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A thwart is a beam that runs laterally across the structure of a boat. In some vessels, rowers may sit on a thwart while rowing. Thwarts are part of a complicated system of framing which helps give vessels strength and rigidity.
In traditional wooden boat building, the shell of the boat consists of long planks called strakes. The strakes are attached to the keel, which makes up the central line or spine of the boat, and to the gunwales, which are the upper edge of the shell. A complex arrangement of timbers supports this shell and makes it hold its shape. Thwarts are part of this framing; supports called thwart risings support them from beneath, while curved fittings called "thward knees," fastened to the gunwales, hold them in place from above.
The number of thwarts in a boat depends on its size. Many small boats have only a single thwart, while longer vessels will have several. A 20-foot (just over six meter) vessel, for example, might have four or vie thwarts.
Modern inflatable boats can also have thwarts. Usually, these are detachable structures which are folded and stowed separately when the boat is deflated. When the boat is in use, the owner fits the thwart into plastic slots in the sides. These temporary thwarts both add to the vessel's rigidity and serve as seats, just as they do in a conventional boat.
Although the term "thwart" most commonly refers to these structural supports, some other beams aboard a boat are also sometimes called thwarts if they run laterally across the vessel. For example, the strut which holds a steering oar in place on a canoe is also called a thwart, even though it does not sit in the same position as the true thwarts. Similarly, when a sail is added to a small vessel, the mast is sometimes supported in place by a crossbeam which is also known as a thwart.
Because the thwarts run from side to side across the line of the boat, their position came to represent that direction. Anything which runs from side to side across a boat or ship is said to be "athwart" it. This position is called "athwartships." This expression entered use in other areas of naval life as well. For instance, an officer in the 18th- or 19th-century British navy who wore his hat with its point forward was said to be wearing it "fore-and-aft," while one who wore it with the points to the sides was wearing it "athwartships."