Jake Kavanagh talks to Sea Start marine engineer Nick Eales about how to avoid the five major causes of an engine breakdown at sea
Engine failure always seems to happen at the most inconvenient time. RNLI statistics show crews were called out to 917 mechanical and equipment failures in 2021, rising to 1,027 in 2022. Figures for last year are currently being collated. Credit: RNLI Credit: RNLI boat gas tank
5 top causes of boat engine failure – and how to avoid them
A modern marine diesel engine won’t let you down – you’ll let it down.
These words from a time-served marine engineer tend to get to the root of the most common breakdowns, especially when underway – little preventative maintenance, poor quality fuel, or the absence of spares (or knowledge) to help fix the problem in situ.
With a few notable exceptions, such as Bukh and Yanmar, marine diesel engines are almost entirely based on commercial or automotive blocks that have been marinised.
This introduces some extra complexity, usually by using seawater as a coolant and fuel that can sit for long periods in the tank between outings.
The environment the engine lives in is also working against it.
Modern marine engines are solidly built, well marinised and fully tested. Here, a small diesel engine from Beta Marine is having a full workup before being shipped to a customer. Credit: Jake Kavanagh
Salty air, lack of use and changes in temperature can age your engine prematurely. ‘Standing’ faults from dormancy can also be an issue.
Research has shown that an average offshore motorboat engine only runs for around 50 hours a year, with a sailing yacht’s engine clocking even less.
A truck engine would do that in a fortnight.
However, while automotive engines are packed tightly under a bonnet and have more electronics than Apollo 9, boat engines are usually easier to access and fix.
The marinisation process is also very robust, with the service items often sensibly placed.
Be prepared. Here Magnus Rassy demonstrates some additions to the engine room of a Hallberg-Rassy 40. Note the tools are ready to hand, the plug-in light (no batteries to remember) and the dual fuel filter arrangement so one filter can be used while the other is cleaned. Credit: Jake Kavanagh
Twin engine installations are sometimes ‘handed’ so dipsticks and filters can all be reached from the central walkway.
To get an idea of the most common breakdowns, who better to ask than Sea Start, the marine equivalent of the AA; it even uses a similar colour scheme.
CEO Nick Eales has been running a fleet of mobile marine mechanics along the South Coast of the UK since 1994, so has pretty much seen it all when it comes to boat engine failure.
In descending order of occurrence, here are the five main reasons for boat engine failure, with Sea Start’s tips on how to stack the odds back in your favour.
“Since the roll-out of bio-based diesel, we’re seeing blockages a lot more,” said Nick. “Fortunately, people realise this is an increasing problem and are taking precautions.”
The fuel system is starved because of blockages in the primary filters caused by the so-called ‘diesel bug.’
This is an organism that lives in the water/fuel interface at the bottom of a tank and forms a glutinous mass which can get sucked into the fuel lines.
The bug loves fuel, especially biodiesel. This sample is petrol from a stalled outboard, and you can see the water and sludge sitting below the fuel. Even small particles in diesel can mess with injectors and fuel pumps, so primary filters are essential to prevent boat engine failure. Credit: Jake Kavanagh
Unlike a car, which has a high-turnover of fuel from equally high-turnover forecourts, marine diesel can sit unused for months.
Water from condensation collects at the bottom of the tank and the bug thrives.
The make up of the new types of biodiesel isn’t helping as the formula retains microscopic water molecules within the material.
Anecdotal evidence suggests synthetic versions are less prone than those with more natural bio content.
“Overheating remains right up there in our list of callouts,” says Nick. “The usual self-fix problem is a clogged raw water filter, but if that’s OK, then the issue is most likely to be the impeller. This is made from flexible rubber, and older blades have been known to crack and sometimes break off. The broken pieces then get sucked into the heat exchanger.
Broken blades can enter the heat exchanger and – if they can be reached – must be carefully removed, in this case with surgical forceps. An inline strainer is a preventative measure. Credit: Jake Kavanagh
“On saildrives, long pieces of stringy weed get drawn up into the intakes in the leg and block the outlet at the top of the drive. It’s difficult to have a filter good enough on the leg itself to prevent this. Spiral weed is the main culprit, and several strands can be drawn in, one after the other, till the water flow stops completely.”
The usual culprit of boat engine failure is an easily cleaned raw water filter, but more serious is a shredded impeller, as seen here. Credit: Jake Kavanagh
While saltwater and electrics are never a good mix, the marine industry has developed ways to make its circuits robust in such a harsh environment.
However, electrical issues remain a major callout for Sea Start.
A common problem is the battery master switch or the changeover version. Often mounted in lockers, these switches can suffer from corrosion issues and get hot. Credit: Jake Kavanagh
“A lot of modern yachts are getting quite complex,” said Nick. “The wiring is not as simple as it should be, and some people run the batteries down too far without realising. A common culprit is solar panels that have not been installed correctly, so draw power out of the battery instead. Another issue is batteries that are well past their prime. A marine lead-acid battery has a useful life of between three to five years. One boat owner we helped was complaining that his batteries were only 18 years old.
Battery health can be easily checked by using a hydrometer for wet types, and indicator windows or voltage drop tests on sealed types. Not regularly checking it risks boat engine failure. Credit: Jake Kavanagh
“Many of the electrical failures we attend are at the quayside rather than out at sea. The boat owner is unable to start their engine due to insufficient charge in the battery, and this can be traced back to problems in the charging loop.
“Sometimes, a battery has just died of old age.”
The other issue Nick encounters, mainly on larger motorboats, is corroded contact problems with fly-by-wire, an increasing addition to the flybridge where joystick control is used.
Slipping drive belts are less common nowadays as engine design has improved over the years, but are still a reason for boat engine failure as water pumps or alternators can’t perform without one.
Belts are key to good engine performance, so the right type of spare must be carried and properly tensioned when fitted. This engine has a mix of a V belt and a flat belt.
“We don’t see belt failure very often,” confirmed Nick, “But when they do go, the engine is pretty much out of action. The problem is often more acute in small sportsboats, as the engine sits very low at the stern. Small amounts of salty bilge water slosh aft and start to corrode the pulleys. These, in turn, rust, and the roughened surface quickly chew through the belts.”
Corrosion on a boat, especially one that is sailed or driven in coastal areas, remains an issue.
The much greater use of electrics on board, with systems left running 24/7 from a shore power connection, has also added more galvanic action to the nearby water.
Corrosion on an engine can make it hard to dismantle for major services, or even just to remove a faulty part. Electric contacts also degrade, especially as they are made from dissimilar metals to the block or head. Credit: Jake Kavanagh
Boats moored in a tightly packed marina are at greater risk, to the point that in some areas even the paint is starting to detach from underwater hulls.
The Italian paint manufacturer Marlin, for example, has recently reformulated its metal primers for much greater adhesion due to this growing problem.
With more yachts on shore power, electrolysis in marinas is increasing. A wasted anode is cheaper to replace than a wasted prop. Credit: Jake Kavanagh
Corrosion can impact an engine in several ways, mainly by degrading fuel lines, detachable filters, and other service items, but can also have an impact on ancillaries such as stern gear, trim tab rams and control cables.
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