A bulbous bow is a bulging bulb at the bow (or front) of a ship just below the waterline. The bulb modifies the way the water flows around the hull, reducing drag and thus increasing speed, range, fuel efficiency, and stability. Large ships with bulbous bows generally have a twelve to fifteen percent better fuel efficiency than similar vessels without them. A bulbous bow also increases the buoyancy of the forward part and hence reduces the pitching of the ship to a small degree.

The greatest amount of hydrodynamic benefits of bulbous bow will be at the high end of the semi-displacement speed range, reducing as the speed decreases. At higher speeds wave making resistance accounts for the greater portion of the drag, and the slower the vessel is proportionally more of the resistance is taken up by wetted surface drag. At low speed, say, 6 knots or lower the bulb will even cause an increase in drag because of the greater wetted surface area. At that low speed the added power consumption is negligible and generally little time is spent in this speed range.

In terms of hydrodynamics, the destructive interference of the primary and secondary wave trains causes an overall reduction in drag which is beneficial to the vessels resistance characteristics. The water flowing over the top of the bulb is exerting a downward pressure that is keeping the stern from squatting, causing the vessel to move with less resistance.

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