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Old 08-11-2013, 09:23 PM   #1
Nomad Willy's Avatar
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 14,278
Reducing blade area of prop

On Willy I have an 18" Michigan "MP" prop driven w a 40hp engine w 2.57-1 reduction. Of the 40hp I usually use about 18 to cruise at 6.15 knots. I'm slightly over propped but one inch pitch reduction would make me under propped.
Ideally I like to be just a tad under propped like about 50rpm. The prop is under loaded at 18hp so I'm think'in that reducing parasitic drag by taking off a bit of the prop edges or edge will gain the rpm I need, reduce the P drag and avoid using too much power to turn the blades through the water. Always a danger w large low pitched props w too much blade area for the power.
One could make an accurate line on the blades on each of the 3 blades, grind off to the lines and dress the edges appropriately. If I took some extra effort while trimming balance should not be affected. So would save a trip to the prop shop.
Could take it all off on one or the other side or the same amount off both. I'm leaning toward all off the trailing edge.
What think all?
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North Western Washington State USA
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Old 08-11-2013, 09:42 PM   #2
Brooksie's Avatar
City: Cape Cod, MA
Country: USA
Vessel Name: Island Seeker
Vessel Model: Willard 36 Sedan
Join Date: Oct 2007
Posts: 975
I wouldn't try it (and i've tried many crazy things)
First the edges would come out too blunt and you will not be able to "fine" them up again yourself. Second any gain from lost drag would be offset by not being "overpropped" any more. Being overpropped only takes place at WOT* and is only harmful if you try to use that power and you can't use WOT to any advantage on your boat anyway so overpropping is a moot point.

* During the remainder of the rpm range, the HP the propeller can absorb falls far below what the engine can produce.
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