Sentence:  A conſiderable number of <WSD>thwarts</WSD> were laid from gunwale to gunwale, to which they were ſecurely laſhed on each ſide, as a ſtrengthening to the boat [a canoe]. 
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.