pitch speed versus actual speed

[FrederickBoater]

On a recent test, I recorded the following data: hand held GPS said 46 mph, analog tachometer read 5600 rpm, stainless steel propeller says 19 inch pitch, engine manufacture (Merc Optimax 125) says 2.07 gear ratio. So the propeller pitch speed was (5600*19)/(2.07*1056) = 48.7 mph, and my propeller slip was 100*(46/48.7) = 94.5%. The guys at the prop shop said that's generally unheard of. My question is about the prop because I consider the GPS, the tach and gear ratio to be accurate. Is it possible that the prop is stamped as a 19 pitch, but because of cupping, rake, camber (and a host of other prop terms that I know nothing about) that the effective pitch is more like 19.5, or 20, or higher? Don't get me wrong. I'm quite pleased with the 94%, I'm just having a hard time believing it! Thanks :confused:

[groundloop]

If ya' wanna' get nit-picky the SLIP is (48.7 - 46) / 48.7 * 100 = 5.5%. (You calculated prop efficiency, which is kinda' sorta' the opposite of slip).

There was a thread on here not too long ago about the fallacy of prop slip numbers. And IMO you've hit the nail on the head about the pitch stamped on the prop maybe not being dead nuts on what the actual pitch is.


edit to add: Here's a link to that thread: http://www.screamandfly.com/showthread.php?273870-Prop-Slippage-Calculator

And here's a quote from Hoss Marine Propellers:

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?

[staylor]

If the prop is stamped 19 and it's a cupped merc prop, you normally add 1 inch to the pitch for the cupping. This puts your slip at 10%. But as groundloop posts, the slip is just a number which indicates if the prop is working on your rig. For example, I had an 18 ft Baja once w/175 hp and the best prop I found was a 14 inch dia Turbo in 24 pitch. It ran the fastest of everything I tested by 2 mph and slip worked out to 9%. I then bought a larger boat, a 21 Checkmate, and put the same engine on it. I assumed the 24 Turbo would run the best- but it was awful. The prop kept venting, hole shot stunk, top end seemed slow- and the prop slip worked out to be nearly 25%- which I knew was just too much. I then swapped to the second best prop I had run on the 18 Baja- a 14.5 dia OMC raker in 22 pitch and this ran much better than the 24 Turbo- the Raker was now the fastest prop- and slip for the Raker was running right around 8%. Slip is just a number that can be used to suggest how a prop is working on your rig, and perhaps give you some ideas as to what you should try for your next prop. In your case- I'd say the prop you have is running well.
Doug