A drydock is a controlled work area that lets engineers take a ship or submarine out of the water without dragging it onto land. This matters because the hull, propellers, rudders, sonar domes, sea chests, and underwater fittings need regular inspection and repair. Biofouling, corrosion, cracks, and damaged coatings can reduce efficiency and safety if they are not treated.
Drydocks make heavy marine maintenance possible while keeping the vessel supported in a stable position.
A graving drydock is a basin with gates that can be flooded to float a vessel in, then drained so the ship settles onto carefully placed blocks. A floating drydock works like a giant buoyant platform with tanks that fill with water to sink and pump out water to rise. Both designs depend on buoyancy, ballast control, structural support, and careful alignment of the ship over keel blocks and side supports.
The goal is to transfer the vessel’s weight safely from water support to solid supports while giving workers dry access to the hull.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = rho_water g V_displaced.
- A vessel floats when its weight equals the buoyant force: W = F_b.
- A floating drydock sinks by filling ballast tanks with water and rises by pumping ballast water out.
- A graving drydock is flooded to admit the ship, sealed with a gate, then pumped dry.
- Keel blocks carry much of the ship’s weight and must line up with strong parts of the hull structure.
- Pressure in water increases with depth: P = rho g h.
Vocabulary
- Drydock
- A drydock is a structure used to lift or isolate a vessel from the water so its underwater parts can be serviced.
- Graving drydock
- A graving drydock is a shore-based basin that can be flooded, sealed, and pumped dry around a ship.
- Floating drydock
- A floating drydock is a buoyant dock with ballast tanks that can sink under a vessel and then rise to lift it.
- Ballast tank
- A ballast tank is a compartment that is filled or emptied with water to control buoyancy, draft, and stability.
- Keel block
- A keel block is a strong support placed under the ship’s keel to hold the vessel after the water is removed.
Common Mistakes to Avoid
- Thinking a drydock uses a crane to lift the whole ship, which is wrong because most drydocks use buoyancy and controlled pumping rather than direct lifting.
- Forgetting that the ship must be aligned before pumping begins, which is wrong because poor alignment can overload keel blocks or damage the hull.
- Assuming a floating drydock rises because it becomes lighter than air, which is wrong because it rises when pumping water out increases its net buoyancy in water.
- Treating water pressure as the same at all depths, which is wrong because pressure increases with depth according to P = rho g h.
Practice Questions
- 1 A floating drydock displaces 60,000 m^3 of seawater when fully supporting a ship. If seawater density is 1025 kg/m^3 and g = 9.8 m/s^2, what buoyant force acts on the dock and ship together?
- 2 A graving drydock contains 180,000 m^3 of water after the gate is closed. If pumps remove water at 3,000 m^3 per minute, how long does it take to pump the dock dry, ignoring seepage and pump slowdown?
- 3 Explain why a ship must be positioned over keel blocks before the water is pumped out of a drydock, and describe what could happen if the supports are placed incorrectly.