The secret of the Viking ship lay in its unique construction. Using a broad ax rather than a saw, expert woodworkers would first split oak tree trunks into long, thin planks. They then fastened the boards with iron nails to a single sturdy keel and then to each other, one plank overlapping the next. The Vikings gave shape to the hull using this "clinker" technique rather than the more conventional method of first building an inner skeleton for the hull.

Next, the boatbuilders affixed evenly spaced floor timbers to the keel and not to the hull; this insured resilience and flexibility. They then added crossbeams to provide a deck and rowing benches, and secured a massive beam along the keel to support the mast.

The longships' light, economic construction was a major factor behind their success. Modern replicas have achieved speeds of up to 14 knots. In marked contrast to modern sailboats, the ships' lack of a big, vertical keel meant that they were highly maneuverable and could easily penetrate shallow surf and river estuaries. Seafarers steered using a single side rudder on the right, the 'starboard' or "steering board" side. (The term 'starboard' is thought to have originated in the Viking era.) They could also reef the square sails in strong winds and adjust them to permit rapid tacking.

From Nova Online - The Vikings