proper prop?

[devindreb]

Help Please....<br />I recently bought aq '72 fiberform trihull open bow 18' boat. I placed an inherited 1978 johnson 85hp outboard motor on it. it runs great and idles fine. however at full throttle it only sounds(no tach) like it is running at 2000 rpm and only reaches about 10-15 mph. the motor came off a old cabin cruiser. is a different prop the answer? I am frusterated with the output and it seems obvious the motor can do more. if so what prop pitch and size is revommended? I have no idea what pitch it is right now but it is a 3 blade 14" prop right now. thanks.

 

[Sherman]

Re: proper prop?

You really need to find out the pitch and dia. of your prop. This should be stamped on the hub at the base of the blades. 14 x 12 is a 14" pitch and a 12" dia. If it is a 14"pitch and the engine will only run 2000 rpm you will have to decrease the pitch to a 12 or less. You should a least hook up a temporary tach. I would think you should be looking for about 5100 rpm.

 

[Sherman]

Re: proper prop?

Sorry had brain fade the pitch is always the last number My 115 evinrude is a 13 x 19 13inch dia. 19 inch pitch.

 

[Hooty]

Re: proper prop?

The next thing ya need is a tachometer.<br /><br />c/6<br /><br />Hooty

 

[walleyehed]

Re: proper prop?

If you are operating this at 1500ft above sealevel or lower, a 13.75"X15" should put you real close to the W.O.T. rpm range if your eng. is in good operating condition.

 

[boating brad]

Re: proper prop?

hooty is right. that "sounds like" stuff is not to reliable after idle. if you want to prop it right you have to get a tach.

 

[devindreb]

Re: proper prop?

Thanks guys I am going to check it out and get a tach. will let you know the results.

 

[Dershman]

Re: proper prop?

New prop is my next improvement project. How do I install a tach on my 1980 merc 40?

 

[Davejr62]

Re: proper prop?

I have a 1978 85 hp johnson v4 on a fairly heavy 1984 sunrunner fiberglass boat,15'. The motor came with a 17 pitch prop, I had to go to a 13 pitch prop for watersports, skiing, etc. It jumps out of the water instantly. I get about 5200 rpms trimmed out. A 15 pitch prop is perfect for me if I'm not doing watersports, for fishing on big lakes.