Transom

A transom is the flat or gently curved transverse surface that closes the stern of a hull, typically rising from at or below the waterline up to the deck. A transom immersed at speed lets the flow separate cleanly off the bottom edge, producing a ventilated dry transom and a rooster-tail wake; the resulting hollow lowers stern wave-making at high Froude number, which is why fast craft favor wide immersed transoms. At low speed the same immersed transom adds a base-drag suction and a wetted dead-water region. Transom shape sets stern buoyancy, deck area, and the flow into the propeller and rudder.