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.