Marine diesel glossary

A sacrificial anode is a block of an electrochemically active alloy fitted to underwater metal or inside an engine's cooling passages, which corrodes preferentially so that the metal it protects does not.

An anti-siphon valve is a device fitted at the top of a raised loop in a marine cooling or exhaust circuit that admits air to break a water column, preventing seawater siphoning back into the engine when it is stopped.

Back pressure is resistance to exhaust flow leaving an engine, measured against the manufacturer's maximum limit; excessive back pressure reduces power, raises exhaust and coolant temperatures, and increases fuel consumption.

Bleeding is the process of removing air from a diesel fuel system by working through bleed points in sequence from the tank towards the engine, and is required after a filter change, a fuel run-out or any repair that opened the fuel circuit.

Brake-specific fuel consumption (BSFC) is the mass of fuel an engine uses per unit of power produced per unit of time, allowing engines of different sizes to be compared on efficiency independently of their output.

A calorifier is a marine hot-water tank heated by engine coolant through an internal coil, by an electric element, or by both, storing domestic hot water for use aboard.

Common rail is an electronically controlled diesel injection system in which a high-pressure pump supplies a shared fuel rail held at very high pressure, from which electronically triggered injectors deliver precisely timed and shaped injections.

Compression ignition is the process by which a diesel engine ignites fuel: air is compressed until it becomes hot enough that fuel injected into it ignites spontaneously, without any spark.

A cutless bearing is a water-lubricated bearing with a grooved rubber lining inside a metal shell that supports a propeller shaft where it exits the hull, relying on water flow through its grooves for lubrication and cooling.

A duty rating is the manufacturer's definition of the maximum output an engine may produce and the operating pattern it may be used in, such as the hours per year and the proportion of time at full throttle it is designed to tolerate.

An ECU is the electronic control unit that manages a modern diesel engine, reading sensors and controlling injection timing and quantity, boost, protection shutdowns and alarms, and storing fault codes when it detects a problem.

A glow plug is an electrical heating element in the combustion chamber or intake of a diesel engine that raises air temperature to help the engine start when cold, since compression alone may not produce enough heat in low temperatures.

A heat exchanger transfers heat from an engine's closed coolant circuit to raw water drawn from outside the vessel, passing both through a shared assembly of tubes so that heat moves between them without the two ever mixing.

An hour meter records the total running time of an engine and is the primary basis for hours-based service intervals, but it records only duration and says nothing about the load the engine carried during that time.

An impeller is the flexible rubber vaned rotor inside a marine raw-water pump that draws cooling water through the engine, and is a consumable item that hardens, cracks and sheds blades with age and use.

An intercooler, also called an aftercooler or charge-air cooler, cools the compressed intake air leaving a turbocharger before it enters the engine, increasing the air density available for combustion.

A lift pump draws fuel from the tank at low pressure and delivers it through the filters to the injection system, and is separate from the high-pressure pump that supplies the injectors.

A mixing elbow is the fitting where raw cooling water is injected into the exhaust gas stream downstream of the engine, cooling the exhaust so it can be carried in rubber hose and silenced in water.

NMEA 2000 is a standardised marine data network that connects instruments, displays and engines on a single backbone, allowing engine measurements and alarms to be shown on a multifunction display alongside navigation data.

A primary filter is the vessel-mounted first-stage fuel filter, usually incorporating a water separator and a clear bowl, that removes water and coarse contamination from fuel before it reaches the engine-mounted filter.

Raw water is untreated water drawn from outside the vessel — seawater, brackish or fresh — and circulated through the engine's cooling and exhaust systems to carry heat away before being discharged, usually through the exhaust.

A raw-water strainer is a housing with a removable basket fitted between the seacock and the raw-water pump, catching weed, plastic, shell and debris before they reach the impeller and heat exchanger.

Reduction ratio is the relationship between engine speed and gearbox output shaft speed, expressed as a figure such as 2.6:1, allowing a relatively fast-running engine to drive a larger, slower-turning and more efficient propeller.

A saildrive is a geared drive unit that bolts to the engine and turns the drive through approximately ninety degrees, passing down a leg through the hull to a horizontal propeller shaft, sealed at the hull by a model-specific diaphragm.

A saildrive diaphragm is the flexible rubber seal between the saildrive leg and the hull, keeping water out of the vessel; it is a safety-critical, time-limited component replaced on an interval set by the drive manufacturer.

A seacock is a valve fitted at a through-hull fitting that opens or closes the connection between the sea and a vessel's systems, and the engine's raw-water seacock is the valve that admits cooling water.

A secondary filter is the fine fuel filter, usually mounted on the engine, that provides final filtration immediately before the injection system, protecting injection components from the contamination the primary filter did not catch.

A stern gland is the assembly that seals around a propeller shaft where it passes through the hull, either as a traditional stuffing box compressing packing material around the shaft, or as a dripless mechanical or lip seal.

A waterlock is a chamber in a marine wet-exhaust system that collects cooling water and reduces noise, sized so it can hold the water that drains back when the engine stops without any of it reaching the engine.

A wet exhaust is the arrangement used on most marine diesels in which raw cooling water is injected into the exhaust gas downstream of the engine, cooling it enough to be carried in rubber hose, silenced in water and discharged overboard.

Wet stacking is the accumulation of unburned fuel and carbon in the exhaust and combustion spaces of a diesel run for long periods at very light load, caused by combustion temperatures staying below the level needed to burn fuel completely.