Question about slip

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domint

Cadet
Joined
Jun 7, 2009
Messages
19
Thanks in advance for all the sage advice:

Questions first:
1.) Is 2.5% slip a "real" number? That's what I get using online calculators
2.) What is appropriate fuel usage for my motor?

Raw data:
1978 Evinrude 115hp on a Parker 18 ft Center Console.
2:1 gear ratio
Propeller is a 4 blade "COMPROP" 13.5"Dia x 13" pitch
Propeller is made of some weird composit material, but it really seems to work well. Good hole shot, decent top speed.
WOT should be and is 5000 RPM (checked with a timing light thing)
Top speed is 30.0 mph (GPS) on flat water
Load is a little lighter than normal, maybe by 250 lbs or so.
Slip came out to be about 2.5%
It cavitates or ventilates (not sure which) when turning sharply at speed.
20" shaft motor is as low as it can be.
Motor is "like new" - compression 125,125,124,125. Carbs cleaned, starts IMMEDIATELY when warm and within 3 seconds when cold.
 
Last edited:
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Question about slip

2.5% is a "real" un likely number. I feel that slip calculation is really sensitive to incorrect unput.30 mph at 5,000 with a 13" prop may result in very low slip numbers.
Either your prop numbers are wrong or the tach is off.A 13" prop on your setup should over rev.And it probably is reving at 6,000 or more.By the way the 115 likes to rev and is rated at 5,500 and some run it higher.
A 17" prop on your motor is more typical and probably would turn between 5,000 and 6,000 depending on the boat. Check your tach.Be sure all connections are clean and tight check the switch position on the back and operate it a few times to clean the contacts .
 
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Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Question about slip

Agree that the tach numbers are suspect. Using the actual gear ratio of 2.29:1 your theoritical top speed at 5000 rpm is about 27 mph. We'll assume your GPS speed is fairly accurate but the rpm according to a "timing light thing" may not be so accurate.
 
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domint

Cadet
Joined
Jun 7, 2009
Messages
19
Re: Question about slip

I feel good about my tach numbers. The timing light has an RPM check on it, and it matches my tacg gauge. And it idles where it should according to my manual. And my Evinrude manual book says the motor should max out at 5000 rpm, so I don't want to over-rev into 5500 rpm range.

Where did you get the 2.29:1 gear ratio? I could not find any reference to it online or in my book, so I simply turned the flywheel and counted prop revs. That's how I got 2:1 ratio. If the gear ratio really is 2.29, then the online calculator says my slip is impossible (negative slip).

But overall, I agree that 2.5% slip is unrealistically low, thus this whole thread.
 
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steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Question about slip

I get 2:1 as well.My info says max is 5,500 most agree the 115 likes to rev and rpms are suggested to combat the effects of ethenol. Typical; prop on a 18 footer is 17" smallest I've seen is 15.
Generally rpm ranges from 5,500 to 6000.It sure seems to me that your 13 is likely to produce more rpm than a 15 or 17. We know the prop is a 13,have settled on 2:1 ratio.We know your speed is gps so it would seem the tach is our problem.
 
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domint

Cadet
Joined
Jun 7, 2009
Messages
19
Re: Question about slip

$#%#*^%@!!!

The box says 13 pitch, but the propeller is 15" pitch.

Doh!
 
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