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A bulbous bow is a ship design with a bulb mounted on the front of the ship below the waterline to reduce drag. This can be built into the initial designs of a ship and implemented at the shipyard, or added on in a retrofit. The design can cut down on fuel costs and make a vessel run more smoothly. It adds to the design and construction costs of the ship, so a careful cost-benefit analysis is advised to determine if it is appropriate for a given vessel.
Though the creation of this design is often credited to the United States Navy, it was also developed independently in the Imperial Japanese Navy and used in other countries. The US naval architect who is associated with this design was looking for a way to decrease operating costs on large vessels like aircraft carriers. These ships operate at a consistent speed throughout most of their deployment, until they come to an anchorage. They also require tremendous amounts of fuel to operate, thanks to their size and weight. Naval architects developed the bulbous bow, and noted considerable efficiency improvements, between five and 15%, which is significant on a vessel of that size.
The bulbous bow works by disrupting the flow of water to limit drag on the ship. This reduces the amount of fuel needed to propel the ship, particularly when its speed is constant. The best results are observed with larger ships that tend to create more drag. Smaller vessels, like recreational sailboats, are less suited to the bulbous bow design. While it might increase efficiency, the savings would be small compared to the costs of implementation.
Bulbous bows tend to cut down on the size of the wave created by the bow of the ship as it moves through the water. In addition to reducing drag, this can help the ship move more smoothly, with less rocking and side to side motion while underway. Cutting down on wake size also makes navigation safer for other craft in the area. A large wake can be very disruptive to smaller boats, an important consideration in naval architecture, as captains want to avoid upsetting surrounding shipping.
Naval architects place the bulb at the base of the prow, the very front of the ship, and mount it below the waterline. While the ship is underway, it is not visible. If repairs or maintenance are necessary, divers can service the bulbous bow. Otherwise, the shipowner will need to wait until the ship is in dry dock and the bulb is visible to enact any necessary repairs.