Ships and submarines sit in water according to buoyancy, weight, and hull shape. Draft is the vertical distance from the waterline down to the lowest part of the hull, while freeboard is the vertical distance from the waterline up to the main deck or top of the hull side. These measurements matter because they affect safety, stability, loading limits, and whether a vessel can travel through shallow water.
A clear waterline diagram helps show which part of the vessel is supported by the water and which part remains above it.
When a ship is loaded with cargo, fuel, or passengers, its weight increases and it sinks lower until it displaces enough water to balance that weight. This increases draft and reduces freeboard, which can make waves more likely to wash over the deck. Submarines use ballast tanks to change their average density, allowing them to float at the surface, hover underwater, or dive.
For both ships and submarines, the key idea is that the vessel’s position in the water is controlled by the balance between weight and buoyant force.
Key Facts
- Draft = vertical distance from the waterline to the lowest point of the hull.
- Freeboard = vertical distance from the waterline to the main deck or upper hull edge.
- Buoyant force equals the weight of displaced water: F_b = ρ_water g V_displaced.
- A floating vessel is in vertical equilibrium when F_b = W.
- Adding cargo increases weight, increases displaced volume, increases draft, and decreases freeboard.
- A submarine dives by taking water into ballast tanks, increasing its average density.
Vocabulary
- Draft
- Draft is the depth of a vessel below the waterline, measured down to the lowest part of the hull.
- Freeboard
- Freeboard is the height of a vessel above the waterline, usually measured up to the deck edge or hull side.
- Waterline
- The waterline is the level where the surface of the water meets the hull of a floating vessel.
- Buoyancy
- Buoyancy is the upward force exerted by a fluid on an object placed in it.
- Ballast tank
- A ballast tank is a compartment that can be filled with water or air to change a submarine’s buoyancy.
Common Mistakes to Avoid
- Confusing draft with freeboard is wrong because draft is measured downward below the waterline, while freeboard is measured upward above the waterline.
- Assuming a heavier ship always sinks completely is wrong because a floating ship sinks only until it displaces enough water to equal its weight.
- Ignoring shallow-water clearance is wrong because a ship with too much draft can hit the seabed even if much of the hull is still above water.
- Thinking submarines dive by pointing downward only is wrong because diving mainly depends on changing buoyancy with ballast tanks, not just changing direction.
Practice Questions
- 1 A cargo ship has a hull depth of 12 m from keel to deck. If its draft is 7.5 m, what is its freeboard?
- 2 A ship floats with a draft of 4.0 m when lightly loaded. After cargo is added, its draft becomes 5.6 m. By how many meters did the draft increase, and what happened to the freeboard if the hull height stayed the same?
- 3 A submarine at the surface fills its ballast tanks with seawater. Explain how this changes its average density, buoyant force balance, and motion in the water.