The bilge keel is a long fin attached externally along the turn of the bilge, running roughly in a fore-and-aft direction. Attachment method:

• The bilge keel is fabricated as a light, flat bar or fabricated plate of length running over the middle portion of the hull. It is attached by one of two methods:

(i) riveted/bolted connection through doubler plates, in which the keel web is connected by a continuous fillet weld to a flat-bar or doubler section on the shell, or

(ii) welded directly to the shell by continuous or intermittent fillet welds.

• Because the attachment is a potential source of high local stress concentration and fatigue cracking (the shell at the bilge is highly loaded in bending and docking), the weld is usually a full-strength continuous fillet or butt weld, and the keel is not welded to the shell with single small tack welds that crack under repeated relative movement (flexing).
• The ends of the bilge keel are tapered and rolled to a fine radius and blended smoothly (scalloped/angled end) so as to reduce stress concentration at their termination; heavy strike plates (flat bar doubler) at the ends carry the connection.
• The keel is set slightly at a small angle to the base plane so that its face lines up with the flow; it is normally fitted parallel to the load waterline and positioned at the turn of the bilge where it is not unduly loaded in docking (kept clear of the keel blocks).
• Doubler plates and drainage are arranged, and the attachment is periodically inspected (especially the weld toes) because the bilge keel experiences severe fluctuating loads in a seaway.

1. To avoid severe local stress concentrations and fatigue cracking at their ends/attachment; a full-length keel would place the highly-loaded ends in regions of the hull girder with large bending stresses, promoting cracking. Pounding/the connection would also be in heavily loaded way of the end bottoms.
2. They would cause high drag/frictional resistance in the deep/fore and aft ends where the bulb and stem flow is disturbed; the ends are kept clear to allow the propeller/shaft area (aft) and bow (forward) flow and to reduce appendage resistance.
3. The ends would foul the raised stake/Dfrenchman? no: the ends would interfere with docking (the keel blocks and cradle), with the propeller and with the sea-chests/ballast pipes, and would not be effective because at very great fineness near the ends the damping of roll is small. They also would add weight and cause the keel to hit the dock blocks or the ground.

1. Fluid drag / hydrodynamic damping: as the ship rolls, the bilge keel body moves transverse through the water, generating a resistance (both pressure and friction) opposing the roll velocity. Because the damping force is proportional to the velocity of the keel (highest at the bilge, which is far from the roll axis), the keel produces large moments that resist and dissipate the rolling energy, arresting and reducing the amplitude of roll. This is the principal roll-damping mechanism.
2. Creation of eddies/vortices and flow separation: the sharp edges of the bilge keel shed eddies and cause wake and separated flow at the bilge, increasing the hydrodynamic damping and inducing a phase lag in the roll so that the passive system resists resonance and limits maximum roll angle. The energy of rolling is converted to eddying motion and hence heat, damping amplitude.

These mechanisms make the bilge keel an economical, passive roll damper that reduces roll amplitude, improves comfort and cargo security and reduces sloshing and parametric roll, without moving parts or power.