A thwart is part of an undecked boat that provides seats for the crew and structural rigidity for the hull. A thwart goes from one side of the hull to the other. There might be just one thwart in a small boat, or many in a larger boat, especially if several oarsmen need to be accommodated. Because they provide structural rigidity, some larger boats even have supporting knees on top or parallel to the thwart, they are made as permanent and not movable\removable.
Today, when the hulls are made from fiberglass, they are made removable and adjustable for the rowers.