What is usual range of prop slippage?

[Chris1956]

I used a GPS to determine my speed in my Avenger speedboat. I run a '77 Merc 1500 inline (2::1) gear ratio, a jackplate that has the AV plate 1-2" above the water, while on plane. I run an old 19" pitch Mercury SS 3 blade standard prop.Here are the results of my test.

WOT = 5000, GPS Speed 44MPH. By my calculation, this MPH would be the result of almost no slip. Is my calculation incorrect, or is my Tach (new last year) inaccurate?

 

[walleyehed]

Re: What is usual range of prop slippage?

I get 4-1/2% slip...highly unlikely.
We have a known GPS out of the equation so lets look at an inaccurate tach and possibly a worked prop or higher pitch than we think.
I would say most hulls that plane will be from 8% to 18/20%.

Tach isn't a teleflex is it?

 

[Chris1956]

Re: What is usual range of prop slippage?

Ken, Tach is Teleflex-Morse Vector Series with the blue accent and white numbers. Why did you guess teleflex? Could I have set it wrong? It seems to read accurately at idle speeds.

 

[walleyehed]

Re: What is usual range of prop slippage?

It "may" read close at Idle, but we've seen way too many of these follow a trend of being way off at WOT...sometimes 300-400+ off.
I don't think you have it set wrong...it's just "OFF" on top-end...possibly...
We still need to verify the prop is truely what we think it is...is there anyway you can get your hands on a known prop to test?

 

[Chris1956]

Re: What is usual range of prop slippage?

Ken, I have a brand-new 21"pitch Rapture I bought from I-boats. When I installed it my RPM readings were so low, i removed it in favor of the 19" prop until I could get the motor to run better. I have been trying to get to 5500RPM+ since then. For the record, the motor sounds like it is pretty revved up with the 19" prop.

 

[walleyehed]

Re: What is usual range of prop slippage?

OK, let's look at this from another point of view...Let's assume the prop IS a 19P, we have a 2:1 ratio, we have a known 44MPH GPS. Let's go backwards, and punch-in 11% slip, which should be "ball-park". With these numbers, I come up with 5355RPM.
What was max RPM with the 21?

 

[Chris1956]

Re: What is usual range of prop slippage?

Ken, Thanks for the guestimate of slip. I will use it a a rule of thumb, until I sort this out. I am currently repairing an exhaust leak. Darndest thing, I adjust the carbs perfectly with the cowl off, and it won't idle and runs poorly with the cowling on. Not enough O2 with the cowling on, which kills power on bottom carb.

Anyway, My tach is showing a low slip (30 MPH at 3600 RPM) with that 19" prop, at medium RPM. At high RPM it shows a more realistic slip (44MPH @ 5200RPM).

I will test with the new 21" prop, when I have fixed the exhaust leak and baslined the motor's performance.

 

[Chris1956]

Re: What is usual range of prop slippage?

Ken, Over the last few days I have been trying to maximize the speed of the boat, and take RPM readings at that speed. I find the WOT RPMs vary by 1-2 hundred RPM, for a single speed (due to trim angle or inconsistant Tach operation??). The best I have done with that 19" pitch SS Mercprop is 45 MPH @ 5300 RPM, which makes me believe my tach is reading low at high RPM. I now have a fuel restriction holding back the RPMs. I will try the remote tank and see if that helps.

 

[walleyehed]

Re: What is usual range of prop slippage?

How'd things turn out, Chris?

 

[Chris1956]

Re: What is usual range of prop slippage?

Ken, I ran a test with a 21" Mich Wheel aluminum prop, just to see how it compared to the Rapture 21. The test was inconclusive due to rough water. The next chance I have to run a test that cooresponds with smooth water conditions, I will give it a whirl. I do seem to have fixed my exhaust leakd