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Diesel engine checks for sailors: WOBBLE and common faults

You don't need to be a mechanic to skipper a yacht, but you do need to keep the engine running and fix the common faults at sea. Most problems come down to cooling water, fuel or a belt, and most can be caught before you leave.

Before you start: WOBBLE

WOBBLE is the usual memory aid for daily engine checks. Versions vary a little between schools, but the checks are the same:

  • W, water: the raw water seacock is open, the strainer is clear, and the freshwater coolant is topped up.
  • O, oil: the engine oil is between the dipstick marks, checked at a consistent temperature. Check the gearbox oil too.
  • B, belts: the alternator and water pump belt is tight, with no fraying or cracking.
  • B, batteries (or bilge): the batteries are switched on and charged, and the bilge under the engine is dry.
  • L, look around: no fuel, oil, water or exhaust leaks, and nothing loose.
  • E, exhaust (or electrics): once it's running, check that cooling water is coming out of the exhaust.

Overheating

Usually the raw water cooling. Work through it in order:

  1. Is water coming out of the exhaust?
  2. Is the raw water seacock open?
  3. Is the strainer clear of weed and debris?
  4. Is the impeller intact? A failed impeller is a common cause, so carry spares.
  5. Is the belt tight and in one piece?
  6. Is the freshwater coolant level correct? Let the engine cool before opening the cap.

Won't start, or stops

If the starter motor won't turn the engine over, look at the batteries, the battery switch and the connections. If it turns over but won't fire, or the engine runs and then dies, it's almost always fuel:

  • an empty tank, or a fuel cock turned off;
  • a blocked fuel filter, often from sediment stirred up in rough weather;
  • water in the fuel, which shows in the filter bowl;
  • air in the fuel system, after running low on fuel or changing a filter.

Bleeding the fuel system means pushing the air out so fuel reaches the injectors. On most yacht diesels you loosen the bleed screws in turn, starting nearest the tank, and pump fuel through with the lift pump until fuel without bubbles comes out. The exact sequence varies by engine, so learn yours from the manual before you need it.

Other things to watch

  • Black smoke: often overloading, for example a fouled propeller, or an air supply problem.
  • Alarms and gauges: know what your oil pressure and temperature alarms sound like, and stop the engine if either goes off.
  • A rope round the prop: stop the engine straight away rather than trying to power through it.

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Questions

What does WOBBLE stand for?

A memory aid for pre-start engine checks: Water, Oil, Belts, Batteries (or bilge), Look around for leaks, and Exhaust (or electrics). Versions vary slightly, but the checks are the same.

Why is my boat engine overheating?

Usually the raw water cooling: a closed seacock, a blocked strainer or a failed impeller. Check for water coming out of the exhaust first, then the belt and the freshwater coolant level.

Why won't my boat's diesel start or keep running?

Most often fuel: an empty tank, a blocked filter, water in the fuel or air in the system. Changing the filter and bleeding the fuel system fixes many stoppages.

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Last checked 5 November 2026. DeckPrep is an independent revision tool and is not affiliated with, endorsed by, or operated by the RYA. Always check current requirements with the RYA.