Propeller slip....

Hightower · 04 December 2007, 17:19

....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.

simmons0 · 04 December 2007, 17:40

Just stuck some rough figures in the calculator for ours and getting about 9% at WOT

H

Hightower · 04 December 2007, 17:51

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?

ccopith · 04 December 2007, 18:28

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

ccopith

Chris · 04 December 2007, 19:57

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

Chris · 04 December 2007, 20:05

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