A catamaran has two hulls instead of one. The result is a more stable ride (think a Rolls Royce compared to a Dune Buggy) without the danger of keeling over.
Kurt Hughes Designs of Seattle
Under sail, the average speed is 8-12 knots (approximately 9-13 miles per hour). Whereas a monohull treads through the water, catamarans skim over the surface.
Prior to departure time, the Captain makes that decision.