instead of a thread, has a tapered hole forged through the end away from the head, into which a tapered pin or cotter is knocked. The taper is in effect a straight thread. In conjunction with a washer, this draws the bolt tight, as a nut does on a screw bolt. The third type of bolt is the clench bolt. It has some of the features of a rivet but was usually much longer than the normal rivet; in a wooden ship, perhaps a metre or more. For a shipwright's use, it is of copper. A head is formed by upsetting one end using a swage. It is then knocked through a hole bored through the work to be fastened, and through a washer. The head is held up with a dolly and the other end is upset over the washer in the same way as the head. Until well into the nineteenth century, this is what held the great ships of the world together, though some may have used iron. Until the late 1950s, the centre-line assembly of British Admiralty twenty-five foot motor cutters were fastened this way. Where suitable metal was not available, it was possible to use treenails (pronounced trennels). They were like clench bolts but made of wood, and instead of being clenched, they had a hardwood wedge knocked into each end to spread it. The surplus was then sawn off. Comparison between clinker and carvel The Vikings used the clinker form of construction to build their longships from split wood planks. Clinker is the most common English term for this construction in both British and American English, though in American English the method is sometimes also known as lapstrake; lapboard was used especially before the 20th century to side buildings, where the right angles of the structure lend themselves to quick assembly. The smoother surface of a carvel boat gives the impression at first sight that it is hydrodynamically more efficient. The lands of the planking are not there to disturb the stream line. This distribution of relative efficiency between the two forms of construction is an illusion because for given hull strength, the clinker boat is lighter. Additionally, the clinker built method as used by the Vikings created a vessel which could twist and flex relative to the line extending length of the vessel, bow to stern. This gave it an advantage in North