Why a ship needs helper machines
Auxiliary machinery means every machine that is not the main engine or the propeller, but is needed to keep the ship working. The ship makes its own cooling water, fresh water, power and cleaning, because it may be at sea for weeks.
Pumps
A pump moves liquid and raises its pressure. Two main groups:
- Centrifugal pump: a spinning impeller flings liquid outward. It gives smooth, large flow at medium pressure and is used for sea water cooling, ballast, fire and bilge. It must be filled with liquid before starting (priming).
- Positive displacement pump (gear, screw, piston): traps a fixed volume in each turn and pushes it on. It gives high pressure and works with thick liquids, so it is used for fuel and lube oil. It can pump air out too, so it is self-priming.
Head is pressure written as a height of liquid: H = P / (ρg). About 10.2 m of water head equals 1 bar. Pump power: P = ρ g Q H / η, where Q is flow (m³/s) and η is the efficiency.
Hydraulic equipment
A hydraulic system uses liquid (oil) in closed pipes to transmit force. Pascal's law: a pressure applied to a liquid in a closed container is passed on equally everywhere. So pressure p = F1/A1 = F2/A2 and the force on the big piston is F2 = F1 x (A2/A1).
The force is multiplied, but the big piston moves less: d2 = d1 x (A1/A2). Energy is not created: work in = work out (ignoring losses). Oil is used because it is almost incompressible and also lubricates.
Ships use hydraulics for steering gear, deck cranes, winches and windlass, hatch covers and stabilisers. A pump supplies pressure, valves send the oil, and rams or motors do the work. Check for leaks and keep oil clean.
Fresh-water generators
A ship cannot carry all the drinking and boiler water it needs. A fresh-water generator (evaporator) makes it from sea water.
- Hot water from the engine cooling system (about 70 to 80 °C), which would otherwise be wasted, warms the sea water.
- The shell is kept at a strong vacuum, so water boils at only about 40 to 45 °C. This saves energy and gives less scale.
- Vapour rises to a condenser cooled by sea water and turns into pure water, collected in a tray. The salt stays behind and the strong brine is pumped overboard.
- A salinometer checks the salt in the product. If too salty, the water is dumped back.
Treated further (minerals and chlorine), it becomes drinking water.
Pollution-control equipment
MARPOL needs ships to clean waste before release.
- Oily-water separator (OWS): oil is lighter than water, so in a still tank it floats up and is removed (gravity separation, often with filters). The water goes overboard only if oil is 15 ppm or less, checked by an oil content meter that stops the discharge if the limit is passed.
- Sewage treatment plant: bacteria break down waste and the water is disinfected.
- Incinerator: burns oily sludge and some garbage.
- Ballast water treatment: filters and UV light kill sea creatures before discharge.
- Exhaust cleaning: scrubbers remove sulphur and catalysts reduce nitrogen oxides.
Records of all discharges are kept in an Oil Record Book.
Try it: four small experiments
In the 3D, use each slider and predict the change before looking. At home: (1) pour oil on water in a glass and watch it rise, (2) join two syringes of different sizes with a thin tube filled with water and push the small one, (3) put a cold steel plate over a steaming pan and catch the drops.
Key formulas and definitions
- Head H = P / (ρ g)
- Pump power P = ρ g Q H / η
- Pascal: F1/A1 = F2/A2, so F2 = F1 x A2/A1
- Distance: d1 x A1 = d2 x A2
- Oil in discharged water ≤ 15 ppm (MARPOL)
- Water ρ = 1000 kg/m³, g = 9.81 m/s²
Worked examples
1. A pump delivers water at 3 bar gauge pressure. What head is this?
P = 3 bar = 300,000 Pa. H = P/(ρg) = 300000 / (1000 x 9.81) = 30.6 m.
2. A pump moves 0.05 m³/s of water to a head of 20 m with efficiency 0.8. Find the power.
P = ρ g Q H / η = 1000 x 9.81 x 0.05 x 20 / 0.8 = 12,262 W, about 12.3 kW.
3. A hydraulic jack has small piston area 2 cm² and big piston area 40 cm². A force of 50 N pushes the small piston. Find the force on the big piston.
F2 = F1 x A2/A1 = 50 x 40/2 = 1000 N.
4. In the jack above, the small piston moves 20 cm. How far does the big piston move?
d2 = d1 x A1/A2 = 20 x 2/40 = 1 cm.
5. A fresh-water generator makes 24 t a day. A crew of 30 uses 150 litres each a day. How many tonnes are left for other uses?
Crew use = 30 x 150 = 4500 L = 4.5 t. Left = 24 - 4.5 = 19.5 t.
Common mistakes
- Thinking a hydraulic jack gives free energy. Force is multiplied but the distance is divided, so work stays the same.
- Using a centrifugal pump for very thick oil. A positive displacement pump is better.
- Forgetting to prime a centrifugal pump. It cannot pull a liquid if it is full of air.
- Thinking vacuum makes water boil hotter. Lower pressure lowers the boiling point.