Marine Engine Oil: Avoiding Viscosity Breakdown

Oil in a marine engine, such as this 4.3-liter GM V6, must be changed at regular intervals to avoid viscosity breakdown, fuel dilution, and moisture contamination.

Complete service records for a used boat you are considering purchasing are a good way to judge how well it has been cared for. Changing oil at the proper interval—every 50 hours of use—prevents “viscosity breakdown” and engine damage that can result, but what does that term mean?

According to Webster’s dictionary, viscosity is the “internal friction of a fluid, which makes it resist flowing past a solid surface or other layers of the fluid.” However, the way it is advertised, viscosity breakdown is often confused with a thinning of engine oil, and that’s not the case at all.

“As an oil is exposed to heat and combustion and all the things that it’s exposed to in an engine, its natural tendency is not to thin out or break down,” said Larry Norris, a technical specialist with Phillips 66. “Oxidation is the primary wear mechanism of the oil molecule itself, and the end result of that is for oil to thicken, not thin.”

Oil companies add petroleum-based polymers—viscosity modifiers—to multigrade oils in particular, say a 10W-40. Basically, a viscosity modifier features molecules that typically tend to have a higher molecular weight and form a longer chain. When oil is cold, the viscosity modifier’s molecules essentially curl up and allow oil to flow fairly easily. It acts like a thinner oil, the 10 in 10W-40. As the oil heats up, the molecules begin to expand to cause the oil to maintain viscosity and act like a thicker oil. That’s the 40 in 10W-40.

In a marine engine, it’s critical to warm your engine to operating temperature before dropping the throttle. Besides heating the oil up so it flows properly, warming the engine also allows engine components such as pistons and rings to expand, which greatly reduces fuel dilution, which also is a significant problem in a marine engine.

“A big part of the job of the oil is to absorb contaminants, such as byproducts of combustion, unburned fuel that gets past the rings, moisture contamination — pretty much anything that gets into the crankcase,” Norris said.

Left unchanged, oil that has absorbed a lot of unburned fuel does, in fact, become thinner and far less suited to lubricating an engine, particularly one that works as hard as it does in a boat. Some fuel dilution is inherent in how an internal combustion engine operates, but in a boat it can be more pronounced than in an automobile.

Another prevalent problem in marine engines is moisture contamination. A wet environment, wet exhaust, and lower coolant temperatures often don’t allow a marine engine to “cook off” moisture that accumulates in the oil.

“Cool water in an automobile is 180 degrees,” Norris said, adding that as a marine engine cools off after you shut it down, it draws in more moisture. “Now, in a marine application, you’ve basically got an unlimited supply of ambient temperature water. So you’ve constantly got a supply of 70 to 90 degree water, which, as you circulate it through the engine, is going to carry heat away from the engine much faster than 180-degree water. The problem you have in a marine application is, can you maintain an adequate oil-sump temperature to actually drive the water off even during normal operating temperature?”

One final challenge in marine engines is foaming, or, bubbles in the oil. Norris said that oils are formulated with enough antifoaming additives to last the life of the change interval. And it doesn’t take much additive, he said, but in addition to a crankshaft whirling around inside the engine block, splashing oil all over the place, remember that an automobile doesn’t normally encounter two-foot waves at 40 mph. The added jarring of an boat landing on wave tops only increases the likelihood of oil foaming, which makes it all the more important to adhere to recommended change intervals.

“From a hydraulic perspective, liquid can’t be compressed, but air can be,” Norris said. “And so if you have air bubbles in the oil that is forming that film, then the film is much weaker and that could introduce wear concerns into the engine.”

Service records won’t tell you everything, but a used boat that has had regular oil changes is likely a safer bet than one that hasn’t.

- Brett Becker

 

 

 

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