Prop Slippage.......fact or fiction

**dreamer428** · 07-01-2019, 08:39 AM

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.

**powerabout** · 07-01-2019, 08:44 AM

Originally Posted by dreamer428

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.

How did you measure the pitch of those props.....

**strat250** · 07-01-2019, 08:45 AM

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.

**dreamer428** · 07-01-2019, 09:02 AM

Originally Posted by strat250

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.

Yes, this is true and the accuracy of the GPS can be at fault depending on the mounting of the antennae and I suppose the tach can also have some error. I'm hoping a few S&F boaters will chime in with their slippage numbers.

**dreamer428** · 07-01-2019, 09:10 AM

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.

**XstreamVking** · 07-01-2019, 10:30 AM

I have to also add sometimes people exaggerate some. It comes with the territory.

**mjw930** · 07-01-2019, 08:49 AM

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.

**JPEROG** · 07-01-2019, 08:49 AM

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

**Euroski** · 07-01-2019, 02:49 PM

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!

**Jimboat** · 07-01-2019, 05:59 PM

Originally Posted by dreamer428

...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?

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.

**dreamer428** · 07-02-2019, 07:25 AM

Originally Posted by Jimboat

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.

Jimboat, Thanks for the very interesting and informative article on prop slip.

**Motv18** · 07-01-2019, 09:52 PM

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.

**allstock** · 07-02-2019, 12:09 PM

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.

**Motv18** · 07-02-2019, 10:09 PM

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

**KIRCHNER** · 07-31-2019, 10:57 AM

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.