Which prop?

[aesgator]

I need a new prop and I need to know if this prop will work. Currently I'm running a 11 3/4 with 17 pitch on 15ft fiberglass cc boat. The tach says its doing 4000 rpm, and the GPS says it used to go 35mph. A local boat shop has a prop that is 12 1/4 13 pitch, and I was wondering if that is too much of a change for this motor. Its an 84 Johnson 60 hp. Any help would be great. Thanks.

 

[Silvertip]

Re: Which prop?

I would check the accuracy of the tach before spending money on props. The numbers you provide don't agree with the prop calculator. I also think a 13P prop is way too little pitch.

 

[aesgator]

Re: Which prop?

I've been wondering about my tach for awhile if it has been as accurate as it says. I'll see if i can check it or see whats going on with it this weekend. I still need a new prop regardless, mine has seen its last oyster bar. The 13 pitch in question just happens to be 50 dollars so I wanted to completely rule it out before I buy a different one that is more expensive.

 

[jerryjerry05]

Re: Which prop?

If you spend that $ 50 then get ready to spend more.That is WAY wrong.
Get the tach checked out.You can buy a small tach that can be hooked to a plug wire and have the results fast.
You should be turning 5200-5600RPM. J

 

[aesgator]

Re: Which prop?

My manual says my max RPMs is 5000. Still I'm going to look at finding a new tach on ebay.

 

[hwsiii]

Re: Which prop?

Gator, I ran your numbers through my software, and if you have a 2 to 1 gear ratio on your motor, I couldn't find the specs on your motor, I found you will be below 4,500 RPM and running somewhere about 25 miles an hour. I also found that the numbers you gave as per speed and RPM is not theoretically possible, as you would be running with a negative -9% Slip. I agree with everybody else I would check your tachometer and make sure you use a GPS for speed measurements as well as be sure you make your speed measurements in 180 degree different runs then add the two numbers together and divide by two, to negate any water current differences.

H

 

[aesgator]

Re: Which prop?

For the past year my boat has run 30 to 35 mph with two to three people in it, with tackle and a cooler. These numbers stay pretty much the same between two different GPS's, so I know the speed is accurate. The tach, I'm all but convinced is wrong, and I pretty sure its not the right tach for my motor, and doesn't have a setting on it that matches the poles on the alternator for my motor. So I'll be replacing it soon, and getting new RPM numbers for yall.