The longships were characterized as a graceful, long, narrow and light, with a shallow-draft hull designed for speed. The ship's shallow draft allowed navigation in waters only one meter deep and permitted arbitrary beach landings, while its light weight enabled it to be carried over portages or used bottom-up for shelter in camps. Longships were also double-ended, the symmetrical bow and stern allowing the ship to reverse direction quickly without a turn around; this trait proved particularly useful at northern latitudes, where icebergs and sea ice posed hazards to navigation. Longships were fitted with oars along almost the entire length of the boat itself. Later versions had a rectangular sail on a single mast, which was used to replace or augment the effort of the rowers, particularly during long journeys. The average speed of Viking ships varied from ship to ship, but lay in the range of 5–10 knots (9.3–18.5 km/h) and the maximum speed of a longship under favourable conditions was around 15 knots (28 km/h).