Sentence:  My barge was sixty feet in length, and not more than twelve in the widest part; by taking away one <WSD>thwart</WSD> beam near the stern, laying a floor two feet below the gunwale, and raising an arched roof about seven feet above the floor, a commodious room was formed, fourteen feet long, and ten wide, with a closet behind it; [...] 
Meaning of the word marked with <WSD> tags:  A brace, perpendicular to the keel, that helps maintain the (t=breadth) of a marine vessel against external water pressure and that may serve to support the rail.