5 top causes of boat engine failure – and how to avoid them

Jake Kavanagh talks to Sea Start marine engineer Nick Eales about how to avoid the five major causes of an engine breakdown at sea

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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.

Two men working on a boat engine

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.

A man on a boat

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.

1. Boat engine failure from fuel starvation

“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.

A man holding a bottle of contaminated fuel responsible for boat engine failure

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.

Top tips:

  • Add bug killer. Diesel bug fuel treatments such as Soltron, Marine 16 and Fuel Set can be routinely fed into the tank in measured doses. This not only kills the bug but can also break up the remains into smaller, combustible pieces. Biofuel has been causing issues for other appliances too, such as diesel-powered heaters and stoves, especially as the biological content of the fuel is being steadily increased by suppliers. These smaller tanks can also be dosed effectively.
  • Scrub the tank. “If you buy a boat that is new to you, the first thing you should do is have the tank cleaned,” advises Nick. “Then dose it with bug killer. This kind of approach is now a way of life for boat owners. As part of my preventative maintenance regime, I use a small pump to remove any water. Some tanks also have a manual drain at the bottom, but this isn’t always practical.”
  • Easy filter change. “Yachts nowadays have more powerful engines as they are running so many ancillaries. They also tend to do more motoring. Once a yacht starts to bash into a heavy sea, the contents of the tank are stirred up and the bug gets into the primary filters. Know how to change the filters and carry spares. Some boats have a dual ‘switch-over’ filter system, so you can still run the engine on one filter while cleaning the other. Practice in the comfort of a marina, so you know what to do in an emergency.”
  • Know some cheats. “There are some quick fixes to get you home if you have a blockage,” says Nick. “On some popular designs of yacht, you can actually pull the fuel pipe physically upwards, and clear the intake from the contaminated area. You’ll reduce the amount of available fuel, but you can keep motoring. Another cheat is to remove the gauze fitted to the pick-up. For some reason, these always seem to block up, as they surround an 8mm pipe. Removing the gauze seems to help.”

2. Boat engine failure from overheating

“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.

A man working on an engine after boat engine failure

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.”

A man holding a broken impeller which has caused boat engine failure

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

Top tips:

  • Have a robust raw water filter. The raw water filter should be easy to check and access. Usually mounted just above the waterline and isolated by a seacock, they contain a mesh or basket that can be quickly lifted out, cleaned, and reinserted. Motorboaters often add a ping-pong ball into the filter, making it easy to detect water flow through a clear plastic lid. (Something stuck in the inlet may not make it to the filter itself, so seeing an unmoving ball is a clue).
  • Check flow after clearance. “Clearing weed blockages on a saildrive can be tricky,” explained Nick. “You can take the inlet pipe off and blow it through with a dinghy pump. Sometimes you can use a coat hanger to dig it all out, but occasionally the blockage is so bad the drive must be partly dismantled. Once the blockage has been removed, it’s important to double-check that the pipe is clear. Take the valve off the top of the drive and make sure you have a good squirt of water under pressure to confirm all the weed has gone.”
  • Replace the impeller annually. Impellers are not particularly expensive, but if left for long periods without use, they can distort or crack. “Take it out at the end of the season, or just change it annually,” advises Nick. “Ones that have been removed but remain in good condition can be kept for emergencies.” Another tip is to fit a suitable mesh filter between the impeller and the heat exchanger. This will catch any bits of blade that may break away.
  • Fit an exhaust alarm. Rising engine temperatures are usually alarmed via a sensor in the cooling loop, but an earlier and more accurate indicator is to detect rises in the exhaust gas. Modern exhaust alarms can be set just above usual operating temperatures, so will quickly detect an overheating engine before any damage can occur.
  • Spares and knowledge. As with fuel filters, carry a spare impeller of the right type and know how to fit it. A good trick is to flatten the fins with a cable tie and use liquid detergent to ease its passage into the pump body. The tie is then slid off, releasing the blades. Again, have a practice when safely moored up, and keep spares where you can find them quickly.

3.  Failing electrics

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 battery master switch on a boat

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.

A hydrometer being used to test the health of a boat battery

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.

Top tips:

  • Replace old batteries. “If I buy a boat that’s new to me, the first thing I change is the batteries,” said Nick. “I do this as a matter of course, replacing them with brand new ones from a certified supplier. Even if the ones that came with the boat are relatively new, I don’t know their history. They may have suffered abuse, such as short circuit or too deep a discharge, which can lead to early failure”.
  • Keep things simple. This is not always easy with so much electrical equipment on board nowadays, but it helps to keep the basic wiring as simple as possible. “We had an issue with a split charger,” said Nick. “But no one knew where it was, or how it had been wired in. It took a long time to trace the wiring back and find the fault.”
  • Mark key systems. It really helps to mark the wires in a circuit. Coloured or numbered sleeves on the critical circuits are a good idea and are widely used in the superyacht sector. The sleeves are made of plastic, click easily into place, and are matched to a wiring diagram kept by the breakers. Knowing the wiring runs on your boat will greatly help with troubleshooting.
  • Check contacts. Keeping all visible battery and key electrical contacts clean and free from corrosion will greatly help avoid breakdowns and short circuits. One boat owner we met would regularly spray his engines with a fine mist of duck oil. However, excessive use of water repellents like WD40 can build up a thick conductive layer so they should be used sparingly.
  • Consider an app. Several companies now offer apps that use sensors to report on a variety of onboard systems. These can include fuel levels and battery state, with the more advanced versions even monitoring the ebb and flow of energy from renewables. These apps can also warn of potential early failures, allowing action to be taken before it becomes a problem on passage.

4. Loose belts

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.

A boat engine

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.”

Top tips:

  •  Carry the right spares. “Belt design has changed a lot,” said Nick. “Volvo, for example, are now opting for flat belts rather than V-belts. However, when it comes to V-belts, it is important to have the right one, not just one of the right length and bought from an automotive store. The profile of the belt is key to it giving long and reliable service. It must fit snugly into the pulley, or it could easily slip under load. Check with your engine supplier for the correct belts.”
  • Remove rust from pulleys. Rusting pulleys can quickly chew through a belt, so make sure they are clean and corrosion-free. If the engine sits close to bilge water, make sure the bilges remain as dry as possible.
  • Keep the tension. The belts need to grip properly to work, so it pays to give them a push test now and again just to ensure they are at the right tension. Adjustment is usually very simple and will make a big difference to the efficiency of pumps and alternators.

5. Boat engine failure from corrosion

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.

A rusted boat engine

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.

A waste anode on an engine

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.

Top tips:

  • Refresh the anodes. Anodes literally sacrifice themselves to protect your valuable underwater gear, so during the annual haul-out, check how much is left. If in doubt, just swap them out. They are much cheaper than a new propeller.
  • Check and clean. Give the engine a visual inspection every so often. Clean those pulleys, remove any corrosion with sandpaper or wire wool, and touch in any exposed metal with metal primer and paint. If the boat is to be left for a while, seal up the exhaust and air inlets to keep corrosion of the internals at bay.
  • Lubricate. It pays to lubricate any linkages or moving parts that require it, but make sure the right lubricants are used. This is because some types of mineral oil can react adversely with salt water. The manufacturers will make a recommendation.
  • Regular servicing. Finally, make sure your engine gets its regular service. With the cost-of-living crisis, insurers have noticed a rise in engine breakdowns as people scrimp on basic maintenance.

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