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A practical introduction to marine diesel engines — maintenance, fault-finding, and emergency repair at sea.
The RYA Diesel Engine Course is a one-day practical course covering the operation, maintenance, and fault-finding of marine diesel engines. It is designed to give you the confidence to keep your engine running and troubleshoot common problems.
Any boat owner, skipper, or crew member who wants to understand the engine they rely on. Especially useful for anyone planning extended cruises or deliveries.
Understand the fundamental principles of diesel combustion, the four-stroke cycle, and the types of marine diesel engines you will encounter on boats.
Marine diesel engines operate on the four-stroke cycle, also known as the Otto cycle adapted for compression ignition. Unlike a petrol engine, a diesel engine does not use spark plugs. Instead, it relies on extremely high compression to raise the temperature of the air in the cylinder until it is hot enough to ignite diesel fuel spontaneously.
The four strokes
The crankshaft converts the linear (up-and-down) motion of the pistons into rotational motion, which is then transmitted through the gearbox to the propeller shaft. In a four-stroke engine, each cylinder fires once every two complete revolutions of the crankshaft.
The four-stroke diesel cycle: intake, compression, power, exhaust
Learn how fuel gets from the tank to the cylinders, how to bleed the system when air gets in, and how to prevent and deal with the most common fuel problems.
The fuel system delivers clean diesel at the correct pressure and volume to the injectors. Any interruption — air in the system, contaminated fuel, or a blocked filter — will cause the engine to run roughly or stop entirely. Understanding the fuel path from tank to injector is essential for diagnosis and repair.
Fuel flow path (in order)
The five main systems of a marine diesel engine
Understand how marine diesel engines are cooled, the difference between raw water and indirect cooling systems, and how to diagnose and fix overheating.
A diesel engine generates enormous heat during combustion. Without effective cooling, the engine would quickly overheat, warp the cylinder head, blow the head gasket, or seize. Marine engines use seawater (raw water) as the ultimate heat sink, but how that water interacts with the engine differs between the two main cooling systems.
Raw water (direct) cooling
Indirect (freshwater) cooling
Understand the batteries, alternator, starter motor, and warning systems that keep a marine diesel engine running and your boat's electrics powered.
The electrical system on a boat serves two critical functions: starting the engine and powering the boat's domestic systems (lights, instruments, refrigeration, navigation equipment). Most boats separate these functions into two battery banks to ensure you can always start the engine, even if you have drained the domestic batteries overnight.
Battery types found on boats
Typical battery bank setup
The alternator charges the batteries whenever the engine is running. It is belt-driven from the engine crankshaft. A voltage regulator controls the charging output to prevent overcharging. Most standard marine alternators produce between 50 and 120 amps. The alternator charges whichever batteries are selected on the battery switch, or a split-charge system (VSR — voltage-sensitive relay, or a battery-to-battery charger) can automatically charge both banks.
Learn the daily checks, routine maintenance tasks, and systematic fault-finding approach that will keep your engine reliable and get it running again when things go wrong.
A two-minute check before you start the engine each day can save you hours of grief at sea. These checks are simple, quick, and should become an automatic habit — like checking your mirrors before driving a car.
Pre-start checks (do these every time)
After starting the engine, confirm: water is flowing from the exhaust, the oil pressure warning light has gone out, the charging light has gone out, and the engine sounds normal. Let the engine warm up at low revs for a minute or two before engaging gear or increasing throttle.