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Old 12-16-2012
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Re: overpropped vs underpropped

You are driving along at 65 mph in a car with a six speed gearbox.

Do you

A drive in 3rd
B drive in 4th
C drive in 5th
D drive in 6th

I hope you all came up with D [ No Ferrari owners need answer ]

Now ask yourself can I get to max rpm in 6th gear? No of course.You are pulling to high a gear but you WILL get more MPG at 65 mph than any other gear.

That is why on a cruising boat you select a prop that is overpitched and you should NOT be able to reach max rpm as quoted by the manufacturer.

This thread contains some inaccurate information. A diesel engine will be at it's most fuel efficient when run at it's peak torque RPM. This is basic mechanical engineering.

Assuming that bearings are designed to cope with the BMEP at that speed then the engine life will not suffer EVEN if it is running at full fuel setting to achieve that speed.

Accurate figures for diesel engine life are notoriously difficult to arrive at but there is a considerable body of empirical evidence that suggests that engines that are run below their max power rpm last longer.

So in practice know what RPM your engine generates max torque and prop it so at full throttle with NO WIND and FLAT WATER your RPM is a bit higher, say 20%.

Now if you know you are going to spend your time running the ICW then go 10%.

Or if you know you will be bashing to windward in heavy chop maybe 30%.

BTW Cruising sailboat diesel engines rarely see enough use to wear out.

Last edited by TQA; 12-16-2012 at 08:43 PM.
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