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Old 04 December 2007, 13:19   #1
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Propeller slip....

....Do you know what yours is?

I'm trying to work out whether my engines performing to it's best. So did some calculations on maximum speed/gear ratios/pitch etc.

What I need is some feedback from you guys as to what your prop slippage is so I can take an average and work out if the engines firing on all cylinders if you know what I mean.
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Old 04 December 2007, 13:40   #2
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Just stuck some rough figures in the calculator for ours and getting about 9% at WOT

H
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Old 04 December 2007, 13:51   #3
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I was kind of working at 10% however when I punch the figues I have obtained with the two diffents props I have run, I get 18% slip.

Anyone else got slip figures from their boats?
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Old 04 December 2007, 14:28   #4
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when i was messing around with different props earlier this year i ended up with the following results,

boat empty (no dive gear) going wot the slip works out at 6%

when were diving with all the gear (alot of heavy gear) onboard doing 22 knots at 3600rpm the slips works to 29%. with no dive gear onboard running at 3600rpm doing 26 knots it goes down to 16 %

cheers

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Old 04 December 2007, 15:57   #5
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Totally Confused

I cannot figure out where I am going wrong on this.

MY RPM at WOT is 5400
Ratio on the etec is 2:1
Prop is an 18" SS

so using the calculation according to Downhilldai...

5800 engine rpm / 2 = 2700 prop rpm
2700 prop rpm x 18 inch pitch = 48600 inches per minute forward travel speed
As there are 63360 inches in a mile (according to my grandfather!)
48600/63360 = 0.767 miles per minute
0.767 x 60 = 46mph
(theoretical mph with your set up at 5400 RPM, assuming nil prop slip.)


Yet my speed according to GPS and Speedo = 47 mph (40.5 knots)

Even if I add a couple of inches to prop size to allow for cupped SS blades then I should only be getting without slip 51mph

So I appear to be getting negative slip or virtually no slip


What all this about then

I am getting my speed by running both with and against tide and then averaging the two readings. I am definate about the prop size I am running. the Tacho appears to be reading correctly and I have checked it against a timing gun with tacho.

C
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Old 04 December 2007, 16:05   #6
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OK i have tried a couple of prop calcs online and I am definately only getting about 3.3% slip at WOT, I have calculated this on a 19" prop although mine is 18" but the instruction on the web page stated allow and extra " for cupped props.

Chris
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Old 04 December 2007, 16:25   #7
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Has anyone else got some data for me, I figured that there would be some variation in prop slippage but not as much as you guys are suggesting.
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Old 04 December 2007, 16:31   #8
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Quote:
Originally Posted by Chris View Post

so using the calculation according to Downhilldai...

5800 engine rpm / 2 = 2700 prop rpm

C
Maths not your strongest subject then.
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Old 04 December 2007, 16:34   #9
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14% with standard alloy prop at 5700 revs with 4 people and full tank.
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Old 04 December 2007, 16:42   #10
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Thanks V666 that's the sort of info I need
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