Papa was strongly in favour of the self-righting principle.  The best boats are diagonally built, and copper-fastened.  The planks are of mahogany, two thicknesses of half-inch board, with painted calico between them.  The keel is of American elm, and the false keel is one piece of cast-iron, two and a half inches in width, by four and a half in depth, weighing nine hundredweight.  The stem is of English oak, and the gunwale of American elm.  The floors are of ash or oak.  The deck is of mahogany, well caulked, and seven-eighths of an inch in thickness. These boats are about thirty-three feet in length over all, eight feet in breadth, four feet in depth.  They pull, when double-banked, ten oars, which are made of ash, or sometimes fir; and they carry five or six pairs of spare oars, to replace any which may be broken.  They are fitted with life-lines outside, by which the men, if thrown out of the boat, can hold on to her, or people swimming can haul themselves on board.  No other boats are built so strongly.  The principle adopted for planking--that of placing the planks diagonally--gives the greatest possible strength and elasticity, while the mahogany used is of the best.