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Old 10-22-2010, 12:53 PM   #20
Scraping Paint
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Operating a twin on one engine and fuel economy

Baker wrote:

Marin, what were the conditions of the MIT experiment?
First of all, trying make a comparison between a thin aircraft propeller in air and* wide marine propellers in water is an apples and oranges situation even if the theories are similar.* Plus the prop in a multi-engine plane is going to be feathered so it makes obvious sense that stopping it in this condition will generate less drag than locked in an unfeathered position or even free-wheeling.

It's been a number of years since I saw the MIT study, but typical for them they did all sorts of tests, most if not all in a test tank with instrumented props and shafts.* They tested at a variety of speeds and with a variety of prop configurations, and the conclusion was that a prop that's allowed to freewheel transmits less drag to what it's attached to (the boat) than a prop that is locked off.* The drag amounts varied a lot depending on the speed through the water, the friction applied to the freewheeling shaft, etc.

But the bottom line of the study was very clear.* All else being equal--- the prop diameters, number and configuration of blades, and the speed and angle*through the water, etc.--- the prop that was allowed to freewheel, even with a high drag on its shaft simulating a transmission or whatever, always produced less measured drag than the prop that was locked off.* The locked off prop produced the maximum turbulence every time.* A freewheeling prop could be slowed by shaft friction to the point where the turbulence from the blades was very high, but it was never quite*as high as the turbulence off the locked prop.

And obviously the less resistance that was applied to the freewheeling shaft, the less turbulence was generated, and the lower the drag as FF pointed out earlier.

That's about all I remember from my one-time reading of the test results.* Someone posted it on one of the GB sites during a similar discussion to this one.*

-- Edited by Marin on Friday 22nd of October 2010 12:58:03 PM
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