What are zinc anodes on a boat and what happens if they are depleted?

A. Zinc anodes are only needed in salt water — fresh water marinas do not require them

B. Depleted zinc anodes cause engine overheating — replace them when the temperature alarm fires

C. Zinc anodes protect the engine from fuel contamination — they are a filter, not a corrosion control

D. Zinc anodes (sacrificial anodes) are bolted to the hull, shaft, propeller, and heat exchanger to prevent galvanic corrosion — zinc is more electronically active than other metals, so it corrodes preferentially, protecting bronze, stainless, and aluminium components. When zinc is depleted, the next most reactive metal (often bronze propeller or stainless shaft) corrodes rapidly. Inspect annually, replace when 50% consumed

A. 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

B. Diesel engines work at lower compression ratios than petrol engines for safety

C. Diesel engines use spark plugs but fire in a different sequence to petrol engines

D. Diesel engines use carburettors to mix fuel and air before compression

A. Compression, ignition, induction, exhaust — the compression happens before air enters

B. 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

C. Ignition, compression, induction, exhaust

D. Induction, power, compression, exhaust