Bilge Keel
A Bilge Keel is a long, narrow fin welded along the bilge — the curved turn between the bottom and the side of the hull — running for much of the length of the ship on each side. Its job is to damp the rolling of the ship by resisting the water movement as the hull rolls.
Bilge keels are a simple, passive and effective way to cut roll, improving comfort and cargo safety, at the cost of a small increase in resistance. Because they project from the hull, they are vulnerable in groundings and berthing contacts and are inspected at docking surveys.
Rolling makes a ship uncomfortable, can shift or damage cargo, and in the worst case threatens stability. The bilge keel is the cheapest, most reliable roll-reduction device on most ships — no moving parts, always working — which is why almost every merchant ship has them.
How does a bilge keel reduce rolling?
As the hull rolls, the projecting fins drag against the water and create damping forces that oppose the roll, reducing its size — a passive effect that needs no power or control.