Calculating prop slip with Rake and Cupping

[marada1]

Tried a new Solas HR Titan 4 14X21 today on my 1990 OMC Cobra 4.3L 4bbl. Performed pretty well but I think I'd like a little more hole shot so I'm thinking about ordering a 19". I was turning 4400 RPM at WOT with a speed of 52 MPH (GPS and speedo agree). This is with only me on board and trimed out to the max. I came home and plugged the numbers into the prop slip calculator and it indicates 0% slip. Can that be? (doubt it) I have read that if your prop has cupping you should add 1" of pitch. Anyone agree with that? This Titan is a high rake design with some cupping. If I plug in 22" pitch into the calculator it says 5% slip. Question is this. To make an estimate for the 19" pitch could I plug in 20" pitch, add 400 RPM, and 5% slip and be prtty close?
If so I should see a much better hole shot and run about 51 MPH @ 4800. Max for my engine is supposed to be 4600 so I'd have to watch that.
What do you guys think?
Another question, How does the Stiletto BP2 compare to the Titan HR as far as rake and cup? They come in a 20" which might be just right.

 

[craze1cars]

Re: Calculating prop slip with Rake and Cupping

Me too...zero slip. See my recent similar question that got no response to-date:

http://forums.iboats.com/showthread.php?t=261902

You and I have different props, but mine is aggressively cupped and raked as well. May have something to do with it, but I have no idea. Hoping someone knowledgeable sees our questions and has a thought on the subject...

Happy boating!

 

[marada1]

Re: Calculating prop slip with Rake and Cupping

Here's the calculator I am using
http://www.go-fast.com/Prop_Slip_Calculator.htm
I did some more reading and found this
http://www.go-fast.com/prop_slip.htm
read the third paragraph its says to add 1" of pitch if prop is cupped.
I think this is the answer. When you add the inch of pitch to the calculator it should give your % slip. Mine was 5%
Hope this helps

 

[John_S]

Re: Calculating prop slip with Rake and Cupping

Yes cupping is equivelent to having more pitch. How much to add depends on particular prop. Just remember, the calculator is just a tool. Put more trust it actual measured data. Sometimes, the drive gear ratio is not what you actually have, unless you have confirmed it.

Swapping brand/type of prop is a guessing game to some degree. Staying with the same brand/type as you change pitch will give you the best theoretical (calc) to actual.

The 19 pitch is probably what you want. Does Solas, like others, have an exchange program?

What was your old prop and wot data?

 

[marada1]

Re: Calculating prop slip with Rake and Cupping

I went into the carb yesterday because I was having issues at idle speed. I tore it down once before trying to solve this but this time I found the problem. There was a blob of some sort of clear expoxy looking material stuck in the bottom of the port side primary (and idle) venturi booster reservoir. The fuel pick-up for the venturi was sitting down on this "blob" therefore restricting fuel flow. Ran it yesterday on muffs and it made a huge difference at idle and also coming off idle. Planning to run it on the river today and I'm expecting a much better hole shot and maybe even better WOT RPM. Basically, all idle fuel was coming from one side of the carb and I feel like it was also restricting primary fuel flow. This fix I'm sure will end the light throttle surging I've been experiencing as well. Hoping I'll pick up 200 rpms or so at WOT. I'll be back with results......