In naval architecture, a transom is the surface that forms the stern of a vessel. Transoms may be flat or curved and they may be vertical, raked forward (known as retroussé), or raked aft. The bottom tip of the transom can be approximately on the waterline, in which case the stern of the vessel is referred to as a "transom stern", or the hull can continue so that the centreline is well above the waterline before terminating in a transom, in which case it is referred to as a "counter stern".
On smaller vessels where an outboard motor is the source of propulsion, the motor is usually mounted on the transom, and held in place either by clamps or metal bolts that go through the transom. In this arrangement, all the power of the motor is transmitted via the transom to the rest of the vessel's structure, making it a very important part of the vessel's construction.
The expression over the transom is rooted in the architectural meaning of the word. See: Transom (architectural).
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