Need prop help

jhicks79

Cadet
Joined
May 1, 2012
Messages
11
I need some direction for a prop. I just bought a 19ft lowe deck boat with a 120h johnson. The motor will easily tur 5,200 rpms, at 29 mph. It never seems to get on the pad like it should, plus when you turn, it tends to blow out. I was running 29 mile an hour, and was attempting to turn around, slowly, when the engine reved up, and the boat slowed down. It has a cheap looking alluminum prop that has been used alot. I don't know anything about pitch. I'm looking for something to pull skiers, and tubes with, so I need something to get up quikly, but I would like to have some speed also. Thanks for any suggestions.
 

JoLin

Vice Admiral
Joined
Aug 18, 2007
Messages
5,146
Re: Need prop help

Sounds to me like your motor is set too high on the transom and turns are pulling the prop out of the water. I wouldn't look at changing my prop until I had that straightened out.

Also, you don't say what Johnson's recommended max rpm is for your motor. My 1996 175hp Johnson was rated to 5800 rpm and I propped it so it'd hit max rpm at full throttle with a normal load of fuel and passengers.

My .02
 

jhicks79

Cadet
Joined
May 1, 2012
Messages
11
Re: Need prop help

My motor is rated for 5800rpm's. The motor doesn't set on a jack plate, but it was put on at the factory where it sits know. I'm still working on some carb issues to help me get to 5800. Now the only boat's I'ver ever owned where bass boats, which is totaly different, but at WOT, I would bump the trim up until the RPM's maxed out. I tryed that with this boat, and it jumped from 5200rpms to 6000rpms but it actually slowed down a little. I'm blowing out bad for some reason. Just as an opinion the prop blades are dull and show some wear, but it just looks small to me. The last motor I had was a 150 black max, with a quik silver prop, which is alot bigger than the one I have now. Thanks
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Need prop help

Its very likely you need to upgrade your prop its possible the po put on just any prop to sell the boat.
Look near the base of the blades or under the prop nut for numbers _ _ X _ _ diameter X pitch.
get us the info in the stickys at the top of the prop forum.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Need prop help

We still need the info requested in the stickys at the top of the prop forum.
Do you know the gear ratio of the motor? If not then we need to know the year.
If we assume the prop is original then it's safe to replace it with a similar size prop.
Pitch is the primary consideration. Diameter follows the design.
A prop is a compromise so your not likely to get best top speed and best holeshot from one prop.
Off the top of my head I don't think a 19 will do both. Usually you can find a prop that will do well at both
but not the best possible of each.
In testing for best speed putting some weight at the stern(a passenger perhaps) might be enough to minimize venting
for s straight line test.From your description it sounds like a new prop may be a good idea.
 

jhicks79

Cadet
Joined
May 1, 2012
Messages
11
Re: Need prop help

I'm not sure of the gear ratio. The motor is a 1993 Johnson 120hp outboard, on a 19' Lowe deck boat. The best I can tell it is a factory oem alluminum prop, which is pretty rough. 13"X19 pitch.

I'm thinking the blowing out problem when I attempt to turn is a spun hub. I'm replacing that for sure.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Need prop help

Usually a spun hub once present occurs at about the same power range and continues to get worse to the point of not being able to plane.
If the prop is almost 20 years old it could actually worn out. Al props can wear down and dings and such cause grip problems.
It might be cost effective to have it repaired and try some new test runs.If a change is in order you can sell it or keep for a spare.
 

jhicks79

Cadet
Joined
May 1, 2012
Messages
11
Re: Need prop help

I found a SS prop that was just recondition at a prop shop. I'm going to compair cost verses a new one and go from there. I will keep my old prop as a spare. Never can tell when you might need one! Thanks, I really appreciate all your help.
 

michaud500

Petty Officer 3rd Class
Joined
Sep 3, 2011
Messages
91
Re: Need prop help

I am having the same issue! When I turn I lost speed and the engine reved. Than when I would try to go WOT my boat would vibrate at high RPMs. The one thing that I noticed with my prop is when I torqued it down the hub seemed to move with the nut. How much flex is there with the rubber hub? Might be worth checking.
 
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