Question 1

What fluid does a marine gearbox typically use and how do you check it is serviceable?

Accepted Answer

Most marine gearboxes use ATF (Automatic Transmission Fluid) or the manufacturer-specified hydraulic/gear oil. Check: dipstick on gearbox casing; level between min/max marks with engine warm; colour — fresh ATF is red/pink, degraded ATF is dark brown/black or smells burnt. Change interval: typically every 2 years or 200h. A gearbox with insufficient or degraded fluid will slip under load or fail to engage Gearbox fluid maintenance is commonly neglected. Most Hurth, ZF, PRM, and Paragon marine gearboxes specify ATF Dexron III or Type F — check the manual. The dipstick check is a 1-minute task. Dark, burnt-smelling fluid indicates oxidation from overheating (excessive slip) or water contamination. Water in ATF: fluid turns milky/pink and foamy — urgent gearbox service required. Gearbox failure at sea means no propulsion. Most gearboxes are inexpensive to maintain and expensive to replace.

Question 2

What is the fundamental operating principle of a diesel engine that distinguishes it from a petrol engine?

Accepted Answer

Compression ignition: diesel fuel is ignited by the heat of highly compressed air (compression ratio ~22:1) — no spark plugs are used. Fuel is injected into the compressed air at the moment of ignition Diesel (compression ignition): air is compressed to ~22:1 ratio, reaching ~500-700°C. Diesel fuel (which ignites at ~250°C) is injected directly into this hot compressed air and spontaneously ignites. Petrol engines use ~8:1 compression and require a spark to ignite. The high compression ratio makes diesel engines more thermally efficient (~40% vs ~25% for petrol) and safer (diesel is much less volatile than petrol).

Question 3

Describe the four strokes of a diesel engine cycle in the correct order

Accepted Answer

Induction (air in): piston down, air drawn into cylinder. Compression: piston up, air compressed to high temperature. Power (ignition): fuel injected, combustion drives piston down. Exhaust: piston up, exhaust gases expelled The four-stroke diesel cycle (suck, squeeze, bang, blow): 1) Induction — intake valve open, piston moves down drawing in air only (no fuel). 2) Compression — both valves closed, piston moves up, air compressed to ~22:1. 3) Power — injector fires, combustion, piston driven down. 4) Exhaust — exhaust valve open, piston moves up expelling gases. Two complete crankshaft revolutions per power stroke per cylinder. Multiple cylinders provide smoother, overlapping power strokes.