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post #27 of Old 12-17-2012
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Re: overpropped vs underpropped

I don't want to get involved in a war over what is the right RPM to achieve with a boat diesel.

The OP asked a specific question related to satisfying the warrenty conditions on a new engine. Rather than tell him which way to go I was trying to give him some advice BASED on his expected boat and engine use.

But there are some basic facts about diesels that people who decide to change things need to know.

The throttle on almost all modern diesels is connected to the governor and sets the RPM, the governor in turn adjusts the fuel supply to achieve that rpm. This means that at half throttle the engine can easily be in a full fuel situation and producing all the power it can AT THAT RPM.

Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP

However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired. The only time I have seen this being a problem was an engine set up to drive a large refrigeration pump and a double alternator set up, when run just above tickover the rear cylinders ran hot. FORD 6D.

In my professional opinion as a engineer an engine installation should allow full fuel operation at ANY REASONABLE RPM without overheating. I would be happy to go to court on this.

Cooling an engine gets more complex if it has a turbo fitted. This is because the cooling is usually done by the engine oil. [ I would NOT have a turbo diesel in a sailboat although I am out of touch with some of the latest developments in the field where they claim that no cool downs are required]

An engine operated at it's max torque RPM will be running at is most fuel efficient RPM. Generally this would also be the most miles to the gallon in a boat. Look up Brake Specific Fuel Consumption.

Just to complicate things engines are more efficient when run hot.

My background is vehicle engineering but I am aware of the large body of research on maximising fuel economy and engine life on ships diesels.

Last edited by TQA; 12-17-2012 at 10:12 AM.
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