A ship floats because its hull pushes water aside and the water pushes back with an upward buoyant force. Ship displacement is the weight of the water that the hull displaces, or pushes out of the way. For a floating ship at rest, this displaced water weighs exactly the same as the ship.
This idea is essential for designing ships that can carry cargo, fuel, passengers, and equipment safely.
As more weight is added to a ship, the hull sinks lower and displaces more water until the upward buoyant force again equals the total weight. Naval architects use displacement to describe a ship's size and loading condition, often in long tons, metric tonnes, or short tons depending on the system used. Submarines use the same principle, but they can change their buoyancy by filling or emptying ballast tanks.
Measuring displacement helps predict draft, stability, carrying capacity, and safe operating limits.
Key Facts
- Displacement is the weight of water pushed aside by a ship or submarine hull.
- For a floating ship in equilibrium, buoyant force = ship weight.
- Archimedes' principle: F_b = ρ_fluid g V_displaced.
- If a ship gains cargo, V_displaced increases and the ship sits lower in the water.
- Displacement tonnage measures weight, while gross tonnage measures internal volume.
- 1 metric tonne = 1000 kg, 1 long ton = 2240 lb, and 1 short ton = 2000 lb.
Vocabulary
- Displacement
- Displacement is the weight of fluid pushed aside by an object placed in that fluid.
- Buoyant force
- Buoyant force is the upward force a fluid exerts on an object immersed in it.
- Hull
- A hull is the main body of a ship that contacts the water and provides floating volume.
- Draft
- Draft is the vertical distance from the waterline to the lowest part of a ship's hull.
- Tonnage
- Tonnage is a shipping measurement that can describe either weight displacement or enclosed volume, depending on the type used.
Common Mistakes to Avoid
- Confusing displacement with the volume of the ship. Displacement is the weight of water pushed aside, although it is calculated using the displaced volume.
- Thinking a floating ship displaces water equal to only its cargo weight. A floating ship displaces water equal to the total weight of the ship, cargo, fuel, crew, and equipment.
- Using gross tonnage as if it were displacement. Gross tonnage is based on internal volume, not the actual weight of the ship.
- Forgetting that saltwater and freshwater have different densities. The same ship floats slightly higher in denser saltwater because less volume must be displaced.
Practice Questions
- 1 A ship has a total mass of 12,000 metric tonnes and floats at rest. What is the mass of the seawater it displaces?
- 2 A barge displaces 500 m³ of freshwater with density 1000 kg/m³. Using g = 9.8 m/s², what buoyant force acts on the barge?
- 3 A submarine takes water into its ballast tanks and begins to sink lower. Explain how this changes its total weight, displaced water, and buoyancy condition.