Bilge keel

A bilge keel is a long, shallow fin welded along the turn of bilge, on each side, set normal to the local shell so it bites into the cross-flow as the ship rolls. It is the cheapest and most common passive roll-damping device, adding roll damping (mostly through vortex shedding and appendage drag) that can cut roll amplitude near resonance by a third or more, at the cost of a small rise in calm-water resistance. It is sized as a fraction of LBP and aligned with the streamlines from a flow study to limit that drag. Do not confuse it with the bulb keel of a sailing yacht: the bilge keel damps roll and carries no ballast.