Marine Engineering and Ship Propulsion Systems
What the exam tests
Marine Engineering and Ship Propulsion Systems covers diesel engine cycles, turbocharging, fuel and lubricating systems, cooling, starting air, shafting, propellers, and the relationship between engine output and ship speed. Expect definitions plus power, fuel and propeller-law calculations.
Core rules
- Most large ships use two-stroke, slow-speed crosshead diesel engines directly coupled to a fixed-pitch propeller; medium-speed four-stroke engines are common in smaller vessels and generator sets.
- Brake power is measured at the output shaft; indicated power is measured inside the cylinders. Mechanical efficiency = brake power / indicated power.
- Shaft power = 2πnQ, with n in revolutions per second and Q in newton-metres.
- For a fixed-pitch propeller in calm water, power approximately varies with speed cubed, and fuel consumption follows the same trend near the design point.
- Turbocharging uses exhaust gas energy to raise charge-air density, allowing more fuel to be burned per cycle.
- Starting air, fuel injection timing, governor action and scavenging are common fault-finding topics.
Worked example
A propeller absorbs 1,000 kW at 80 r/min. Using the propeller law, doubling speed to 160 r/min gives a power ratio of (160/80)^3 = 8, so the new power is 8,000 kW. This is a scaling estimate, not a safe operating instruction: engine margins, cavitation, shaft strength and governor limits must be checked. The point is that small speed increases demand large power increases.
Traps
Do not mix rpm with revolutions per second in 2πnQ. Do not confuse indicated with brake power. Propeller law is not exact in heavy weather or at very low load. A higher engine rating does not automatically give the same speed increase if the hull is fouled.