Prop Slippage Calculator

[Hoss Marine Propellers]

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?

[groundloop]

What if your tach is off? What if the "effective pitch" of the prop is different from what is stamped on the prop

I've tried to explain that exact same concept to some guys who try to compare slip between different props down to the gnat's hair. And I've always wondered how accurate the stated prop pitch is. Within 1%, within 10%?

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.

[baja200merk]

How many 28p props lay flat on a pitch block? :nonod:

[pyro]

Right on, Tim.

[transomstand]

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.

[Euroski]

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.




I know my Gaffrig tach is off close to 200RPM. That said what do I need to check how off it is? what type of signal source do I need? Can I build one from radio shack parts (LOL)?

[1FASTLASER]

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.
For how long? Lmao

[transomstand]

For how long? Lmao

Some have been there for years, they just won't learn:nonod:

[groundloop]

I know my Gaffrig tach is off close to 200RPM. That said what do I need to check how off it is? what type of signal source do I need? Can I build one from radio shack parts (LOL)?

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-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-/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.

[ssv1761982]

Is the tach on a smartcraft monitor accurate? I see a diference in it and my analog tach.

[Euroski]

Is the tach on a smartcraft monitor accurate? I see a difference in it and my analog tach.


On mine the SM is 200rpm lower than the Gaffrig. I would say the SC is correct. But would like to check the tach...

[LaveyT]

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.

[JWTjr.]

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. 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.

[LaveyT]

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. IPhttp://forums.iboats.com/clear.gifhttp://forums.iboats.com/clear.gifSlip 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

[pyro]

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.

[transomstand]

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

A REALLY smart (and handsome) guy posted it here a couple times, maybe he'll post it again:D

[transomstand]

285379

[LaveyT]

A REALLY smart (and handsome) guy posted it here a couple times, maybe he'll post it again:D

Stoker Nick???:p

[LaveyT]

That's pretty cool stuff, Thanks!

[transomstand]

Stoker Nick???:p

I didn't say old and broke down:nonod::D

[whipper]

Thats a good info base. Its along the same lines as the book you can get from Aeromarine research. The Secrets Of Prop Design. Great stuff.

[Fasprop]

Is the tach on a smartcraft monitor accurate? I see a diference in it and my analog tach.
My sc1000 is about 100-150 rpm less than the new Lavorsi I have.

[jaydee]

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.
that is interesting. does it really work? On 4 cyl. would it be a multiplier of 4?I want to try that on different motor applications as well. I have the same accuracy variabilty with a differnt type of instrament. I have 5 thermometers (some mercury others didgital) that are critical in my business and they vary about 2%. As long as I know which way they are off, then no problem. I would expect the 2% variance would follow with tachs.