40 hp and 16 ' alum prop question

[airshot]

Many of you have posted with questions about your 40 hp motors on 16' aluminium boats, my question for you all is what pitch prop are you running?
This motor I recently purchased (1991 Johnson) has a 12 1/2 X 15 pitch prop
and I wanted to compare it to what others are running, I am concerned that it may be to high of pitch as I am having difficulty with the boat taking a long time to get up on plane. I have posted a number of question on here trying to solve this problem and wish to thank the good folks for all their help !!

Airshot

 

[tashasdaddy]

Re: 40 hp and 16 ' alum prop question

i run that prop, no problems. where do you have to tilt pin set. the angle the motor sets has a lot to do with planing. also you need to find out what your wide open throttle rpms are. you can use an inductive timing light with the tach feature.

 

[ezeke]

Re: 40 hp and 16 ' alum prop question

I no longer own a 16' boat, but I do know of another member of the forum who does.

You will certainly find the following thread worth reading even if it is to chuckle at some of the misconceptions by others (including myself) that the owner was able to disqualify. In the end he was able to get his boat on plane readily with a 28HP motor and you can see how it was done.

http://forums.iboats.com/showthread.php?t=235855&highlight=raise+motor

When you are ready, you may find that the information about propellers on the iboats propeller forum will be helpful.

 

[airshot]

Re: 40 hp and 16 ' alum prop question

Have been thru the tilt back and forth, the boat does plane, the motor is not getting up to speed, I am looking at all the posibilities. If I were starting with a pitch way to high, then along with motor corrections I would change the prop also. I am just trying to narrow the possible problems I may have.
Thanks

Airshot

 

[ezeke]

Re: 40 hp and 16 ' alum prop question

Once you are on plane the top end is pretty much limited to simple mathematics. A 15 pitch propeller can only move the boat a theoretical maximum of 15 inches per revolution of the propeller and is subject to imperfections in setup, plus drag. Your gear ratio is probably 0.414 and your prop should be selected to turn up to 5,800. RPM at WOT. If I did the math correctly that sets your maximum theoretical top end at 34 MPH which is not really possible.

Lowering pitch will decrease the top end, but will get you on plane more quickly.