Why should boat engine oil be changed? What if we didn’t change it?

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PBO motor whisperer Stu Davies ponders the not-so-silly question of why we need to change boat motor oil…

Why do you change your boat’s engine oil? A seemingly innocuous question I was asked a few months ago. Because we do? Because it’s dirty? Because the manual says so? This got me thinking and researching because it’s not such a silly question!

Boat engine oil is there to lubricate, to prevent wear and tear on our engines, to cool it down. Basically, the moving parts of the engine “float” on a thin film of oil which is pumped around the engine by a sump pump.

This oil is also sprayed on the underside of the pistons to help cool them and to lubricate the piston rings and where they contact the cylinder walls.

To do all of these things it must remain stable under extreme pressure in the bearing areas, and it is also used in some boat engines to cool them. It must also be able to nullify acid combustion products.

Seasonal grades of engine oil

Going back to my youth, oil was available in certain grades…different viscosities for different temperatures. So 30 or 40 weight for the summer, then when winter comes change it to a 20 weight, less viscosity for cooler temperatures so the oil can flow easily through a cold engine.

Just when I started riding came Duckhams 20/50, a revelation, attractive green color and no need to change it for the seasons! It acted as a weight of 20 during the winter and 50 during the summer. Before multigrades came along, 30, 40, 50 weight straight oil was just that.

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Generally speaking, they had no detergents or additives. They did not hold combustion products in suspension and anyone who worked on engines at the time will remember the black sludge that accumulated inside these products.

Today’s multigrade oils contain all sorts of nifty additives for clean running. The oil filter removes the largest particles formed by the combustion process, starting at around 15 microns. The others are suspended.

Additives, such as potassium hydroxide, are used to keep the oil’s TBN (total base number) high (i.e. dating back to our school chemistry days, an environment alkaline). It negates the acids formed by the combustion process. These acids can and do attack the metals used in the bearings of our motors.

Simplistically, our oil becomes “full” of suspended combustion products like carbon and acids “wear down” the TBN products in it. So going back to “because it’s dirty”, you can add to that “because it wears out”. We are changing the boat’s engine oil as it has reached the end of its productive life.


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This feature first appeared in the Summer 2022 edition of Practical boat owner. For more articles like this, including DIY, money-saving tips, great boat projects, expert advice and ways to improve your boat’s performance, subscribe to Britain’s best-selling sailing magazine.

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