Marine diesel engine glossary

The terms a marine mechanic will use on the phone, in plain language.

Cooling

Raw water — the seawater or river water drawn in from outside the boat to carry heat away. It never touches the inside of the engine on an indirectly-cooled installation.

Coolant / fresh water circuit — the closed loop of treated water and antifreeze that circulates through the engine itself.

Heat exchanger — the component where the two circuits meet. Coolant passes over a bundle of tubes carrying raw water.

Aftercooler / intercooler — cools the air after the turbocharger, before it enters the cylinders. Cooler air is denser air.

Keel cooler — an external cooling arrangement where coolant is circulated through pipes or a plate in contact with the water outside the hull, with no raw water pumped aboard.

Impeller — the flexible rubber vaned wheel inside the raw water pump. A wearing item, and the most common single cause of overheating.

Anti-siphon / vented loop — a fitting that prevents seawater siphoning back into the engine through the exhaust when the engine is stopped.

Exhaust

Wet exhaust — the standard arrangement where raw water is injected into the exhaust to cool and silence it.

Mixing elbow — where that water joins the exhaust gas. A known corrosion point that needs periodic inspection.

Dry exhaust — an insulated exhaust with no water injection, common on commercial vessels.

Waterlock / muffler — a chamber that holds water and stops it running back towards the engine.

Drive

Shaft drive — the propeller shaft passes through the hull at an angle, in a stern tube, sealed by a stern gland.

Stern gland / shaft seal — the seal where the shaft passes through the hull. Traditional packing glands drip slightly by design; dripless seals do not.

Cutless bearing — the water-lubricated bearing supporting the shaft near the propeller.

Saildrive — a sealed vertical leg through the hull with the propeller on the bottom, sealed by a rubber diaphragm.

Sterndrive / outdrive — an inboard engine driving a steerable leg through the transom.

IPS / pod drive — forward-facing pod propulsion mounted through the hull, steering by rotating the pods.

Reduction ratio — how much the gearbox slows the propeller relative to the engine, e.g. 2:1.

Thrust bearing — the bearing that takes the propeller’s push and transfers it into the boat, usually inside the gearbox.

Fuel and combustion

Injector — sprays atomised fuel into the cylinder. Wears, and can be tested and reconditioned.

Injection pump — meters and delivers fuel at high pressure to the injectors.

Common rail — a modern system holding fuel at very high pressure in a shared rail, with electronically controlled injectors.

Lift pump — the low-pressure pump that brings fuel from the tank.

Water separator — the primary filter that removes water before it reaches the engine.

Bleeding — purging air from the fuel system after a filter change or a run dry.

Blow-by — combustion gas escaping past the piston rings into the crankcase. Excessive blow-by indicates bore or ring wear.

Condition and testing

Compression test — measures cylinder pressure; a broad indicator of engine condition.

Leak-down test — pressurises a cylinder and measures where the air escapes, which localises the problem to rings, valves or head gasket.

Oil analysis — laboratory testing of a used oil sample for wear metals, fuel, coolant and water.

Bore glazing — a polished cylinder wall from prolonged light loading, which stops the rings sealing and increases oil consumption.

Survey — a formal inspection of a vessel, often required for insurance, sale, or commercial operation.

Corrosion

Sacrificial anode — a metal deliberately chosen to corrode instead of the components around it.

Galvanic corrosion — corrosion driven by two dissimilar metals connected in seawater.

Stray current corrosion — corrosion driven by an electrical fault, much faster and more destructive than galvanic corrosion.

Pencil anode — the small anode threaded into a heat exchanger or cooler.

Frequently asked questions

What is the difference between raw water and coolant? Raw water is the seawater drawn in from outside to remove heat. Coolant is the treated closed-circuit fluid inside the engine. They meet only in the heat exchanger.

What is a mixing elbow? The fitting where raw water is injected into the exhaust to cool it. It corrodes over time and is a routine inspection item.

What does 2:1 reduction mean? The propeller turns at half engine speed.

What is blow-by? Combustion gas leaking past the piston rings into the crankcase — a sign of bore or ring wear when excessive.

What is bore glazing? Cylinder walls polished smooth by long periods of light load, so the rings no longer seal properly.