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Let’s do the WOBBLES: 7 Checks for Diesel Engines

Let’s do the WOBBLES: 7 Checks for Diesel Engines

A reliable engine is key to safe boating. Like most diligent boat owners, you probably get your engine serviced at manufacturer-prescribed frequency by a competent marine diesel mechanic. But what do you do in the intervals?

There are some easy pre-departure checks you can safely perform yourself to detect common problems. It is always cheaper to remedy problems in their early stages, and keep your next family outing trouble-free. Let’s take a look at the WOBBLES (Water-Oil-Belts-Battery-Leaks-Exhaust-Sound).

At the Dock: 


✔    Raw water side: Make sure the seacock is open and the strainer is free of debris with the lid properly sealed (hand-tightened only), water- and air-tight.

✔    Fresh water side: Check the level and colour of the coolant. Top up if necessary (also see “Leaks” below)


✔    Level: Refer to the manufacturer’s instructions for the correct procedure and check that the crankcase and gearbox oil levels are on their marks. Top up if necessary (also see “Leaks” )

✔    Not enough or too much crankcase oil is equally bad. The engine will not be lubricated adequately and may seize.

✔    Colour: Diesel engine oil normally becomes quite dark after only a few hours of operation.

● A milky appearance indicates water ingress.

Consistency: Rub the crankcase oil between your fingers – too thick and that oil change might be overdue.

● Gearbox oil (or ATF) should not change colour even towards the end of its lifespan.


If you observe any of the following, call a marine diesel engine mechanic and do not start the engine: 

X discolouration of the coolant; 

X diesel smell in the crankcase oil; 

X darkening of the gearbox oil (or ATF); 

X emulsified crankcase oil (milky/grayish oil deposit inside the oil filler cap).




✔    Check their general condition (cracks, worn edges).

✔    Check their tension. Slack belts slip and age prematurely. Push down on the belt with your thumb in the middle of the longest distance between two pulleys. It should not deflect by more than 1/2’’, and you should not be able to twist it more than . turn between your fingers.

● Black dust around the engine room indicates belts are slipping and need tensioning. If they break, your engine will overheat quickly and stop charging the batteries.


X Do not start the engine if you observe signs of venting and/or deformation of the battery housing (bulging or cracks). These may indicate over-charging or failed cells inside the battery, which carry serious risks (over-heating, explosion and fire, acid burns) and are best investigated by a professional marine electrician.




✔    Electrical cables should be securely fastened to the battery terminals and should be free from corrosion and traces of moisture.

✔    The battery (or its ventilated container) should be securely fastened to the boat to avoid shifting or tipping over under way.

✔    If using a wet-cell battery, don goggles and acid-resistant gloves, and check the level of the electrolyte. Top-up with distilled water as required.

✔    The battery isolator switch (if fitted) should operate smoothly and be in the correct position.

✔    Check for an adequate battery voltage.


✔    Inspect the engine block and hoses, and the bilge underneath, for signs of saltwater leaks (rust/corrosion marks, salt crystals), oil leaks and coolant leaks. Trace and fix them.

● Keep your engine room bilge clean to spot leaks easily.

✔    A few final safety checks before turning the key:

✔    Walk around the deck and make sure there are no lines in the water that could get entangled in the propeller(s).

✔    Ensure the engine throttle is in the neutral position.

✔    Obviously, check that you have enough fuel for your trip, including a 25% safety margin if things do not go as planned.



Seamanship Tips 

● Make sure at least one crew member, in addition to yourself, is familiar with the procedure to start and stop the engine(s). As a responsible skipper, this should be part of your safety briefing with your crew and you should practice at the dock before heading out. 

● Carry spare raw water impellers and engine belts in your boat toolkit.



Before Casting Off: 


✔    Smoke colour: A bit of white smoke upon start is usually nothing to worry about and should clear as the engine warms up.

✔    Water flow: Check twice – immediately upon start and after a couple of minutes. (The cranking burst of water could just be the exhaust muffler clearing itself, even if your seacock is closed.)


✔    Listen for drops and spikes in the revs (“hunting”) and screeching sounds (usually belts slipping).

● Let your engine warm up gently. Do not over-rev a cold engine.



Stop the engine immediately if you notice any of the following: 

X black smoke (usually injection issues or a blockage in the air intake with significant loss of engine power); 

X blue smoke (engine burning oil, a potentially serious safety concern, which can lead to a “runaway engine” with catastrophic consequences); 

X any unusual “knocking” sound, or excessive engine vibration. 

Call a marine diesel engine mechanic immediately.




Even if you use your boat regularly, it is good practice to perform these checks and run the engine to operating temperature at the dock on a regular basis. Most boat owners find that once a month is sufficient.

If the checklist is too much to take in, just remember that your boat WOBBLES (Water- Oil-Belts-Battery-Leaks-Exhaust-Sound).


By Celine Flamain

*Celine is a qualified marine surveyor for small leisure and commercial craft, and a commercially endorsed Yachtmaster® Offshore and RYA Cruising Instructor. She also holds a Post-Graduate Diploma in Marine Insurance and has logged many miles as a professional and recreational skipper.