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Clinker (boat building)

Clinker built (also known as lapstrake) is a method of boat building where the edges of hull planks overlap each other. Where necessary in larger craft, shorter planks can be joined end to end into a longer strake or hull plank. The technique developed in northern Europe and was successfully used by the Anglo-Saxons, Frisians, Scandinavians, and typical for the Hanseatic cog. A contrasting method, where plank edges are butted smoothly seam to seam, is known as carvel construction.
The technique of clinker developed in the Nordic shipbuilding tradition as distinct from the Mediterranean mortise and tenon planking technique which was introduced to the provinces of the north in the wake of Roman expansion. Overlapping seams already appear in the 4th century BC Hjortspring boat. The oldest evidence for a clinker-built vessel, dendrochronologically dated to 190 AD, are boat fragments which were found in recent excavations at the site of the famous Nydam Boat. The Nydam Boat itself, built ca. 320 AD, is the oldest preserved clinker-built boat. Clinker-built ships were a trademark of Nordic navigation throughout the Middle Ages, particularly of the longships of the Viking raiders and the trading cogs of the Hanseatic League.
Once the shell of planking is assembled, transverse battens of oak, ash or elm, called timbers are steam-bent to fit the internal, concave side. Elm species are not durable where the boat is used frequently in fresh water. As the timbers are bent in, they are copper riveted to the shell, through the lands of the planking.
With the timbers all fitted, longitudinal members are bent in. The thwart risings are fastened through the timbers with its upper edge on the level of the undersides of the thwarts. Bilge keels are added to the outside of the land on which the boat would lie on a hard surface to stiffen it and protect it from wear. A stringer is usually fitted round the inside of each bilge to strengthen it. In a small boat, this is usually arranged to serve also as a means of retaining the bottom boards. These are removable assemblies, shaped to lie over the bottom timbers and be walked upon. They spread the stresses from the crew's weight across the bottom structure.
That more or less finishes the boatbuilder's work but the painter has yet to varnish or paint it. At stages along the way, he will have been called in to prime the timber, particularly immediately before the timbering is done. The boatbuilder will clean up the inside of the planking and the painter will prime it and probably more, partly because it is easier that way and partly so as to put some preservative on the planking behind the timbers. Similarly, it is best to have the varnishing done after the fittings are fitted but before they are shipped. Thus, the keel band will be shaped and drilled and the screw holes drilled in the wood of keel and stem then the band will be put aside while the varnishing is done.
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 because it does not rely upon the compressive forces of the caulking and the resulting friction to bind the skin together. It therefore displaces less water so it has less to push aside while moving. The reduced displacement could be used to make the lines finer so as to make the passage through the water easier still. Of course, displacement was increased as cargo was loaded but still, the clinker vessel had the advantage in efficiency as the structure can be less bulky. Therefore, for a given internal volume, there was a smaller external one, meaning that a bulkier cargo could be carried if need be, given sufficient freeboard.
There is an upper limit to the size of clinker built vessels, which could be and was exceeded by several orders of magnitude in later large sailing vessels incorporating carvel-built construction. Clinker requires relatively wide planking stock compared to carvel, as carvel can employ stealers to reduce plank widths amidships where the girth is greatest. The need for sufficient lap to accept the clench fastenings drives towards wider planks in proportion to thickness than can be employed in carvel. In all other areas of construction, including framing, deck, etc., clinker is as capable as carvel. Clinker construction remains to this day a valuable method of construction for small wooden vessels.