Differences Between Oil-Lubricated and Water-Lubricated Stern Tube Bearings

Blog | July 5th, 2019

Without wishing to state the obvious, water-lubricated stern tubes use H2O as a shaft lubrication fluid. For oil-lubricated solutions, a greasy fluid provides a slippery shaft greasing feature. These are the clear-cut differences, as plainly assigned by their technical labels. So far so good, the principal difference is right there, right in the fluid medium. From this central liquid-based discrepancy, though, all sorts of key differences take flight.


Oil-Lubricated Sterns

The oily fluid forms between the propeller shaft and the stern tube. Back further, a lubricant reservoir and return line are installed as a closed circuit. When the oil is pumped to the propeller tube, a lubricating film coats the shaft. But wait, isn’t this hydrocarbon more slippery than water? Shouldn’t this be the preferred stern tube lubricant? Well, before we even get to the environmental issues, think about the system complexities. If water gets into the oil, it’ll emulsify and become less slippery. Special settling tanks separate the seawater. And, although oil is a superior vibration dampener, contaminated lubricants can cause abrasive shaft/tube damage. Using that last sentence as a segue, oil is itself a contaminant. If this hydrocarbon-based fluid leaks, it will poison the sea or lake waters. Fortunately, although not yet commonly available, biodegradable lubricants are currently under development.


Water-Lubricated Stern Tubes

It’s true, biodegradable lubricants are on their way, but even they’ll incorporate a few chemicals weaknesses. Feasibility studies need to be carried out on such substances to see if they’ll have long-term effects on sensitive biospheres. In the meantime, why not simply draw water in from beyond the hull and use this universally available fluid as the tube lubricant? Less material abrasive than oil, smaller ships can even utilize inflatable seals if seawater is sourced as the tube “grease.” Tube-to-shaft clearance gaps do widen slightly when using water, but that’s a small price to pay, especially when pumping a liquid that can’t contaminate the environment. By the way, seawater seals also tend to be less sophisticated, so there’s less chance of a leak anyway.

Environmentally friendly, that’s the advantage that draws the most attention. At a time when environmental policies are ever-tightening, especially at sea, that feature matters. All the same, not every water-lubricated stern tube system is created equal. The tube-to-shaft clearances must be carefully adjusted. Then there’s the fact that water is a poorer vibration and heat manager. Oil-lubricated stern tubes, on the other hand, can achieve full hydrodynamic lubrication action, plus they dampen vibration and remove thermal losses. Therefore, in conclusion, in order to use an environmentally friendly water-lubricated stern tube, the engine design and tube architecture must exhibit high-tolerance engineering limits.

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