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How to Care for Your Boat’s Diesel Engine

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Small diesel engines tend to be remarkably reliable. Even in the harsh saltwater environment, marine diesels can last for decades and thousands of hours, largely because they are entirely mechanical (other than the gauges, alternator, and starter motor). If you can crank it started, it will go: There are no computer or electrical components to fail.

The precise timing and supreme efficiency of mechanical diesel engines is a kind of engineering marvel, yet they are wonderfully simple. They need only fuel, air, lubrication, and cooling to run. They are so simple and dependable that many function despite poor (or no) maintenance. However, their high level of reliability can only be sustained if the engine is properly maintained.

Each of the engine’s systems is, of course, crucial to its smooth running. It won’t work well (or at all) if the fuel system has air or particulates in it. Similarly, if the air intake or the exhaust are blocked, you have problems. Overheating and serious damage can occur if the oil system or the cooling system are not functioning correctly.

Most diesel engine manufacturers make it easy for owners to ensure they’re doing the right maintenance at the right time. Most operating manuals have tables for periodic maintenance, spelling out when you should do what. My husband, Seth, and I have a Yanmar 3YM30 on our wooden cold-molded sloop Celeste, and it’s a measure of the little Yanmar’s simplicity that its maintenance table is only two pages long.

Changing the secondary fuel filter is one of the easiest of the regular maintenance jobs.

Changing the secondary fuel filter is one of the easiest of the regular maintenance jobs.

Photo by Ellen Massey Leonard

Checking it Out

Almost all sailors do one item on the maintenance table reflexively—leaning over the side immediately after engine start to check that water is flowing from the exhaust. But most of us don’t do the first two items on the list before every engine start: a visual inspection of the engine’s exterior and a fuel level check.

This is because the engine is often hidden under or behind companionway stairs or other cabin furniture. Disassembling all of that to examine the engine before every start can be a pain, although at the least, if you can easily remove the steps to see the front of the engine and inspect the sides somewhat, it can pay to make the effort before getting underway. Seth and I are lucky to have a hatch (with a gasket) in the cockpit, so that we can at least get a view of the engine from aft and above.

Laying eyes on our fuel levels, though, requires more rummaging under berths than is practical, and we do not have a fuel gauge, so we deal with this by keeping track of when we fueled last and how many gallons we’ve used since. We have a very good idea of how many gallons we use per hour from many hours of use over the years. (For Celeste’s engine, it’s about a third of a gallon, rounding up to be conservative.)

Yanmar recommends that various other checks and maintenance be done at certain intervals, 50 hours, 150 hours, 250 hours, or one year. Coolant is supposed to be replaced every year, for example, and the seawater pump impeller needs to be replaced every 1,000 hours or four years, whichever comes first. Seth and I got a little bit of a rude awakening on that one. Having forgotten about that item, we turned over the engine one morning in Alaska and found seawater spraying over the front of the engine. We quickly shut it down and tried to wipe off the corrosive saltwater. Then Seth set to work dismantling the pump to replace the impeller.

The Racor turbine type of fuel filter was a worthy upgrade from the spin-on type. 

The Racor turbine type of fuel filter was a worthy upgrade from the spin-on type. 

Photo by Ellen Massey Leonard

Keeping it Clean

Most people only make a good visual inspection when they have the engine exposed for oil and filter changes. For our 3YM30, Yanmar recommends draining the fuel/water separator every 50 hours (or monthly) and replacing the fuel filters every 250 hours (or every year, whichever comes first).

They also recommend draining the water and sediment from the fuel tank every 250 hours. This is difficult, if not impossible, to do on many sailboats, but at least you can keep the tanks topped off to minimize condensation developing in the tanks. If you notice water and sediment in your fuel filters and fuel/water separator bowl, it might not be a bad idea to replace the fuel filters more often. If things get really bad, hire a fuel polisher to run all the fuel in your tanks through a filter/water separator.

Racor is the primary manufacturer of marine diesel engine fuel filters, and usually the default fuel/water separator that’s installed is the spin-on type. To replace it, you first must drain fuel out of the bottom of the glass bowl (having shut off the fuel supply, of course), so that when you unscrew the filter itself, it doesn’t spill fuel everywhere. When we had the spin-on type, I used a wide-mouthed glass jar to catch the drained fuel, which I’d return to the fuel tank after pouring it through a coffee filter to catch any particulates.

Then you remove the entire filter assembly and screw on a new one, having lubricated the gaskets with clean fuel. These replacement filters are relatively expensive, and sometimes it takes some serious elbow grease (or even a filter wrench) to get the old ones off. Even if you are very careful, you’re almost bound to spill some fuel.

This is why, once we had used all our spares for the spin-on type, Seth and I upgraded to Racor’s turbine type, despite the higher cost. Replacing the filter is merely a matter of removing the top, pulling out the old one and dropping in a new one. The replacement filters are also cheaper than the spin-on type filters, so if you use your engine often or plan to use your boat for many years, this can eventually make up for the high initial price of the turbine separator.

Most diesels, including our 30-hp Yanmar, also have a secondary fuel filter, and this one is easy to remove and replace: You simply unscrew the bowl, pull the filter out, drop a new one in, and screw it back on.

Once you’ve replaced the filters, you need to bleed the air out of the system (force diesel through the system) before the engine will start. Priming the big primary fuel filter will speed this up; simply fill it with clean diesel. Remember to turn on the fuel supply at the tank again. Then loosen the bleed screw on top of the secondary filter about half a turn, while pumping the little lever on the fuel lift pump. Keep pumping until you no longer see bubbles coming out under the bleed screw, then tighten down the bleed screw (while continuing to pump so that no air gets in).

Normally, you should not have lost pressure in the high-pressure fuel lines on the injector pump and injectors, so this should be the end of the job. If you ever do get air into the high-pressure lines, you’ll need to bleed them, too. Most manuals will tell you how to do this. (Note: Many newer diesels are common-rail, rather than mechanical, with much higher pressures at the injectors. If your engine is one of these, it will probably self-bleed on the high-pressure side, but if it doesn’t call a professional to assess the problem rather than trying to bleed it yourself.)

Draining the spin-on fuel filter required some tricky maneuvering with a jar to prevent spills. Ellen subsequently switched to the turbine type of Racor fuel filter.

Draining the spin-on fuel filter required some tricky maneuvering with a jar to prevent spills. Ellen subsequently switched to the turbine type of Racor fuel filter.

Photo by Ellen Massey Leonard

Keeping it Lubed

Maybe the most important job to keep your engine functioning well for a long time is changing the oil frequently. Oil lubricates the engine’s moving parts, preventing friction and keeping the temperature down. It also filters and removes particulates, keeping the engine clean and running smoothly.

This job can be messy but it’s not difficult, and it is critically important. Whatever you can do to make it easy—so you’ll do it regularly—is worth it, and various portable oil change pumps are available. If you have the space in the engine compartment and the money, I’ve heard good things about the Reverso oil change pump, although I’ve never used one personally. Because it is plumbed into the engine, it removes old oil cleanly, adds new oil, and can be adapted to service more than one engine or generator.

On Celeste, which is quite a small and basic boat, we stick to the tried-and-true manual method. Yanmar recommends changing the oil every 150 hours and the oil filter every 250 hours for our engine, and I start with plenty of oil absorbent cloths at hand and around the engine. I snake a pick-up tube on a little oil pump down into the dipstick tube, place the larger tube from the “out” end of the pump into an empty oil container, and pump until nothing more will come out. Then I pour new oil into the filler opening. I change the filter, which is a spin-on type on the side of the engine, every time I change the oil, even though that’s more often than Yanmar recommends.

It’s also very important to change the oil if you plan to leave your boat sitting for a long time and when you resume using it. Dirty oil sitting there can do a lot to hasten the demise of your costly and otherwise reliable engine.

One of the most important things to remember when you change the oil is to run the engine beforehand. This not only makes the process easier, because warm oil is much easier to suck out through that tiny pick-up tube, but it also makes your oil change more effective. All those contaminants you are trying to remove by changing the oil won’t actually be removed (or at least not well) if you don’t run the oil through the engine first. They will have separated from the oil like water separates from fuel, settled to the bottom, and will be left behind as the cold oil is removed. Then all your labor will have been for naught as those contaminants mix with the fresh oil the next time you run the engine.

While you have the engine accessible for these maintenance tasks, you might as well make a thorough inspection looking for leaks, rust, and things like cracks or wear. Check, clean, or possibly replace the air filter, the exhaust/water mixing elbow, the belts for the raw water pump and alternator, and the electrical connections. Check the coolant level and look at the fuel lines, making sure they aren’t chafing or hanging loose.

Although diesel engines on sailboats are auxiliary to the sails, they are supremely useful machines, and it’s worth putting in the effort to keep them in good shape. If you keep to a regular schedule, it becomes second nature, and every oil and filter change is easier and quicker than the last. With regular maintenance, you can have a dependable engine working for you as long as you own your boat.

The Value of Visual Inspections

Seth pushed the prop shaft out of the coupler to remove the shaft seal after the bellows failed. It was a tricky job in the water and underscored the value of visual inspections.

Seth pushed the prop shaft out of the coupler to remove the shaft seal after the bellows failed. It was a tricky job in the water and underscored the value of visual inspections.

Photo by Ellen Massey Leonard

That impeller mishap in Alaska unfortunately wasn’t the only time that our lovely little Yanmar got a saltwater shower. The second time was about three years later, in the Marquesas Islands of French Polynesia.

Our trouble this time did not have to do with the engine, but with the propeller shaft seal. Ours is not a traditional stuffing box with flax packing but a mechanical dripless shaft seal. It has two polished surfaces, one that rotates and one that’s stationary, to create a seal around the propeller shaft. A bellows attached to the stern tube holds the stationary disc in place against the rotating doughnut.

The bellows on ours had grown old and brittle and had finally cracked, which we had not realized. When we started the engine, seawater sprayed all over the transmission and aft end of the engine. We quickly shut down and tried to clean up the mess.

Then we tackled the shaft seal. We couldn’t haul out—about 1,000 nautical miles northeast of Tahiti, the Marquesas to some extent remain one of those remote regions where cruising becomes fixing boats in exotic places. Little is available, and you must use all the ingenuity and spare parts you brought with you. So, we had to make the repair at anchor.

First, Seth unbolted the coupler to insert a bolt to act as a press. Then, by tightening the coupler together again, Seth was able to push the shaft aft and out of the coupler with his improvised press. That way he could slip the doughnut and the leaking bellows off the shaft. Of course, this let water gush into the boat from the stern tube, so he had the new bellows standing by, ready to go immediately. Once that and the doughnut were on, he pulled the prop shaft forward and back into the coupler.

We don’t recommend this technique; it’s obviously much better and safer to do a job like this out of the water. This was certainly an illustration of the value of carrying lots of spare parts, but it was also a lesson in being careful with your visual inspections.

Ellen Massey Leonard has sailed 60,000 miles on rudimentary classic boats, including a circumnavigation and a voyage to the polar pack ice.

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