Though the surviving evidence has mostly to do with the birlinn in a naval context, there is independent evidence of mercantile activity for which such shipping would have been essential. There is some evidence for mercantile centres in Islay, Gigha, Kintyre and Knapdale, and in the fourteenth century there was constant trade between the Isles, Ireland and England under the patronage of local lords. Otherwise the chief uses of the birlinn would have been troop-carrying, fishing and cattle transport. In some ways the birlinn paralleled the more robust ocean-going craft of Norse design. Viking ships were double-ended, with a keel scarfed to stems fore and aft. A shell of thin planking (strake) was constructed on the basis of the keel, the planks being edge-joined and clenched with iron nails. Symmetrical ribs or frames were then lashed to the strakes or secured with trenails. Over most of the ribs was laid a slender crossbeam and a thwart. The mast was stepped amidships or nearly so, and oars, including a steering oar, were also used. The stem and stern post sometimes had carefully carved notches for plank ends, with knees securing the thwarts to the strakes and beams joining the heads of the frames. Oak was the wood favoured both in Western Scotland and in Scandinavia, being tough and resistant to decay. Other types of timber were less often used. It is likely that the Outer Isles of Western Scotland had always been short of timber, but birch, oak and pine abounded in the Inner Isles and on the mainland. The abundance of timber at Lochaber was proverbial: ""B'e sin fiodh a chur do Loch Abar"" ("Bringing wood to Lochaber") was said of any superfluous undertaking. The tools used are likely to have included adzes, axes, augers and spoon bits, auls, planes, draw knives and moulding irons, together with other tools typical of the Northern European carpenter's kit. As in traditional shipbuilding, generally, measurements were largely by eye. The traditional practice of sheltering boats in bank-cuttings ("nausts") – small artificial harbours – was probably also employed with the birlinn. There is evidence in fortified sites of constructed harbours, boat-landings and sea-gates. The influence of Norse shipbuilding techniques, though plausible, is conjectural, since to date no substantial remnants of a birlinn have been found.