propeller slip

[74Baja]

hi, I have a 1974 16 foot baja speed boat (shallow hull) with a 1999 Johnson 115 outboard. I was wondering how I could find out my slip %. If there is a formula or anybody knows I would greatly appreciate the information.

 

[Dhadley]

Re: propeller slip

There are a couple of places on the net. Youll need the pitch of the prop, true speed, top rpm and the gear ratio. Try http://www.rbbi.com/folders/prop/propcalc.htm <br /><br />If youll post the pitch, speed and rpm I'll be glad to run it for you.

 

[74Baja]

Re: propeller slip

the pitch is 18, speed is 56mph ( I think, does this speed make sense?) and 5500 RPM<br />thanks Dhadley

 

[walleyehed]

Re: propeller slip

If that is the correct pitch and the tach is accurate, your speed is way off..looks like 41-44 would be close. Is the 18P a Raker ??

 

[74Baja]

Re: propeller slip

sorry, yes it is an 18 pitch raker. I am kind of confused though, about the speed, the previous owner told me it went 56 mph with an old 115 with the same prop. when I go full throttle at about 5200-5500, the boats built in speedometer says about 47 or so. It feels like I'm going faster though. when I run the load is one 170 pound man, one 90 pound boy, a 12 gallon fuel tank and probably about 25 extra pounds of stuff. Does this make sense?

 

[74Baja]

Re: propeller slip

Any advice?

 

[Dhadley]

Re: propeller slip

What Kenny is getting at is that if your tach is accurate (5500) your theoritical speed with no slip is 48 mph with an 18" prop. There cannot be 0% slip. <br /><br />The r's need to come up. The Baja's are good boats and Rakers are good props. Start by trying the motor mounted higher. That should reduce drag and free up the motor some.<br /><br />What brand tach is it? Make sure its set on position 6.

 

[Dhadley]

Re: propeller slip

 

[74Baja]

Re: propeller slip

the tach is a teleflex, where could I change the setting if it is not on 6 ( I am not sure. also are you saying that I( need a higher full throttle RPM? if so, what is the max RPM on a 1999 Johnson 115? my motor is mounted in the second hole from the top. how high is too high? are there any ways for me to get more speed if my tach is correct? Thanks

 

[walleyehed]

Re: propeller slip

60* looper....6000-6200 I believe would be your target RPM.<br />On the back of your tach there is a small dial with numbers around it, and in the center there is a slot forming an arrow on 1 end, it should be pointing to 6P on a teleflex.<br />Too high with the engine is when it will ventilate with neutral trim, or water pressure drops, which I don't think will be an issue. I would raise it one more hole and test....That Raker will run fairly high without ventilating.