If you are considering buying a boat and are undecided about the choice of gasoline or diesel engines, this article will provide some useful information on deciding in favor of a diesel. If you have recently purchased a diesel-powered boat, here are some ideas on getting the best from your diesel engine.
Why diesel
More than 50 years ago, marine diesel engines overtook their gasoline-burning counterparts in popularity and desirability in the pleasure-boat market. Although the initial cost is higher, service life is longer and maintenance expenses are usually lower. Diesel engines operate more efficiently than gasoline engines and yield more miles per gallon. Because diesel fuel has a much higher ignition temperature than gasoline, the risk of fire and explosion aboard is significantly reduced, often resulting in lower insurance premiums.
Diesel Technology
Diesel engines for pleasure use range from one cylinder up to eight or more and are further classified as mechanical injection or common rail technology. Diesel engines have no ignition system because the fuel is ignited by the high temperature developed during the compression stroke. Gasoline engines have spark plugs, wires, coils and distributors that must be periodically replaced. Ignition systems are also the most frequent points of failure.
Fuel Injection Systems
Diesel engines with “mechanical” injection systems have one pump per injector (the camshaft-operated high-pressure pump is part of each injector) or an assembly containing the high-pressure pump or pumps for each injector. By the late 1990s, common-rail diesel engines with electronic injectors were developed to meet stricter automotive emission regulations. One high-pressure pump supplies fuel to a common header and each injector is actuated electronically by the engine control computer. Easier starting, better fuel economy, quieter operation and lower pollution levels are among the benefits of common-rail technology. Caterpillar, Cummins, MerCruiser, Yanmar and Volvo Penta produce common rail diesel engines for the pleasureboat market.
Safety Considerations
Diesels are constant-pressure ignition engines and always operate in the lean fuel mixture regime, preventing carbon monoxide from forming – an important safety feature. Gasoline engines are constant volume ignition engines and always operate in the rich fuel mixture regime. This lets dangerous toxic carbon monoxide form in the exhaust gases.
Diesel Efficiency
Diesel engines operate most efficiently in variable speed ranges below maximum power – the regime at which most vessels are operated. Gasoline engines operate most efficiently at wide-open-throttle conditions, not normally encountered on cruising vessels.
Totaling up the plusses, the diesel engine excels in safety, economy, service life, frequency of maintenance and reliability.
Perceived minuses First cost and repair costs.
Per horsepower, a diesel engine is more expensive and heavier than a gasoline engine. However, this added cost will very likely be offset by the longer service life of the diesel engine.
Periodic Maintenance
Diesel injection pumps and injectors are high precision devices and only can be serviced and repaired by specialist shops with “clean rooms,” while the typical gasoline engine can be serviced and repaired aboard the vessel by much less skilled (and paid) persons. Diesel injectors have much longer service lives (measured in thousands of hours) than gasoline engine spark plugs and ignition system components, resulting in additional savings in replacement parts and labor.
The diesel engine is sensitive to fuel cleanliness due to the high-precision components in the fuel injection system. Diesel fuel filters must be changed more frequently than on the typical gasoline engine. Racor fuel filters are the gold standard for fuel filters.
Owning a diesel engine
As a boat owner, what you need to know about your diesel engine depends on your interests, abilities and budget.
First of all, start off with a known good engine. That means paying for a competent engine surveyor that has a demonstrated knowledge of your particular engine. We did this when we purchased our 2004 Nordic Tug 32 in 2020. Or simply replace a suspect engine with a new one. We did that when we purchased our sailboat in 1997.
Purchase a maintenance log and keep it up to date. I create separate sections for fuel filters, oil changes, oil filters and oil analysis reports, battery maintenance, etc. The maintenance log is a powerful selling tool when that awful day arrives. Conversely, be wary of any vessel not accompanied by a maintenance log.
Familiarize yourself with the engine’s operating manual. Use it to construct a start-up check list and use the checklist faithfully.
Learn to listen to your engine and if it begins to sound differently, do not delay determining the cause. An acquaintance recently noticed that one of his engines began making an unusual sound. He wisely shut down the engine and a mechanic found a blown head gasket. His prompt action resulted in no damage to the block or cylinder walls.
Make careful note of the engine oil pressure and coolant temperature at idle and at operating speeds. Write down these values and clearly post them where you and any guest operator can view these important operating parameters. Changes in these values will most likely indicate trouble brewing.
Keep the engine and engine compartment clean. Any leaks will be easily seen and should be promptly remedied.
Inspect your engine raw water strainer before each startup and clean it before it clogs up.
Replace the impeller in your engine raw water pump at the start of each season or no longer than two years of service. If the impeller is missing any vanes, you will probably find them in the heat exchanger. Remove them before you have an overheated engine. Hint: lubricate the impeller with personal lubricant and tie it with nylon wire ties to make insertion much easier. We use Sherwood impellers on our Cummins engine. Inspect and replace any anodes in the cooling system prior to replacing the engine coolant.
Replace the engine coolant per your engine manufacturer’s recommendations. Sleeved engines require a different coolant than non-sleeved engines. Flush the engine with deionized or distilled water before adding the new coolant. We use Xerex Original Green coolant in our Cummins Model 6BTA 5.9 M1 engine. Consult your engine manufacture’s manual for coolant specifications.
Inspect and replace the alternator/engine water pump belt when it shows signs of wear. It’s a good idea to keep a spare aboard. Take a photo or two of the belt before you remove it. It will make replacement less daunting. While the belt is off, rotate the alternator, coolant pump, idler and belt tensioner, listening and feeling for roughness or play, indications of failed bearings. A good bearing will have a slight drag and will be smooth and noiseless. Gates is a recognized brand of belts. Purchase on Amazon or at NAPA auto parts stores.
Change the engine oil and oil filter per your engine manufacturer’s recommendations or at the end of the season. We use Valvoline Premium Blue 15W-40 in our Cummins engine. You can also use Fleetguard engine oil. Diesel engine oil will turn black within 50 hours or less of engine operation. This is normal.
Do not lay up your vessel for the winter with used oil in the engine. The accumulated acids and products of combustion are not conducive to long engine life. Take an engine oil sample as you drain out the used oil. An oil analysis is currently $35 and a series of oil reports will provide peace of mind. The expense will be recovered when you eventually sell your boat because the new owner will happily pay a premium to have proof of a sound engine. Blackstone Laboratories is a well-recognized testing laboratory that we have used for four years.
Pay particular attention to the engine transmission or gear box. Learn its sounds and do something promptly if it changes its song. Replace the fluid per the transmission manufacturer’s recommendations and pull an oil sample every couple of years. Carefully inspect the fluid screen for metal particles which will indicate impending failure. We use CAT TDTO 30 in our Twin Disc gearbox.
Finally, shortly after engine shutdown, take a minute or two to feel the coupling between the gear box and propeller shaft. If the coupling is noticeably warmer than the gearbox, it is time to have your engine and gearbox realigned with the propeller shaft.
Within a short while these details will become second nature to you. The extra effort you’ve expended will pay handsome dividends in peace of mind and possibly save large sums in repair expenses. n
Harry Hungate and his wife Jane Lothrop circumnavigated the world aboard their Corbin 39 Cormorant in 2012 and completed the Great American Loop aboard their Nordic Tugs 32 Bessie in 2023. He retired from the Air National Guard and Air Force Reserve as a diesel generator mechanic.