A windlass and a capstan are deck machines that help ships and submarines handle loads far too heavy for sailors to pull by hand. The windlass usually handles anchor chain on the foredeck, while the capstan usually hauls ropes or mooring lines around a vertical drum. These machines matter because an anchor, chain, and mooring line can carry thousands of newtons of tension.
Safe operation depends on understanding force, torque, friction, and mechanical advantage.
Key Facts
- Torque is turning effect: τ = rF, where r is drum radius and F is tangential force.
- Power relates force and speed: P = Fv, where v is line or chain speed.
- Work done while hauling is W = Fd, where d is the distance the line or chain moves.
- Mechanical advantage reduces input force but usually increases the distance or time needed.
- Capstan friction increases with wrap angle: T2 = T1e^(μθ), where θ is in radians.
- Anchor chain tension can include anchor weight, chain weight, drag forces, and the ship's motion.
Vocabulary
- Windlass
- A deck machine with a horizontal shaft used mainly to raise, lower, and secure an anchor chain.
- Capstan
- A vertical rotating drum used to haul mooring lines, towing lines, or other heavy ropes.
- Hawse pipe
- A reinforced opening that guides the anchor chain from the deck through the hull toward the anchor.
- Chain locker
- A storage compartment below deck where anchor chain piles up when it is hauled aboard.
- Bollard
- A strong deck fitting used to secure mooring lines after they have been positioned and tensioned.
Common Mistakes to Avoid
- Treating a windlass and a capstan as the same machine is wrong because they are shaped and used differently, with the windlass mainly handling chain and the capstan mainly handling rope.
- Ignoring drum radius is wrong because torque depends on radius, so a larger drum needs more torque for the same line tension.
- Assuming the motor force equals the chain tension is wrong because gears, brakes, friction losses, and mechanical advantage change the force delivered to the chain or line.
- Standing in the bight of a line is wrong because a tensioned line can snap back violently if it breaks or slips from the capstan.
Practice Questions
- 1 A windlass drum has a radius of 0.25 m and pulls an anchor chain with a tension of 12,000 N. What torque must the drum provide, ignoring losses?
- 2 A capstan hauls a mooring line at 0.40 m/s while maintaining a line tension of 5,000 N. What power is delivered to the line?
- 3 A ship is docking in a strong current. Explain why sailors might use a capstan to tension a mooring line but then secure the line on a bollard instead of leaving all the load on the capstan.