Marine Propulsion Machinery and Power Plants
Core concepts
A Class 4 coastal candidate must be fluent in the two-stroke slow-speed crosshead diesel and the four-stroke medium-speed trunk engine, including scavenging (uniflow, loop), turbocharging and charge air cooling, fuel injection timing, and the indicator diagram. Know how a propeller law load curve (power varies with speed cubed) governs sea margin and overload. Understand reduction gearing, controllable-pitch propellers, shafting, stern tube bearings and thrust blocks.
Traps in questions
- Confusing scavenge air pressure with compression pressure; scavenge is only slightly above atmospheric.
- Mixing up brake mean effective pressure (BMEP) and indicated mean effective pressure (IMEP); mechanical efficiency links them.
- Assuming fuel rack position equals load; exhaust temperature and turbocharger speed matter.
- Forgetting that a CPP allows constant-speed running, unlike a fixed-pitch propeller.
Rule of thumb and worked example
Propeller law: P = k·N³. If a vessel needs 1,200 kW at 120 rpm, at 90 rpm power is 1,200 × (90/120)³ = 1,200 × 0.4219 = 506 kW. This predicts the light-running load and explains why small rpm changes give large power changes. For fuel, specific fuel consumption of about 175 g/kWh for a large two-stroke gives daily consumption = 506 kW × 175 g/kWh × 24 h = 2,125 kg, roughly 2.1 tonnes per day. Always check units and whether the question asks indicated or brake power.
Study focus
Revise starting air systems, governor action, crankcase explosions and their prevention, and cooling water treatment. Link machinery behaviour to watchkeeping observations such as exhaust colour, scavenge drain, and turbocharger vibration.