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Thread: Prop Slippage Calculator
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10-13-2013, 01:56 PM #1
Prop Slippage Calculator
Occasionally, someone will post a question about the "prop slip calculator".
Here it is with an in-depth explanation that follows. ( for you guys that need to know how it works?)
"RPM" divided by "Gear Ratio" times "Pitch" divided by "1056" = "Theoretical Speed"
"Actual Speed" divided by "Theoretical Speed" = "Efficiency"
"100" minus "Efficiency" = "Slip"
There are on-line calculators where you just fill in the data & it gives you the result.
Example: Engine RPM of 6000 / 1.87 Gear Ratio = 3208.56 prop RPM
3208.56 prop rpm times 26" pitch = 83422.46 inches per minute
Now you have to convert "inches per minute" into "miles per hour".
12" to a foot & 5280 feet in a mile
60 minutes in one hour
So ........ 12 times 5280 divided by 60 = a factor of 1056
Back to Example: 83422.46 inches per minute divided by factor of 1056 = 78.998 Theoretical Speed in MPH with NO SLIP!!!!
Say actual speed (GPS?) was 72 mph? 72 divided by 79 mph = 0.9114 expressed as 91.14%
100 minus 91.14 = 8.86% slip
But here's where the problems begin........ What if your tach is off? What if the "effective pitch" of the prop is different from what is stamped on the prop or what you think it is?...... then your results are going to be off. Back in the day, before GPS, the measured speed with a speedometer was another variable that could throw your results off.
So, in conclusion, don't get "hung-up" on slippage numbers! Use them for what they're worth.... as a comparison tool & realize that your results may not be as accurate as you think?Thanks,
Tim Bostic
Hoss Marine Propellers
www.hossprops.com
918-479-5167 (shop)
Pugh Hydro w/260 Merc - 12" mid
1987 XB2002 Allison w/2.4 EFI Merc - 15" mid
1985 XR2002 Allison w/2.4 EFI Merc - 12" mid (Restoration Project)
"New" - 1987 XTB21 Allison (Refurbished at the Factory!!!!) w/'99-225 ProMax Merc
Top Speed Blog - http://www.screamandfly.com/blog.php...OSS-quot-Style
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10-13-2013, 07:40 PM #2
What if your tach is off? What if the "effective pitch" of the prop is different from what is stamped on the prop
OH, I've also actually measured the accuracy of a couple of tachs before I put them in the boat (using a laboratory signal source where I work). One was only off by about 50 rpm at 6000, the other was off by about 200 rpm at 6000.
1979 Checkmate Eluder
rebuild thread http://forums.iboats.com/boat-restor...er-541058.html
1993 Rude 175
1990 Johnson 200 - died
Where NOT to buy an engine in Savannah, GA http://forums.iboats.com/showthread.php?t=537238
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10-13-2013, 08:02 PM #3
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10-13-2013, 08:53 PM #4
Right on, Tim.
'89 Hydrostream Vegas XT, '90 Merc 2.4 Bridgeport PCU EFI
My YouTube videos________My Flickr photo gallery
http://www.facebook.com/profile.php?id=1706097519
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10-13-2013, 09:17 PM #5
I was always glad to be born stupid. Never figured out all that scientific stuff. If a prop is fast, I call it "good", if it's slow I call it "bad prop" and make it sit in the corner.
Living in the Freedom provided by Bud Conner and his fellow warriors.
R.I.P. my Heathen Brother
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10-13-2013, 09:33 PM #6Checkmate 2002 Convincor 270 496 MAG-HO
2003 Cougar 22MTR w/300xs SOLD
90 21Skater w/300xs - sold
98 STV Euroski w/280 - sold
2006 Trailblazer SS 6.0l 395hp
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10-14-2013, 12:31 PM #7
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10-14-2013, 01:30 PM #8
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10-14-2013, 01:54 PM #9
It takes a function generator that can put out a square wave. I just did a quick search and saw this one: http://www.ebay.com/itm/New-DDS-Func...-/251334144274
I have no experience with this unit (I have a much more expensive laboratory one at work), but it might be ok - especially if you have a good multi-meter with the frequency counter function (like a Fluke) so you could verify the frequency.1979 Checkmate Eluder
rebuild thread http://forums.iboats.com/boat-restor...er-541058.html
1993 Rude 175
1990 Johnson 200 - died
Where NOT to buy an engine in Savannah, GA http://forums.iboats.com/showthread.php?t=537238
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10-14-2013, 03:11 PM #10
Is the tach on a smartcraft monitor accurate? I see a diference in it and my analog tach.
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10-14-2013, 08:17 PM #11Checkmate 2002 Convincor 270 496 MAG-HO
2003 Cougar 22MTR w/300xs SOLD
90 21Skater w/300xs - sold
98 STV Euroski w/280 - sold
2006 Trailblazer SS 6.0l 395hp
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10-15-2013, 07:09 PM #12
HUUUmmmmm... I suppose if you could input gps speed, enter prop pitch and gear ratio and input tach signal, a slippage Gage could be marketed
The GPS speed and Rpm would be the only variable inputs.L.T.
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10-15-2013, 10:27 PM #13
Not much of a market (small slice of a small pie). If you could market a guide that would help people to understand slip and it's function in the equation, that might be more helpful. Wait, Mercury did that for years ("Everything You Always Wanted to Know About Propellers" but it still doesn't stick.
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10-16-2013, 10:22 AM #14
John,Is that information on the web? I was able to copy the stuff below but I don't think I have seen the pamphlet that you speak of.Thanks. IPSlip is not a measurement of prop efficiency, rather slip is the difference between actual and theoretical travel resulting from the necessary prop angle of attack. A 10 " prop moves 8 1/2" in one revolution leaving a slip of 15%. If the blade had no angle of attack there would be no slip The objective of prop design is to achieve the right amount of slip or angle of attack which is around 4 degrees. This is accomplished by matching the right amount of blade diameter and blade area to the existing hp and prop shaft rpm. Too much diameter or blade area will lower slip but will also lower prop efficiency. resulting in reduced performance
L.T.
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10-16-2013, 11:47 AM #15
If you have a smart phone, you can download a musical instrument tuner app with a frequency counter readout, and use audio of the running engine to measure RPM. Just multiply the Hz by 60 to get RPM. Overtones may sometimes cause the resulting number to be half or twice the actual RPM, but it will still be accurate.
My neighbor's Johnson 15 hummed along at 98.5 Hz, or 5910 RPM.
I use "Pano Tuner" on my iPhone. There's a free version with banner ads, but it works just fine.'89 Hydrostream Vegas XT, '90 Merc 2.4 Bridgeport PCU EFI
My YouTube videos________My Flickr photo gallery
http://www.facebook.com/profile.php?id=1706097519