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07-01-2019, 08:39 AM #1Junior Member
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Prop Slippage.......fact or fiction
Lately I've been playing with Mercury's Prop Slippage Calculator online and using some of the numbers (RPM and GPS speed) posted by some of the members on S&F, I'm amazed. Plugging in the prop pitch, rpm, gear ratio and posted speed in the Calculator, I'm getting zero % slippage and a couple of cases got 1% slippage. These are single engines on performance boats but can this be possible......zero slippage?
Hopefully someone will share with me their real world experience on this subject. My personal knowledge is with mainly bass boats, V hulls and heavy and I have never seen such low slippage numbers like these on my boats.
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07-01-2019, 08:44 AM #2Screaming And Flying!
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07-01-2019, 08:45 AM #35000 RPM
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Depending if the prop is actually the pitch stated and if it has been worked... eg extra cup etc can have an effect on the actually pitch of the prop. And throw numbers off.
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07-01-2019, 08:49 AM #4
Highly unlikely the actual slip is that low. What it’s probably telling you is the stamped pitch is not accurate. With the progressive pitch and tip cupping being used these days the effective pitch can vary by as much as 10%. It sounds like the boats you are using data from are pretty well dialed in so if you want to calculate the effective pitch leave the pitch field blank and put 5% in the slip field then calculate pitch. Even then it’s generous as it takes a nearly perfect prop and setup to achieve true 5% slip.
Mark
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07-01-2019, 08:49 AM #5Supporting Member
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Does not happen on a single. Very efficient twin cats will get down there but it happens with guys that work their set up death and spend big money on propellers. Then the other side of the coin is exactly what Mark said-most guys don't know what propeller they really have.
Joe
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07-01-2019, 09:02 AM #6Junior Member
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07-01-2019, 09:10 AM #7Junior Member
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My Skeeter bass boat with a 200 Yami 4 stroke with factory prop calculates at 6% slippage which is about what others are getting with the same rig.
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07-01-2019, 10:30 AM #8Screaming And Flying!
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I have to also add sometimes people exaggerate some. It comes with the territory.
83 V-King, 96 Mariner, 200 hp ff block 2.5 w/a 28p choppa
We gotta clean this liberal mess up, VOTE TRUMP TO MAGA!
Rebuild thread:
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Videos
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07-01-2019, 02:49 PM #9
And... tach RPM could be wrong too. Most tachs have a +/-%. According to my Rinda software on my 496HO the tach was 150 RPM high which will change slip calculations too! It should of set off the guardian BEEPING!
Checkmate 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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07-01-2019, 05:59 PM #10
Prop slip calculation is a great way to compare different setups on your boat, but it's not as good when trying to compare different boats and setups. Differences in tach readings, inaccuracy in propeller pitch measurement, and even the inaccuracies of gps speedometer measures can confuse the comparison of slip % between boat setups. For example, tachometer readings are often inaccurate unless well calibrated.
0% slip is unlikely in real applications.
Here's an article on 'Prop Slip' that may be of interest.Last edited by Jimboat; 07-01-2019 at 06:50 PM.
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07-01-2019, 09:52 PM #116000 RPM
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If you’re actually flying it will really throw prop slip out of wack. Some of the persons here have less then 6” of pad in contact with the water with no lip on it.
you also have weird effects from the prop at engine rpm over 7k.Last edited by Motv18; 07-01-2019 at 09:56 PM.
'06 Tracker All-Fish/Tournament V-18 90 Optimax, 46.8 gps Goal one complete reach 45 mph.
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07-02-2019, 07:25 AM #12Junior Member
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07-02-2019, 12:09 PM #135000 RPM
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Yes pitch stampings can be inaccurate. Solus Scorpion all run near zero or even into the negative. I'm not sure why but I have three of them and they all run 2" bigger than advertised.
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07-02-2019, 10:09 PM #146000 RPM
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Solas rubex sane company make some heavy heavy heavy cup in their props. A hr-4 will turn 3-5% even on my boat where a laser2 runs 8-15 depending on load
'06 Tracker All-Fish/Tournament V-18 90 Optimax, 46.8 gps Goal one complete reach 45 mph.
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07-31-2019, 10:57 AM #156000 RPM
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After reading extensive technical pages on prop slip, its impossible to get a true number. progressive prop design. rpm, mfg # rating all contribute to BOGUS slip #. just because u come up with a super low # doesnt mean your at optimun efficiency. some super hp big dollar record setting riggs come up with 17-20 slip figure!? its such a variable i wouldnt loose much sleep over it. my personal boat shows less than 0 slip. which is impossible.
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