How to Hug Your Diesel

And other tips to not letting engine troubles foul up your time on the water.

October 4, 2007

An hour from the dock and ahead of schedule, calm seas, clear skies, the destination a family favorite-worth the long boat ride, even to the kids. Yep, just another day in boating paradise. Everything is perfect-or would be, if the port engine hadn’t been running several degrees above normal since leaving the harbor this morning.

So now the suspense begins, the tension that changes the type of day you’re having. You know, long before anyone else on board, that this perfect day may end with your boat limping into an alternate port on one engine, hours past dinnertime, kids crying, wife exhausted and you with little hope of finding a mechanic before Monday.

That’s why you ought to become-no, will become-an expert in your engine’s health. Because it is inevitable, as certain as the fact that one day your engine will fail. Even with regular maintenance by a qualified mechanic, high performance diesel engines, like all complex mechanical things, sometimes quit unexpectedly. But just as our own routine checkups often head off health problems, a quick but thorough engine-room check on the morning of your departure, plus a few regular tests while underway, will usually illuminate developing problems long before an actual failure ruins a cruise.


For marine engine reliability, perhaps the most important part of the drive train is also the most often overlooked-the propellers. The diameter of the props and their pitch, i.e., the angle of the blades to the water, determine how much horsepower is needed to turn the propellers on a given hull. If your engines can’t reach their rated RPM, either they are not performing to spec or the propellers require more muscle than the engines are designed to provide. And they aren’t just working too hard at maximum throttle, but huffing and puffing at cruising speed as well, pumping more fuel into the cylinders than the turbos can provide air to burn. The result is waste and lower performance.

To remedy the problem, don’t assume the props are correct based on the pre-purchase sea trial. Remember all the money you spent on toys after you bought the boat? That’s all RPM-robbing weight now. Just as a few extra pounds put more demand on a person’s heart, adding weight to a boat makes it harder to move, requiring more horsepower.

First, test to see that your engines turn their rated RPM with a full load of fuel and equipment. If they don’t, either lighten the boat or have a propeller shop reduce the pitch of the props, which will lower the boat’s top-end speed a bit but should also increase fuel economy and extend engine life.


Once they are propped right, check your engines often at wide-open throttle (with the synchronizer turned off). Bentley Collins, marketing director of Sabreline Yachts, recommends running them full bore for three to five minutes at least once a month. “Ninety percent of boat owners are afraid to push their engines to maximum RPM, he says, “but the engines won’t break in a few minutes of hard running, and it’s necessary to ensure they are performing properly. With a clean bottom and normal load marine diesels should always exceed their rated RPM at wide-open throttle. If they don’t, a problem is developing. Just as doctors compare stress-test results against previous records, you should keep normal gauge readings handy for both cruising RPM and maximum throttle and refer to them when testing the engines. A fouled heat exchanger or worn seawater impeller, for example, will elevate coolant temperature at wide-open throttle long before the engine overheats at cruising RPM. Catch it now and the problem can be corrected with a routine service call rather than emergency repairs.

Displays on some electronic engines may seem overwhelmingly complex, but you just need to know a few parameters to get a good snapshot of overall engine health. One of the best indicators of health is the engine load at cruising RPM, which shows how hard the engine is working as a percentage of what it is capable of at that time. While load may vary with the boat’s weight or condition of the bottom, any significant increase may be an early indicator that one engine isn’t running properly. This is true even if both engines show an increase, as the “good engine’s load will rise to carry the weak engine’s. Boost pressure or exhaust temperature are also good indicators of load, so adding these gauges is worthwhile on engines that lack electronic displays. Any change in oil pressure or coolant temperature should raise eyebrows as well. Other readings, such as that for transmission oil temperature, can change dramatically with varying seawater temperature; that’s why you should ask your mechanic what to monitor on your particular engines, and consult him when something seems amiss.

In addition to gauge readings and maximum RPM, there are many other signs that an engine may need a mechanic’s attention. “Take a look around. The boat will tell you what’s going on, says Ron Doerr, a Ft. Lauderdale engine surveyor. “If one side of the transom is dirtier than the other, that’s a good indicator of a developing problem. If you smell antifreeze, exhaust or fuel in the engine room, find out why.


Watch the exhaust while running too. Many engine problems will show up behind the boat long before they affect performance. A dribbling injector, for example, may show only small amounts of bluish or grayish smoke, but all the while that unburned fuel is accelerating piston wear. A hint of white smoke could be the start of a cracked cylinder head or exhaust manifold-in which case the engine may run fine for dozens of hours, but sooner or later may suffer a catastrophic failure. A skipper in tune with the boat will notice subtle changes like these and call a mechanic before a breakdown. And, just as in preventive medicine, it is always better to catch a problem at the first sign of trouble rather than waiting until the symptoms cannot be ignored.

That means looking under the hood, something we all have gotten out of the practice of doing since automobiles have become so reliable. You can’t expect the same reliability from thousand-horsepower diesel engines, but a daily engine-room inspection should only take about five minutes, and could prevent an expensive breakdown-or the misery of attempting offshore repairs in a hot oil-perfumed sauna.

Obviously, engine oil level is crucial- engines won’t run very long without it-but don’t obsess over keeping the oil exactly at the full mark. Some engines seem to immediately use a quart or two, but then level off. Too much oil, on the other hand, can mean it is being diluted by diesel fuel, and any cloudiness or milkiness indicates water contamination; either requires immediate attention. Engine oil shouldn’t smell like fuel, nor should it feel gritty, and while it is normal for diesel engines to turn their oil black, transmission oil should remain clear and be free of water, grit or other contaminates.


When working properly, clear plastic reservoirs on most boats provide a visual reference of engine coolant level, but an improperly functioning heat exchanger cap or a pinhole in the hose that connects the reservoir to the heat exchanger may prevent fluid from siphoning from the reservoir, even when the heat exchanger is dangerously low. If the reservoir is low in the morning when the engine is cold and near full in the afternoon when the engine is hot, it is likely working properly. If in doubt, check the level inside the heat exchanger when the engine is cool, either visually or with an index finger.

While in the engine room, look over the engine and transmission for oil, fuel, seawater or coolant leaks. A mirror and flashlight can help your search. Black soot around the manifolds, turbos or exhaust risers indicates exhaust leaks, which are particularly damaging as the abrasive soot clogs air filters and wears cylinder linings. Inspect fuel filter bowls with a flashlight, looking for the telltale light-coffee-colored layer of sludge in the bottom that indicates water in the fuel or the heavy sediment that suggests clogged filters; also check seawater strainers for debris.

Like the doctor doing bloodwork during a physical, an engine oil analysis measures the quantity of metals, water, salt and other contaminates in the oil to warn of impending problems. For maximum comparative value, oil samples must be taken at regular intervals with exact engine hours carefully noted. A regular oil analysis program is a powerful tool, but an occasional random sample doesn’t show much.

Routine maintenance includes more than just oil and filter changes. As with people, engines with a few years on them require a bit more poking and prodding, preferably by the same trusted mechanic on each visit. John Wheatley, the owner of Florida Marine Tech in West Palm Beach, has been keeping my engines running well for nearly 20 years. This relationship gives me confidence in my engine maintenance plan.

Wheatley recommends servicing transmissions and seawater pumps annually, and cleaning and testing cooling systems every two to three years. Hoses and clamps must be inspected regularly and probably need to be replaced every three to five years. Other maintenance items, such as tune-ups and valve adjustments, vary by engine manufacturer, so perhaps the best advice is to develop a good relationship with a mechanic and have him aboard at least once a year for recommended maintenance and a thorough sea trial. Preventive maintenance isn’t cheap, but just like health issues, a little sacrifice now may save lots of pain, or worse, later.


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