From the "Bulbous Bow" entry in Wikipedia:

A bulbous bow, a feature of many modern ship hulls, is a protruding bulb at the bow (or front) below the waterline. Usually visible only when a ship is in drydock, the bulb modifies how water flows around the hull, reducing drag and increasing in speed, range, and fuel efficiency. Ships with bulbous bows generally have 12 to 15 percent better fuel efficiency than similar vessels without them. Bulbous bows achieve maximum effect at a narrow range of speeds over 6 knots (Bray, website). At other speeds, they can increase drag. They have the greatest effect on large ships such as freighters, navy vessels and various passenger ships. They are rarer on recreational boats designed for wide speed ranges and planing over the water.