A Viking ship, especially a warship. They were built usually of fir planks, and differed from the vessels of the Angles, Saxons, and Frisians in having a massive vertical keel of oak instead of a shallow horizontal one; this enabled them to carry a mast and sail. The clinker‐built construction of overlapping planks secured by clench nails conferred great strength with flexibility, and the hulls were waterproofed with tar, seams between the planks being caulked with wool and hair. Later examples were over 46 m (150 feet) long and could carry hundreds of warriors who were also rowers. Longships were of extremely seaworthy design, and the addition of sails made very long voyages feasible, while the shallow draught meant that raiders could penetrate far inland by river. The violent expansion of the Norse peoples was dependent on the skilful use of such vessels.