Bilge keel
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A bilge keel is a strake, or small keel or blister, running along much of the length of the hull. It is used to reduce the hull's tendency to roll. They are typically fitted one on each side, low down on the side of the hull, so as not to increase the draft of the vessel. In battleships they were often quite large and used as part of the torpedo protection system.
A bilge keel is often in a "V" shape, welded along the length of the ship at the turn of the bilge. Bilge keels are employed in pairs (one for each side of the ship). A ship may have more than one bilge keel per side, but this is rare. Bilge keels increases hydrodynamic resistance to rolling, making the ship roll less. Bilge keels are passive stability systems.
Although not as effective as stabilizing fins, bilge keels have a major advantage in their low impact on internal ship arrangements. Unlike fins, bilge keels do not have any components inside the hull that would adversely affect cargo or mission spaces. As a disadvantage, bilge keels increase the hydrodynamic resistance of the vessel, thus hindering forward motion. (This is also true for stabilizing fins).
When designing a bilge keel, there are important decisions to consider. To minimize hydrodynamic drag, the bilge keel should be placed in way of a flowline where it does not oppose crossflow. The ends of the bilge keel should be tapered and properly faired into the hull. Also, a bilge keel should not protrude from the hull so far that the device could be damaged when the vessel is alongside a pier, even with a few degrees of adverse heel. Bilge keels should not protrude below the baseline either, where they could be damaged or fouled by grounding.
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