Marine Systems, Propulsion, Electrical and Mechanical Systems
Core concepts
Understand propulsion options: outboard, inboard, sterndrive, shaft and propeller, waterjet, and sail. Know propeller terminology - diameter, pitch, blade area, slip, cavitation, handedness - and the relationship between pitch, rpm and theoretical speed. Learn engine types (two-stroke, four-stroke, diesel, petrol), the four-stroke cycle, cooling (raw water, closed circuit, heat exchanger), lubrication, fuel systems, exhaust and starting systems.
Electrical content includes DC systems (batteries, alternators, isolators, fuses, circuit breakers), series versus parallel battery connection, and shore power with galvanic isolation. Know Ohm's law and power equations. Understand cathodic protection (sacrificial anodes, impressed current) and galvanic corrosion. Cover steering, bilge and pumping systems, hydraulics, refrigeration, and basic maintenance schedules and fault-finding logic.
Traps in questions
- Confusing pitch (theoretical advance per revolution) with actual advance, which is reduced by slip.
- Connecting batteries in series increases voltage, not capacity; parallel increases capacity, not voltage.
- Assuming a two-stroke engine has separate lubrication sumps; it typically uses oil mixed with fuel.
- Ignoring that cavitation causes vibration, noise and erosion, not just lost thrust.
- Forgetting that anodes must be less noble than the protected metal.
Rule of thumb: theoretical speed
Theoretical speed (kn) ≈ (pitch in inches × rpm × 60) ÷ 72 912, or more simply pitch (in) × rpm ÷ 1 215.
Worked example: Pitch 19 in, 3 000 rpm. Theoretical speed = 19 × 3 000 ÷ 1 215 ≈ 46.9 kn. With 20% slip, actual ≈ 46.9 × 0.80 ≈ 37.5 kn. Always state that slip depends on load and hull form.