 Complete the following phrase:  Galleys were not suitable for the colder and more turbulent North Sea and the Atlantic Ocean, although they saw occasional use. Bulkier ships were developed which were primarily sail-driven, although the long lowboard Viking-style rowed longship saw use well into the 15th century. Their main purpose in the north remained the transportation of soldiers to fight on the decks of the opposing ship as, for example, at the Battle of Svolder or the 