Question 1

What are the signs of a worn cutlass bearing (the rubber bearing that supports the propeller shaft)?

Accepted Answer

Signs: vibration at all speeds; a regular "knock" or "clunk" as the shaft rotates; the propeller shaft can be moved up/down by hand more than ~1.5mm. If severe, the shaft seal will wear and allow water ingress. Inspect annually; replace when play exceeds ~2mm. Cutlass bearings are rubber-lined, water-lubricated, and typically replaced every 5-7 years The cutlass bearing supports the propeller shaft at the point it exits the hull (inside the P-bracket or stern tube). The bearing is rubber-lined, water-lubricated — excellent design but subject to wear from grit and debris in the water. A worn cutlass bearing causes shaft vibration that transmits into the hull (felt as whole-boat vibration), wear on the shaft itself, and eventually wear on the stern gland. Check for play by pushing the propeller shaft up and down with the engine off. Replace is a haul-out job — normally done on the annual lift.

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.