. Fuel lines, battery cables, bilge pump hoses, steering and trim lines often run along the inside of the transom. Move a hole rather than risk drilling through any of them.

Put masking tape over each mark. BoatUS starts each hole with the drill running in reverse so it doesn't chip the gelcoat, then switches to forward; Boating Magazine stresses keeping the drill square to the transom. If the drillings show wood or balsa core, protect it: drill oversize, fill the hole with thickened epoxy, let it cure, then drill the bolt hole through the epoxy plug (the through-bolting accessories guide shows the method). A countersink on each hole's edge helps prevent gelcoat cracks and gives the sealant a place to seat.

Washers alone are not enough. BoatUS notes that backing plates are often needed inside the transom to spread the load and keep the bolts from crushing the core, and the kit maker it quoted recommends a backing plate whenever the transom is less than 2 inches thick. Boating Magazine lists aluminum, marine plywood or a marine polymer board for the plate. Seal plywood with epoxy, and make the plate larger than the bracket's footprint. Full-width kits may come with their own plastic reinforcement blocks for the inside; use them as directed.

Fill each hole and coat the mounting feet with the sealant the platform maker recommends. Push stainless bolts through from outside, with a washer under the head; inside, use the backing plate (or a large fender washer on top of it) and a nylon-insert locknut. A coat of anti-seize on the threads, which Discover Boating's install also uses, keeps stainless nuts from seizing. Have your helper hold the bolt heads while you tighten evenly. Snug them firmly but don't crush the gelcoat; follow any tightening guidance in the kit instructions. Wipe away the squeeze-out.

Attach the ladder to the platform as instructed, then deploy and stow it a few times. BoatUS stresses that the bottom step must reach deep enough for a swimmer to get a foot on it, because most people climb with their legs, not their arms, and that handholds must be in the right place. Test it for real in calm, shallow water with the engine off and help nearby: can every regular crew member climb out unassisted? If not, add steps or a grab handle at the stern. Remember the ladder must be reachable and deployable by the person in the water.

Make it a habit before anyone goes near the stern:

• Engine off and the cut-off lanyard pulled whenever someone is on the platform, on the ladder or in the water nearby (Boating Magazine and BoatUS both insist on this).
• Generator off too, if it exhausts at the stern. The Coast Guard warns CO can build up around a boat when it is idling, moving slowly or stopped, can linger after engines shut down, and can come from the boat beside you.
• No riding on the platform while underway, and no "teak surfing" behind a running boat. The Coast Guard says never to sit, teak surf or hang on the swim platform while engines are running. The National Park Service reports two nine-year-olds who passed out from CO while holding a ski boat's platform on Jackson Lake in 2009 and had to be revived; teak surfing is illegal in all national park units and in a number of states.
• Nobody goes into the space under a swim platform where exhaust outlets are (to free a line, check a fitting or play) unless the area has been aired out first, as the Coast Guard advises.

See the CO detector and kill switch lanyard guides for more.

After the first few outings, check every bolt for tightness and look inside the transom for water around the holes. Rinse the platform and ladder with fresh water after use (Boating Magazine recommends soap and fresh water after every trip and an occasional polish of the stainless), and keep the ladder hinge and lock working smoothly. Any crack in the gelcoat around a mount, or a platform that moves when you stand on it, means stop using it until the mount is repaired.