Bilge Keel is a longitudinal structural member of ship’s hull.
It’s also called as passive anti-rolling (stability) system.
It increases the hydrodynamic resistance to rolling motion of ship.
It is used for resisting or dampening the rolling.
It increases the longitudinal strength.
It also protects grounding and bilges.
Note: It disturbs the transverse streamlines of ship’s hull and make eddy currents and increase the wave making resistance. It increase the water pressure over a large area of ship’s hull and this pressure acts in such a way that rolling motion is damped.
Bilge keels are always arranged to lie within the line of ship’s side and that of the bottom of floors. But if they were to project beyond these lines, it will be more liable to damage. Generally, it is constructed both side of ships.
Flat plates are used for construction for effective obstruction for rolling motion. It is attached to hull with ground plate (bar) only i.e. supported by a backing plate or ground plate. On this plate, bilge keel is welded and the thickness of ground plate is equal to keel plate or less than equal to 14 mm. It is extended aft side more and less in forward side. It should not be extended more than ship’s breadth and depth for safe operation of ship during docking. It is made tapered gradually at the ends for minimizing the drag force.
Bilge keels may be riveted, welded or riveted-welded, but welded construction is preferred.
Ground plate (bar) is welded on ship’s shell plating by continuous fillet welds.
Keel plate (bilge keel) is welded on the ground plate by light or intermittent welds.
Ends of bilge keels tend to produce stress concentration and that may lead to cracks in bilge keels. That’s why, to prevent cracks, ends of bilge keels are tapered-off gradually and end over a floor or tank side bracket.