Zero slip

[mwither2]

This is my current setup. 06 crestliner Jon model C1756SC. Length is 17' . It has a 84' 70hp. evinrude. Trolling motor two batteries,fishing gear,6 gallons of fuel & me. The prop is 13-1/4" x 17". The motor turns at 5700 rpm. New tach.
The motor runs great. The gear ratio is 2.42/1. The cavitation plate is 1/4" above the bottom of the boat. I used the prop slip calculator and it showed
0 slip. It runs at 38mph GPS. Am I doing the calculation right?
I have a raker stainless prop 13-1/2 x 18" I will be getting back on Thursday. How do you think that will run?
Thanks

 

[45Auto]

Re: Zero slip

You're doing the calculation correctly. One of your inputs is incorrect, most likely prop pitch, since it's impossible for your boat to make any thrust with 0 slip.

Prop slip is really irrelevent to your boat and propellor. It's just an indicator of the average angle of attack of the propellor blades. It has NOTHING to do with prop efficiency, which is entirely different.

Your stamped prop pitch is only an average that will be correct at one RPM and one load (weight and drag of your boat). The pitch of your prop actually varies from the hub to the tip. In reality, your prop most likely has an average or "effective" pitch of 18" or 19" due to blade geometry and the light weight of your boat. This would give you a slip number of 5% or 10%. In the ranges our pleasure boats operate at, cup or washout in your prop can make an effective difference of several inches from what is stamped into it. At 17" pitch, each inch of pitch you are off makes about a 5% difference in the slip calculation.

Mercury has a basic explanation of angle of attack and prop slip at:

http://sites.mercurymarine.com/port...26_49291:126_49299&_dad=portal&_schema=PORTAL

You have to understand angle of attack to understand prop slip. I've copied that part of the Mercury page below:

 

[trendsetter240]

Re: Zero slip

Yes, slip can be a tricky number to deal with, as explained by 45auto above.

In my experience you can only use it as an indicator of prop efficiency when you are in as calm and stable a body as possible. Aiming for a 5 to 15% slip works in most cases for general pleasure boat performance purposes.

When you use GPS to calculate speed and your boat is affected by wind, wave and current forces you can end up with irregular slip numbers. Essentially changes in drag. In my case I experienced -13% slip up to +10% slip on the same boat, same prop during the same test.

Search for "4 way aluminium prop test" if you are interested in seeing the results of my trials.

Cheers.

 

[hwsiii]

Re: Zero slip

mwither2, you might try reading this for a better understanding of prop slip.

http://forums.iboats.com/showthread.php?t=381820

45Auto, you might read it as well, to see if there is anything there that you might disagree with. I am always trying to improve my knowledge and understanding of props. And if I have the theory wrong I certainly want to correct it.



H

 

[45Auto]

Re: Zero slip

Excellent post about prop slip you referenced above, Hwsiii! Good job!

 

[hwsiii]

Re: Zero slip

Thank you very much 45.



H

 

[mwither2]

Re: Zero slip

Thanks for all the info guys. I can't wait to try the stainless prop next weekend. This forum rocks!!!