What is ship design?
Ship design is the work of deciding how big a ship should be, what shape it should have, and how strong it should be, before a single steel plate is cut. The people who do this work are called naval architects.
A designer starts with a job: "carry 50,000 tonnes of grain" or "carry 300 passengers". Then the designer chooses the length, width, depth and shape of the hull. The designer checks four things: will it float, will it move easily, can an engine push it, and is it strong enough? If one answer is no, the design is changed and checked again.
Words to know: the hull is the body of the ship. The draft is how deep the ship sits below the water. The waterline is the line where water meets the hull.
Ship calculations: floating, displacement and draft
A ship floats because the water pushes it up. This push is called buoyancy. A floating ship sinks until the buoyancy equals its weight. At that moment the ship has pushed aside water whose weight is the same as the ship's weight. This weight is called the displacement.
Weight of ship = weight of water pushed aside = density of water × volume under water × g.
Add cargo, and the weight goes up. The ship sinks until more water is pushed aside. So the draft increases. Sea water is a little denser (about 1025 kg/m³) than river water (about 1000 kg/m³). So the same ship sits a bit higher in the sea than in a river.
Stability means the ship comes back upright after a tilt. A ship with heavy things placed low is more stable. Heavy things placed high can make a ship tip over. Designers calculate where the centre of weight is, and keep it low enough.
Ship size is also measured as tonnage. Deadweight is how much cargo, fuel and stores the ship can carry. Gross tonnage is a number for the inside volume of the ship, used for rules and fees.
Ship resistance and propulsion
When a ship moves, the water pushes back on it. This push is the resistance. It has two main parts. Friction: water rubs along the hull. Wave making: the ship pushes up waves, and making waves takes energy. At higher speed, resistance grows much faster than speed. As a rough idea, doubling the speed makes resistance about four times.
A smooth, long, slim hull gives less resistance. A bulb (a round bulge) at the bow of big ships reduces the waves they make. Clean paint on the hull reduces friction.
Propulsion is how the ship is pushed forward. Usually an engine turns a propeller. The propeller throws water backward, and the water pushes the ship forward. This forward push is the thrust. At steady speed thrust = resistance. If thrust is bigger, the ship speeds up. If it is smaller, the ship slows down.
Power needed = thrust × speed. This is why a fast ship needs a very big engine: both thrust and speed are large.
Structural mechanics of ships: strength
A ship hull is like a very long, hollow beam floating on waves. If a wave crest is under the middle of the ship, the middle is pushed up while the bow and stern hang down. The hull bends like a hill. This is called hogging. If wave crests lift the two ends and the middle hangs over a trough, the hull bends like a bowl. This is called sagging.
The weight of the ship is not spread the same everywhere, and the water push is not spread the same either. Where they do not match, the hull is bent and sheared. Designers add strong frames (ribs) and thick plates at the places that bend most, mostly the middle of the ship and the keel at the bottom.
A design is checked against the worst waves the ship may meet. The steel must not bend for good and must not crack.
Try it
At home: float a small plastic box in a bucket of water. Add coins one by one. Watch the box sink lower. Mark the water level on the box each time. You are measuring the draft. Now add a little salt to the water and stir. Does the box rise a little? Sea water is denser, so it does.
In the 3D: before you move the sliders, guess what will happen to the draft and the red resistance arrow when you add cargo and when you raise speed. Then check.
Key formulas and definitions
- Floating: weight of ship = weight of water displaced = ρ × V × g
- Displaced volume V = weight of ship ÷ (ρ × g); ρ(sea) ≈ 1025 kg/m³, ρ(fresh) ≈ 1000 kg/m³
- Box-shaped hull: V = length × breadth × draft
- Resistance ∝ speed² (rough idea)
- Steady speed: thrust = resistance
- Power = thrust × speed
Worked examples
1. A box-shaped barge is 20 m long and 5 m wide. It floats in river water with a draft of 2 m. Find its mass.
Volume under water = 20 × 5 × 2 = 200 m³. Mass of water pushed aside = 1000 × 200 = 200,000 kg. A floating barge has the same mass, so the barge weighs 200 tonnes.
2. A ship of mass 12,000 tonnes floats in sea water (1025 kg/m³). How much water does it displace (volume)?
Mass = 12,000,000 kg. Volume = mass ÷ density = 12,000,000 ÷ 1025 ≈ 11,700 m³.
3. The barge of Example 1 loads 40 tonnes of sand. By how much does its draft increase?
Extra mass 40,000 kg needs extra volume = 40,000 ÷ 1000 = 40 m³. Floor area = 20 × 5 = 100 m². Extra draft = 40 ÷ 100 = 0.4 m.
4. A ship sails from the sea into a river. Does it sit higher or lower? Why?
Lower. River water is less dense, so the ship must push aside more volume of water to get the same weight. It sinks a little deeper.
5. The resistance on a boat is 50 kN at 5 m/s. Estimate the resistance at 10 m/s.
Resistance is roughly proportional to speed². Speed doubles, so resistance becomes 4 times: 4 × 50 = 200 kN.
6. At 10 m/s the ship of Example 5 moves at steady speed. What thrust and power does the propeller need?
Steady speed means thrust = resistance = 200 kN. Power = thrust × speed = 200,000 × 10 = 2,000,000 W = 2 MW.
Common mistakes
- Thinking a ship floats because it is light. A steel ship is heavy; it floats because its hollow shape pushes aside a lot of water.
- Saying "heavier cargo makes the ship go up". More cargo makes the draft larger (the ship sinks lower).
- Thinking resistance doubles when speed doubles. It grows much faster, about four times.
- Mixing up thrust and resistance. Thrust pushes the ship forward; resistance holds it back. At steady speed they are equal, not zero.