How a ship moves
A ship moves by pushing something backward. The propeller pushes water backward and the water pushes the ship forward. This is Newton's third law: action and reaction. The pushing force is called thrust. The path of power is: fuel burns in the engine, the engine turns a shaft, the shaft turns the propeller, the propeller makes thrust.
Shafting
The shafting (shaft line) joins the engine to the propeller. It has long steel pipes joined end to end: the intermediate shaft and the propeller shaft (tail shaft). It also has:
- a thrust bearing, which passes the pushing force from the shaft to the hull, so the ship moves and the engine is not pushed out of place;
- support bearings that keep the long shaft straight;
- a stern tube with a seal where the shaft leaves the hull, so sea water stays out.
The shaft turns at the same rpm as the propeller on a slow-speed diesel. With a faster engine, a gearbox reduces the speed first, because big propellers work best when turning slowly.
Propellers
A propeller has 3 to 6 twisted blades on a hub. Each blade works like a screw biting into water.
- Diameter: bigger and slower is usually more efficient.
- Pitch (P): how far the propeller would move forward in one turn, if it were a screw in solid material.
- Theoretical speed = P x rpm. Water is soft, so the real speed is smaller. The difference is the slip: slip = (P x n - Vs) / (P x n). Slip is usually 5 to 25%.
- Fixed-pitch propeller (FPP): blades are fixed. To go astern the engine must reverse. Controllable-pitch propeller (CPP): blades turn on the hub, so the pitch changes and the ship can go astern with the shaft turning one way.
- Cavitation: at high speed, low pressure on the blade makes bubbles that collapse and damage the metal, and cause noise.
Steering gear
The rudder is a plate behind the propeller. The steering gear turns it. When the rudder turns, water hits it at an angle and pushes the stern the other way, so the bow turns. The rudder only works when water flows past it, so a ship at rest cannot steer. The rudder force grows roughly with speed squared.
A modern steering gear is hydraulic: oil pumps push rams that turn the rudder stock. It is controlled from the bridge by the helm or autopilot. Most rudders turn up to 35 degrees each side. Safety rules ask for two independent power units and an emergency way to steer, and the rudder must move from 35 degrees one side to 30 degrees the other side within 28 seconds at full speed.
Various propulsion systems
- Single screw: one shaft, one propeller. Simple and efficient. Most large cargo ships.
- Twin screw: two shafts. Gives more power, a safety backup and better turning. Ferries and cruise ships.
- Controllable-pitch propeller: change thrust and direction by turning blades.
- Azimuth thruster / pod: the whole propeller unit turns round, so there is no rudder and the ship can push in any direction. Used on tugs and cruise ships.
- Water jet: a pump throws a jet of water out the back and a nozzle steers it. For fast ferries and shallow water.
- Bow thruster: a small side propeller in the bow for docking.
- Diesel-electric: engines make electricity and electric motors turn the propellers. Easy to place, quiet.
- Sails: rotor sails and wing sails now add some push and save fuel.
Speed and economy
Ship speed is measured in knots (nautical miles per hour; 1 knot = 1.852 km/h). Water resistance grows fast with speed, so the power needed is roughly proportional to speed cubed: P ∝ v³. Fuel burnt per hour follows power. Fuel per mile follows v² because the ship covers more miles per hour at higher speed.
This is why shipping companies use slow steaming: sailing 20% slower uses about half the power. The trip takes 25% longer, but fuel for the whole voyage falls by about a third and pollution falls too. Clean hull, correct trim and a good propeller also save fuel.
Try it: feel the cube
In the 3D, set the engine speed to 25%, then 50%. Read the speed and power percent in the box. Predict first: what will power be at 100%? Then check it. At home, ride a bicycle slowly and then twice as fast. Notice how much harder the second one is. That is the same speed-cubed rule for air.
Key formulas and definitions
- Theoretical speed = pitch x rpm
- Slip % = (P x n - Vs) / (P x n) x 100
- Power ∝ speed³ (P2/P1 = (v2/v1)³)
- Fuel per mile ∝ speed²
- 1 knot = 1 nautical mile per hour = 1.852 km/h
- Pitch speed (knots) = P(m) x rpm x 60 / 1852
Worked examples
1. A propeller has pitch 5 m and turns at 100 rpm. Find its theoretical speed in knots.
5 x 100 = 500 m per minute. In one hour: 500 x 60 = 30000 m. 30000 / 1852 = 16.2 knots.
2. The theoretical speed is 16 knots, but the ship does 14 knots. Find the slip.
Slip = (16 - 14)/16 = 0.125 = 12.5%.
3. A ship slows from 20 knots to 10 knots. How does the power change?
Speed halves, so power = (1/2)³ = 1/8. Power becomes one eighth.
4. A ship burns 80 tonnes a day at 16 knots. How much at 12 knots? (use P ∝ v³)
Ratio = (12/16)³ = 0.75³ = 0.4219. Fuel = 80 x 0.4219 = 33.75 tonnes a day.
5. A ship must sail 1920 nm. At 16 knots it burns 80 t per day. At 12 knots 33.75 t per day. Compare the total fuel.
At 16 kn: 1920/16 = 120 h = 5 days, 5 x 80 = 400 t. At 12 kn: 1920/12 = 160 h = 6.67 days, 6.67 x 33.75 = 225 t. Slower saves 175 t (44%), but takes 1.67 days longer.
Common mistakes
- Thinking the propeller pulls the ship through the water by "screwing". It pushes water backward and the water pushes the ship forward.
- Thinking power is proportional to speed. It is closer to speed cubed.
- Believing a rudder works at rest. It needs water flowing past it.
- Mixing up pitch and diameter. Pitch is how far it would advance in one turn; diameter is the width of the circle the blades make.