prop question on 1983 johnson 50hp

[jwilkey84]

my buddy and I have a boat with a 1983 johnson 50hp motor on it. The prop it has on it right now is a 12-1/2x13. I don't know what this number means for sure. Previously this particular boat and motor have not been together as a unit. The boat is a pretty light 14' fiberglass runabout. It has a ton of power and gets out of the hole quick, but we were just wondering if putting a diffenent pitch prop on would give us some more top end speed. Any recommendations would be appreciated.

 

[steelespike]

Re: prop question on 1983 johnson 50hp

We really can't help much without your max rpm and speed.
It all depends on those figures.The health of your motor depends on operating within its proper rpm range.We would usually reccomend optimizing your setup before making any prop suggestions.Vertical height has the most affect.
raising as little as an inch can gain 2 to 3 mph depending on its present setup.Be sure it pumps water whan if you raise it. Too much height will result in ventilation( prop loses its grip)Prop needs to be in good shape.
Prop size is measured in inches 12 1/2 is diameter 13 is pitch.Pitch is the number of inches it moves forward in one revolution,disregarding slip.

 

[jwilkey84]

Re: prop question on 1983 johnson 50hp

I will try this weekend to figure out max speed and rpm's if possible. I just got a new handheld GPS so the mph will be easy, but I am not sure how I will measure my rpm's

 

[steelespike]

Re: prop question on 1983 johnson 50hp

Sometimes a marina will let you borrow a shop tach.or the tech will go with you for a reasonable charge. You could get a Tiny Tach very easy to install and will work on any similar motor without regard to pole settings etc. about $40 there is a web site.

 

[jwilkey84]

Re: prop question on 1983 johnson 50hp

Well we tried out the boat yesterday. It has a new coat of frest paint on it. Same 50 hp motor. The boat by itself weighs 570lbs. My buddy and I weigh 300lbs together. So 870lbs. plus the weight of a 1983 johnson 50hp and 6gal. of gas and a battery, and we clocked it on the GPS at 29 mph. Are we getting the maximum speed out of this motor?

 

[jalapeno]

Re: prop question on 1983 johnson 50hp

My 83 50hp Johnson has a 12 1/4 X 15, and that seems to be a pretty standard size according to all of the after market manufacturers. There is even a 17 pitch one I saw, but I don't know much about props, other than what is on my boat and what I have researched concerning that. I don't know anything about RPMs vs Prop Pitch like these other fellas that are obviously more educated in the outboard motor world.