Transom on a boat

Seen from astern, the transom is the whole face across the back of the hull. US boat regulations define it as the surface at the stern that projects or faces aft, which is why the stern view shades the entire aft face, from the bottom to the top edge. A hull with a transom stern stops at a flat or nearly flat surface there, and that surface is what the word names.

Transom height

On small outboard boats the regulations use the transom as a reference line, so its height has an exact definition. Transom height runs from the lowest point along the top of the transom where water could come in, down to a line that extends the bottom of the hull at the centerline. The rule leaves any keel out of the measurement, and the drawing follows it.

Full, notched and Euro transoms

Builders usually call a transom full when it spans the stern at side height, and notched when its top has an outboard cut-out. An industry guide says outboards on full transoms usually need a bracket, and describes a Euro transom as that bracket built into the hull. The regulations use “full transom” in another sense, for a transom wider than half the boat’s maximum beam.

On many outboard boats the top edge has a lowered cut-out where the motor mounts. Inside it, a walled splashwell, which the regulations call a motorwell, stops water that comes through the cut-out from reaching the passenger area. The centerline section on the right shows the well sitting forward of the transom.

Fittings on the transom

The outboard motor, a complete engine, gear case and propeller unit, is mounted on the transom itself. Low down on each side, the drawing adds stern eyes, attaching points for a boat on a trailer, and a drain plug controls the draining of water from the hull. The swim platform, where fitted, gives a low step at the stern for boarding from the water.

What it is made of

Boating writers describe fiberglass transoms as usually built around a core of marine plywood or a composite board, sandwiched between layers of fiberglass. BoatUS editors describe rebuilding a rotted core as usually a job for a professional, since it must come out from between the fiberglass skins. This site does not cover repair.

The word

The same word names a crossbar over a door and, in American usage, the small window above it. Dictionaries trace it to Latin by different routes: Merriam-Webster through Old French traversain, “set crosswise”, and Etymonline from transtrum, “crossbeam”.