Sailing 101: Diesel Engine Oil Changes and Oil Filters Paramount to the Engine’s Life

Ahoy sailors, last time we discussed how to read a nautical chart. This time we will be discussing the importance of changing the oil in your diesel engine and the oil filters that are very crucial for a long healthy engine life. When swapping out engine oil and filters, consider these tips. Next week we will discuss ideas for boat interior designs for those looking to refurbish their boat. 


Because diesel engines generate a considerable amount of soot, they are hard on oil. If you aren’t conditioned to monitoring how fast the oil turns black after being changed, you might be surprised.


Soot is a natural by-product of the internal combustion. Soot is why diesel engine oil turns black, sometimes only after a few miles. When it becomes excessive, it can thicken the viscosity, leave deposits on wearing components, and ultimately clog a filter or, worse, an oil passage. When soot thickens the oil, it makes it more difficult for oil to flow into tight tolerances, especially when cold.


New oil is reinforced with additives that help avoid this deterioration to an extent. However, acid formation is the most crucial driver of oil changes, which is offset by a base additive that neutralizes the acid. Oil analysis calculates this acid-neutralizing ability as the total base number or TBN. Most new oil starts with a TBN from eight to ten, and as acid production is offset, that number falls. So you’ll know it’s time for an oil change when that number hits 2.5.


Technically, oil changes should be driven by this figure. However, unless you are equipped to carry out an oil analysis, which is strongly recommended, you have no idea of your oil’s condition. So instead, replacing it based on engine hours or the calendar is best. For example, some oil requires changing every 150 hours or six months, while other oils allow for as many as 450 hours or annual replacement. 


Of course, the oil you use must meet or exceed the ­specification in your engine owner’s manual. The oil used in diesel engines has an American Petroleum Institute-designated C prefix, as opposed to the oil used with gasoline engines, which has an S prefix. The C prefix is followed by a second letter indicating the additive package type.


As a boat owner, you must also use the weight of oil that your engine manufacturer appoints. For example, this may include a 15W-40 or a straight 30 weight. But, again, stay consistent, and avoid mixing letters or weights.


Also, oil filters are just as important as the oil itself. Quality and construction run the scope, so don’t be frugal. Usually, you can’t go wrong using the engine ­manufacturer’s brand, but these can run more costly than common retail brands readily available at auto-parts stores.


It’s important to ensure the area around the filter is clean before and after removing the old filter, and essential to remove the old gasket with the filter. An old gasket will not make a proper seal with a new filter. It can squirt out the entire contents of the crankcase very quickly.


Most sail auxiliary engines have a drain hose attached to the bottom of the oil pan, where an oil pump can be connected. Some sailors prefer the manual-vacuum variety. To pump efficiently, engine oil should be warm but can be handled at a manageable operating temperature. Upon completion, make sure the drain hose cap is reinstalled. You’ll need two wrenches for this—one to hold the hex base nut and another to turn the cap nut. 


Lastly, refill the engine to the full mark on the dipstick, run the engine for about 30 seconds, turn it off, wait a minute for the oil to run entirely into the crankcase, recheck the dipstick, and add oil, if necessary to top off. Then you’re all set!


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