A bilge keel is an appendage the runs along a ships side, serving to dampen the rolling motion incurred by waves in a seaway. The bilge keel is welded on at the turn of the bilge and faired or tapered down to take the curvature of the hull at the ends. Typical bilge keel constructions are of plate or bulb-flat.
Bilge keels are static devices, whose properties of roll dampening are primarily caused by vortex shedding. Bilge keels effect on roll are generally more effective at speed, as a result. Bilge keels can also have a positive effect on vessel tracking by providing increased lateral area. Conversely, they are sources of increased wetted area and drag, both of which increase resistance.
Bilge keels should be situated so they will not strike the wharf or another vessel when tying alongside. The bilge keels should also not extend below the baseline of the vessel so as not to be damaged if the vessel runs aground. The only exception to this is seen on vessels that are designed to be loaded/unloaded while aground, in this case the bilge keels are backed with more structure to help support the vessel (a feature on some sailboats, were the vessels prominent bilge keels will self-supported the boat when beached). The bilge keel itself should be aligned with the vessels flow lines, to minimize drag.
Bilge keels, particularly on steel vessels, are "lightly welded" along a portion of the vessels length. This allows the bilge keel to be deformed or detached in case of impact without risking the vessels hull. Typically, short sections will be welded, with gaps between. The bilge keel will be attached to a backing strip - a strip of metal which prevents the bilge keel from propogating cracks into the hull when damaged.