A bilge keel is a long, narrow fin fixed to the lower outer side of a ship or submarine hull. Its job is to reduce rolling, which is the side-to-side rotation caused by waves, turns, or uneven forces. Rolling can make a vessel uncomfortable, reduce crew performance, and make equipment harder to use.
Because a bilge keel has no moving parts, it is a simple and reliable passive stabilizer.
Key Facts
- Rolling motion is rotation about a vessel's long axis.
- A bilge keel increases hydrodynamic drag when the hull rolls through water.
- Damping force often scales with speed: Fd is proportional to v or Fd is proportional to v^2, depending on flow conditions.
- Rotational damping reduces angular speed: torque = I alpha and damping torque opposes the roll direction.
- A longer or wider bilge keel usually gives more roll damping, but also increases resistance during forward motion.
- Bilge keels are usually placed near the turn of the bilge, where the side of the hull curves into the bottom.
Vocabulary
- Bilge keel
- A fixed strip or fin along the lower outer hull that increases water resistance during rolling motion.
- Roll
- The side-to-side angular motion of a vessel about its lengthwise axis.
- Hydrodynamic drag
- A resistive force from water that acts opposite the motion of an object moving through it.
- Damping
- The reduction of oscillation amplitude by removing mechanical energy from the motion.
- Hull
- The main watertight body of a ship or submarine that displaces water and supports the vessel.
Common Mistakes to Avoid
- Thinking a bilge keel lifts the ship like an airplane wing. It mainly adds drag during roll, not upward lift for normal support.
- Drawing the bilge keel on the centerline bottom of the hull. It is usually mounted along the lower outer sides near the curve between the side and bottom.
- Assuming bigger bilge keels are always better. Larger fins can damp roll more, but they can also add drag, stress, and vulnerability to damage.
- Confusing roll with pitch. Roll is side-to-side rotation about the vessel's length, while pitch is bow-up and bow-down rotation about a sideways axis.
Practice Questions
- 1 A ship rolls with an angular speed of 0.20 rad/s. A bilge keel produces a damping torque modeled as tau = -120000 omega. What is the damping torque at that instant, including its sign?
- 2 A model hull without bilge keels has a roll amplitude of 12 degrees. After bilge keels are added, the amplitude is 7 degrees. What is the percent reduction in roll amplitude?
- 3 Explain why a bilge keel reduces rolling most strongly when the hull is moving sideways through the water during a roll, but has less effect when the ship is steady and upright.