I received this today in an e-mail from a friend whose career is installing, servicing, and maintaining marine engines of all kinds in everything from recreational production boats to custom yachts. I had commented to him on the topic of this thread and here is his reply.
You all can certainly comment on what he wrote but as I possess but one ten thousandth of one percent of his engine experience and knowledge I am in no position to either defend or refute it other than to say it sure makes sense to me.
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Many people believe that almost all of the heat produced by a diesel engine comes from combustion. In fact, they may be interested to learn that friction losses can amount to as high as 10 percent of an engine's heat rejection.
Synthetics flow much easier at low temperatures. They also flow much easier at operating temperature so, yes, they do reduce friction by a measurable amount and require less power to pump the stuff around. That reduction is reflected in less heat produced by friction and fuel burn. The piston ring to cylinder contact is responsible for about 50 percent of the friction losses (friction produced heat) and the bearings provide another 30 percent or so.
The bottom line is that synthetic lube will lower engine heat production and by extension,temperature. But, and this is the big but, if the cooling system is functioning properly with adequate margins for fouling, high sea temperature, and loss of efficiency from pump wear or other normal conditions, the difference would be lost in the noise.
The two or three degrees you mentioned is what I would call noise. There are other issues that make that visible and those issues could be a cooling system or installation that has zero margin, a difference in bearing clearances, ring pressure (how tight in the cylinders)block casting roughness, a single bearing running hotter than the others (we measure individual bearing temps in large engines rather than assume an average based on measuring oil temp) or just normal manufacturing tolerances. That is why there is a range of allowable temperatures rather than a single figure.
Someone else changing to synthetic might see a temperature change if their engine is operating at the upper limits of its margin of heat rejection, or see nothing at all if the system is operating at the lower margin or in the middle. By system, I don't mean just cooling system, I mean the boat and its auxiliaries as a complete system in the sense of physics.
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