Ships float because they displace water, but the shape and number of hulls strongly affect how they move and feel at sea. A monohull has one main body in the water, while a multihull uses two hulls in a catamaran or three hulls in a trimaran. These designs create different tradeoffs in stability, speed, deck space, drag, and cost.
Understanding these differences helps explain why cargo ships, sailboats, ferries, racing vessels, and submarines are shaped so differently.
A monohull often has deep displacement and may rely on ballast or a heavy keel to resist rolling, while a multihull gains stability from its wide spacing between hulls. Catamarans and trimarans can have lower wave-making drag at some speeds because each hull is long and narrow, but they may have more wetted surface area. Monohulls usually handle heavy loads well and can be easier to build, dock, and protect in rough conditions.
Submarines are usually monohull pressure vessels because a strong single cylinder is efficient for resisting deep ocean pressure.
Key Facts
- Buoyant force equals the weight of displaced water: F_b = rho_water g V_displaced.
- A vessel floats when buoyant force equals its weight: F_b = W.
- Initial stability increases when the center of buoyancy shifts sideways to create a restoring torque after the vessel tilts.
- Restoring torque can be estimated by tau = W d, where d is the horizontal distance between weight and buoyant force lines.
- Long, narrow hulls usually reduce wave-making drag at moderate to high speeds.
- A catamaran has two hulls, a trimaran has three hulls, and a monohull has one hull.
Vocabulary
- Monohull
- A vessel design with one main hull that provides buoyancy and holds the main structure.
- Multihull
- A vessel design with two or more hulls connected by a deck or frame.
- Catamaran
- A multihull vessel with two parallel hulls that are spaced apart for stability.
- Trimaran
- A multihull vessel with one central hull and two smaller side hulls called outriggers.
- Drag
- The resistive force from water and waves that opposes a vessel's motion.
Common Mistakes to Avoid
- Assuming wider always means safer is wrong because very wide multihulls can have high initial stability but may be harder to recover if capsized.
- Thinking a catamaran has less drag in every situation is wrong because two hulls can increase wetted surface area, especially at low speeds.
- Ignoring load distribution is wrong because uneven weight on a multihull can change trim, increase drag, and reduce stability.
- Treating submarines like surface ships is wrong because a submarine must resist water pressure at depth, so pressure-hull strength is often more important than deck space or wave drag.
Practice Questions
- 1 A small boat displaces 2.5 m^3 of seawater with density 1025 kg/m^3. Using g = 9.8 m/s^2, calculate the buoyant force on the boat.
- 2 A vessel has weight 80,000 N. When it heels, the horizontal distance between the line of action of its weight and buoyant force is 0.45 m. Calculate the restoring torque.
- 3 A ferry company must choose between a monohull and a catamaran for short coastal trips carrying passengers. Explain which design might offer better deck space and comfort, and name one tradeoff the company should consider.