Prop size for 18' 120 johnson

[Nurrenbern]

I have a 1979 baja runabout with a 1990 120 johnson. I have a 4 blade 12 3/4 by 21pitch prop on it now. Just wondering what would be the best size and pitch for this boat setup. I would like to have more speed only running about 45mph now.<br />Any suggestions would be appreciated.

 

[Walker]

Re: Prop size for 18' 120 johnson

Do you have a tach? What rpm's are you turning?

 

[Dhadley]

Re: Prop size for 18' 120 johnson

To give any useful advise we have to build that map. Tell us where you are now. We have the speed and pitch but we need the rpm. What brand is the prop? From that we can calculate the slip %. <br /><br />Tell us about your set up. We assume there is no jackplate. Are you comptemplating one? Generally speaking youll need a jackplate to get the most out of a good 4 blade.<br /><br />Then tell us where you want to go -- light load, top end only -- skiing -- holeshot etc.

 

[Nurrenbern]

Re: Prop size for 18' 120 johnson

Thanks for the fast reply only running about 5200 rpm wot dont have a jackplate not for sure on the brand but is a Stanless prop. Not planning on getting a jack plate.

 

[Dhadley]

Re: Prop size for 18' 120 johnson

Youre at about 14% slip now. Not great but not bad. Can you raise the motor any? IE -- is it mounted all the way down on the transom now?

 

[Nurrenbern]

Re: Prop size for 18' 120 johnson

Im pretty sure I can raise it up some not sure how much. Would a bigger diameter prop help any?

 

[Dhadley]

Re: Prop size for 18' 120 johnson

I'm a big fan of larger diameter simply because we find that the larger the diameter, the higher you can mount the motor. Actually anytime you increase blade area you can increase the X dimension. Obviously raising the motor now would be a good idea. It will tell you how that hull, with that prop, responds. <br /><br />Now, without a jackplate the smaller diameter 4 blades are kinda what youre stuck with in a 4 blade. I'd still try anything in a larger diameter if you get the chance but as far as spending money on technology, I'd concentrate on 3 blade props. Try something in the 13.5" range like a Raker or Turbo Lightning Jr. Probably in the 20" pitch. <br /><br />Get the motor as high as you can and watch the water pressure. You should be fine. Dont worry if the small 4 blade vents in a tight turn. Right now we're looking at how the rpms respond to height in holeshot and top end.