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Cavitation problem

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Rudy63

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I have had my boat 5yrs and have never experienced this problem. This year I had the motor rebuilt. Bored over with new marine cam. It definitely has a ton more power. It will jump on plane in a instant. But now it will cavitate when I try to accelerate quickly. I was told I need to have my prop. cupped.
Is this true and can you add any addition info that would be helpful. Thanks
 
Put a line through the center of the prop and hub and them when it does it again check to see if the line has moved. It could very well be the hub is slipping under extreme loads.
 
Check your WOT rpms now would be the time to adjust pitch at the same time. If your running too high on the rpms just get a prop with more pitch.
 
i don't understand how it can slip since prop is on a spline shaft.
Click to expand...

Ayuh,.... The splined hub is pressed into the prop, it can slip at the press fit intersection,....
 
There is a rubber bushing bonded to the splined hub and pressed into the prop. Mainly for if you hit the bottom with the prop it dont destroy the drive just the prop. If it was not ventilating before the rebuild it probably is not after either. The added ponies and a old prop can very likely cause the hub to slip. The chance the hub slips and the line staye the same is very unlikely. Did you check WOT rpms?
 
No have not chkd rpms since tach was not functioning . I've pulled the prop. Should there be any visible sign that of slippage ? Thx
 
Do you have pics of the prop?I doubt you will be able to tell unless you scribe a line or use a felt marker and see if the line moves.
 
Ayuh,.... No pictures, but, Ya, they'll press out the old drive hub, 'n press in a new drive hub,....
 
Sorry , dont know why the did not upload BUT here is where i'm at: just noticed metal filing near where my prop has been setting on my counter. It seems that I did more than spin the rubber hub but tore something up. My question: I understand your not able to see the actual damage done but with this limited info is the prop toast? Should I just replace it new , which I can for about $160.00 or so. Thanks
 
Rudy, hopefully you have a good spare propeller that should be identical to the prop that you normally run.
This must be an AQ series propeller without any "thru-prop" exhaust porting.
Also, make sure that you are using the correct "spinner" and not a prop nut!

Install the spare prop and go give it another try.

Also, we will normally see an OEM specified WOT RPM "range" that should become our target. I do not believe that the high side will be 5,500 RPM. I would think that this will be more in the range of 4,400 to 4,800 RPM.
Double check your OEM service manual!

In order to test for WOT RPM, the hull must be clean, engine in full tune, and the final gear reduction must be correct for the Engine.
Your Engine will require the 1.61:1 over-all reduction.

To find the gear ratio (the over-all reduction), remove your propeller, remove the ring anode and take a wire brush to the AFT-most area of the prop shaft bearing carrier. The ratio will be stamped into the carrier and should read 1.61 (the :1 will be omitted).
(no other ratio will work for this engine)



While on the topic of a rebuild that renders more power........ make dang sure that your ignition system is delivering the correct "Progressive" spark advance and the correct "Total Advance" at the correct RPM.

Short of no oil pressure and/or over-heating........ Ignition Induced Detonation is the gasoline Marine Engine's worst enemy!


.
 
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Re... buy or fix prop. A good HONEST prop rebuilder will tell you up front what it will cost to rehab prop. If you don't have a spare, then consider buying a new one and keeping this one, rebuilt as a spare. A spare cone, bolt and line cutter, as well as the plastic washer that needs to be in there is a prudent investment.
 
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