In naval architecture, a transom is either the surface that forms the stern of a vessel or one of the many horizontal beams that make up that surface (e.g., the "wing transom", etc.). Transoms may be flat or curved and they may be vertical, raked forward, also known as a retroussé or reverse transom, angling forward (toward the bow) from the waterline to the deck, or raked aft, often simply called "raked", angling in the other direction. 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" or "cutaway 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 critical part of the vessel's construction.
The transom is at the stern of the boat where the two sides of the vessel curve together, meet at the bow, and are combined because of the transom. Most small vessel's transom have to support an outboard motor as seen in the image. This means that the transom transfers the energy from the motor, to the hull, and then moves the vessel through the water. When the motor is turned, the angle is changed of the motor pushing the vessel, and the energy is redistributed on the transom to turn the vessel. All of this means that the transom is under a lot of stress and in response, it is the thickest and strongest part of the hull. 
Maintenance of Transom
Some older fiberglass vessels have fiberglass over wood transoms and some have only wood transoms. Wood can rot overtime which will destroy the strength of the transom. This can be a very big problem and catastrophic. The transom needs to be inspected periodically and it can be repaired if rot is caught early. 
A flat transom is plain and it is what it says (flat). It can be slightly raked, but it usually looks like a semi circle.
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