Entry & progression
Entry: Module 2. Progression: the plant a DDE runs is high-speed, hard-worked and unforgiving of neglect. Almost everything that stops a small vessel starts in the fuel or the cooling water.
Fuel — the largest single cause of everything
Fuel systems on small vessels fail because of water and dirt, and yachts are unusually exposed: tanks that are rarely emptied, long lay-ups, part-filled tanks that breathe and condense, and bunkering from whoever was on the fuel dock. Water in the bottom of a tank grows microbial contamination — the black sludge that blocks a pre-filter in an hour. It sits there quietly all season and then lifts the first time you take a beam sea.
- Drain the water separators on a schedule, not on a hunch. Look at what comes out.
- Change the note of the vacuum gauge into a habit. A rising restriction is the plant telling you what is coming.
- Polish or clean the tanks if the boat has sat, and know when they were last opened.
- Log every bunkering and keep a sample. If the fuel was bad, the sample is the only argument you have.
- Know the sequence to change a filter on a running engine — or to change it fast and bleed it — because you will do it in a seaway.
Cooling — the second cause
Raw water is the boat's most hostile fluid, and the failures repeat in the same order everywhere: strainer blocked (weed, plastic, a jellyfish), impeller shredded — and the pieces gone downstream into the heat exchanger, which is why you account for every blade — exchanger fouled, anodes eaten and then the tube stack behind them, pump seal weeping salt onto the engine below.
The closed circuit gets neglected because it is simple: coolant that has lost its inhibitor is an acid bath with a nice colour. Test it, or change it on the calendar.
Air, exhaust and the turbo
A dirty air filter and a fouled charge-air cooler quietly cost you power and put temperature into everything downstream. And the exhaust deserves genuine fear: wet exhausts run hot rubber hose within inches of hot casing, and an elbow that corrodes through pumps water back where it must never go. Mixing elbows are a scheduled item, not a run-to-failure item. Support the hangers. Soot tracks and salt crust are the leak announcing itself before it opens.
Lubrication
Oil is your cheapest diagnostic. Send it away and read the report. Rising iron, copper, silicon, fuel dilution or water each tell a different story, and they tell it months before anything shows on a gauge. A boat with a two-year oil-analysis history has a health record. A boat without one has an opinion.
Reading the fault, not the alarm
A high-temperature alarm is a symptom with a dozen parents: blocked strainer, failed impeller, fouled exchanger, dead thermostat, low coolant, an air lock, a slipping belt, a fouled hull, a fishing net on the running gear. The engineer's job is not to acknowledge the alarm. It is to find the parent — and to do it before the temperature makes the decision for you.
TIP — The most useful three minutes in the day A daily check, in the same order, every day: levels, leaks, belts, colour of the exhaust, sound of the pumps, temperature of things that should not be hot, and one look under each engine at the drip tray. It costs nothing and it catches almost everything early. It is also the habit that a periodically unmanned space most depends on, because it is the only time a human being is guaranteed to look.
CAUTION — Manufacturer's data beats general guidance, always Torque figures, clearances, service intervals, coolant specifications and fuel-quality limits come from the engine builder's manual for your engine, in your rating. Nothing on this page overrides it. Where the manual and habit disagree, the manual wins and habit changes.