SEAMANSHIP · Lesson 6 of 6
Engines and Boat Systems
25 min · Beginner friendly
You’ll learn toCarry out daily engine checks using the WOBBLE mnemonic · Describe basic diesel fuel and cooling systems and common faults · Manage batteries, gas safety and fresh water aboard
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About 10 min
A marine diesel is simple and robust, and it fails for predictable reasons — almost always dirty fuel, blocked cooling water or a flat battery. Nearly every failure announces itself in advance to a skipper who looks. This lesson teaches you where to look, on the engine and on the other systems aboard.
Take a guess
Coming below to make tea, you smell gas. Decide: which of these must you not do?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Daily Checks: WOBBLE
Before the first start each day, work through six checks — five minutes with the engine cold and off. The mnemonic WOBBLE keeps them in order; do them at the same time every day and faults have nowhere to hide.
| Letter | Check | What you are looking for |
|---|---|---|
| W — Water | Coolant level in the header tank; raw water seacock open; strainer clear | Coolant between min and max; no debris in the strainer basket |
| O — Oil | Engine oil and gearbox oil dipsticks | Level between the marks; oil not milky (milky oil means water contamination) |
| B — Belts | Drive belt tension and condition | About 1 cm of give under firm thumb pressure; no cracks, fraying or black dust |
| B — Bilges | Look and sniff in the engine bilge | Dry and clean; no new oil, fuel or water, and no fuel smell |
| L — Leaks | Visual sweep of hoses, clips, pumps and fuel lines | No drips, stains, salt crusts or weeping joints |
| E — Electrics | Battery state, terminals, isolator switches | Good voltage, clean tight terminals, isolators on for starting |
Knowledge check
Thirty seconds after starting the engine you notice there is no water coming from the exhaust. What should you do?
Fuel: Clean or Nothing
Knowledge check
You turn the cockpit key to off — and an older diesel keeps running happily. Why?
No spark plugs makes a diesel wonderfully reliable — and utterly intolerant of dirty fuel. The defence is a chain of traps along the fuel's path:
- Diesel bug
- Microbial growth at the diesel-water interface in the tank; its black sludge blocks filters. Prevention: full tanks, clean fuel.
- Bleeding the system
- Removing air from the fuel lines after running dry or changing a filter — air in the lines stops a diesel dead. Many modern engines self-bleed.
Keeping It Cool
Most yacht diesels use indirect cooling: a sealed circuit of fresh coolant, like a car's, itself cooled by raw water drawn from the sea. Lose that raw-water flow and the engine overheats within minutes — so what matters is knowing where along its path the flow can fail.
Knowledge check
The overheat alarm sounds; there is no water at the exhaust, and you have shut down. Which component do you check first?
| Symptom | Most likely cause | First action |
|---|---|---|
| Engine cranks but will not start | Fuel: empty tank, closed fuel cock, air in lines, blocked filter | Check fuel level and fuel cock; suspect the filters; bleed if needed |
| Will not crank at all | Electrical: isolator off, flat start battery, loose terminal | Check isolator switches and battery terminals; try the domestic bank if switchable |
| Overheat alarm sounds | Raw water flow lost: seacock, strainer, impeller | Throttle back, check exhaust for water; shut down; work through seacock, strainer, impeller |
| Black smoke, sluggish | Overloaded engine or fouled propeller | Reduce revs; check for rope or weed around the prop when safe |
| Engine stops suddenly | Fuel starvation or rope around the propeller | Note what changed; check the separator bowl and filters; look astern for a trailing line |
Your turn
Motoring out of Westhaven, your engine overheat alarm sounds. Describe your first four actions, in order, and say why each comes where it does.
Work it through on paper first — then check yourself against the solution. Reading a worked answer feels like learning; producing one is learning.
Batteries
Knowledge check
Yachts carry at least two separate battery banks. What is the point of the split?
Learn what the battery monitor's numbers mean: a healthy 12-volt battery at rest reads about 12.7 volts full and roughly 12.2 volts at half charge — and conventional batteries age quickly if regularly pulled below half. With the engine charging you should see around 13.8 to 14.4 volts. The daily disciplines: know where the isolators are and leave them in the right positions, keep terminals clean and tight, and match your electrical appetite to your charging — the fridge and the autopilot are the big eaters aboard.
Gas Safety
Knowledge check
Why must a gas cylinder locker on a yacht drain overboard rather than into the bilge?
One fact drives every gas rule aboard: LPG is heavier than air. That is why the cylinder lives in that overboard-draining locker, why the gas alarm's sensor is mounted low, and why the habit that matters most is turning the gas off at the cylinder after use — so the supply pipe spends most of its life unpressurised, and a failed joint has nothing to release. Gas installations themselves are work for qualified engineers.
Fresh Water and the Heads
Fresh water is the consumable that limits time away from shore: plan a few litres per person per day, more in hot weather, and check the tank gauge before any passage. The marine toilet — the heads — pumps to a holding tank or overboard through seacocks, and it clogs on anything it was not designed for. And know where every seacock on the boat is: each one is a hole below the waterline, and knowing them by heart is basic seamanship.
Knowledge check
End on an easy one: what may go into the marine heads?
Key points
- WOBBLE every day before starting: Water, Oil, Belts, Bilges, Leaks, Electrics — then confirm water at the exhaust after starting.
- Diesels need clean fuel and air-free lines: watch the water separator, know about , and stop the engine with the fuel stop control.
- Cooling is raw water through seacock, strainer, impeller and heat exchanger — overheating almost always means that chain is blocked.
- Keep the engine battery sacred for starting; run domestic loads from the domestic bank and watch the voltage.
- LPG is heavier than air: cylinders live in overboard-draining lockers, and the gas goes off at the cylinder after use.
- Know every seacock on the boat, brief crew on the heads, and treat fresh water as the consumable that limits your .
Finished this lesson?
Working through the sections marks it complete — or mark it yourself.