Help with Prop - calculators show 0% slip ??

[Dughuts]

I have 1999 DF70 with a 13 1/2 x 15 prop mounted on a 16ft aluminum bass boat. Its a aluminum 3 blade and I believe the prop is a progressive pitch. Just looking for some prop help and suggestions.

I need to get a new prop and have been checking out these online calculators and they all say I have very little to no slip. This seems wierd but maybe its the progressive pitch prop that screws these calculators up???

I have tried a couple different mounting heights and I have some test results. All the tests are done with the Suzuki Multi-tack and GPS.

Weird thing to me is the RPM to speed ratio didn't really change with raising the motor to the third hole. The only thing that changed is that I can easily over rev it now. I also thought it seemed like there was less boat drag, more boat out of the water. Basically, I would like the best prop for 1) gas usage 2) Top End and 3) Hole shot. That being said I,m thrilled with the gas usage and Hole shot right know. It planes instantly, like 2.6 milliseconds.

Here's the results:

RMP____Hole_MPH__Hole_MPH
3000-----1----12----3----12
3500-----1----20----3----20
4000-----1----22----3----23
4500-----1----27----3----28
5000-----1----30----3----30
5800-----1----35----3----35
6000-----1----36----3----36

***Hole 1 WOT hits 6000 RPM just over the 5800 max.

***Hole 3 at 6000 IS NOT at WOT and I didn't push it further.


I was thinking about going 17, 19, 21 pitch, 3 blade, hopefully with the same progressive pitch.

Let me know what you all think.

Thanks

 

[steelespike]

Re: Help with Prop - calculators show 0% slip ??

You have a number of options.I can't give too much detail but:
First prop calculators are only as accurate as their input and hull design/shape is the trickiest to figure.Its pretty obvious your setup is pretty close.
A good prop shop may be able to add a little cup to the prop.Should give better grip,lower the rpm slightly and increase the speed a little.You could go to a 17 probably knock off 200 to 400 rpm slow hole shot and increase speed.Sticking with your present design should produce more predictable results. A prop ike the Turning Point Hustler in the same pitch could also lower rpm while improving performance across the board.The Hustler is a very agressive prop;unfortunately
it has had some unpredictable results. It is an aluminum prop with close to stainless perfortmance.

 

[craze1cars]

Re: Help with Prop - calculators show 0% slip ??

Almost every prop calculator makes reference to the need to ADD one inch of pitch before doing calculations in any of the following: If your prop is cupped, heavily raked, 4 blades, and presumably "progressive" pitch....

Anytime a mfr takes a prop that is a given pitch, and adds cup, they are effectively increasing the pitch. So for your progressive pitch prop, you probably just need to add a number or two to the pitch figure...

So when you do your calculations, simply type in 16 instead of 15. Likely this will give you realistic numbers.

Anytime I put straight numbers for the aggressive 4 blade props I run on my boat into a calculator, I actually get negative numbers. So your results are not uncommon. For my boat, adding one isn't even enough. I always have to add TWO to my actual pitch before doing the math, in order to get a realistic figure. Don't treat prop calculators as an end-all...they are simply one tool to help compare one prop to another on the same boat, get a rough prediction of speed/RPM, and determine whether one prop may be more efficient than the other.

However comparing slip figures from one style or even brand of boat to another, is almost always a meaningless exercize.