Prop help for a 1989 20hp Johnson

[psteurer]

Hello,

I am running a 1989 20hp Johnson with the standard OMC aluminum 10 x 13 prop that likely came with the motor. My WOT with a fairly light load is 4600 rpm and speed is 21 mph according to a GPS. According to my OMC manual the motor is rated to run WOT between 4500-5500 rpm. The motor is on a 1963 14 foot aluminum Sea King runabout boat that weighs about 275 pounds.

I would like to get my rpm's higher. I have been looking at 10.5x12, 10.5x11, and 11x9 prop likely in aluminum. I could also go to stainless steel. I am thinking that the 10.5x11 would get me in the low 5000 rpm range and would be good. What would be a good prop diameter, pitch, and material should I go with?

 

[junior1113]

Re: Prop help for a 1989 20hp Johnson

i would run that close to 5500 to maximize engine life. a 2" drop in pitch will result in an increase of 300-350 rpms. id put a 9" pitch and get about 5300. good luck

 

[hwsiii]

Re: Prop help for a 1989 20hp Johnson

If your gear ratio is 2.15:1 as I show you should buy a 10" pitch and it will let you run maximum RPM at 5,500 and increase your load carrying ability as well as your hole shot. Just check the gear ratio though.

H

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks for the replies. I will check the gear ratio.

Are you recommending a 10x10? The only OMC props I think that are available are 10.5X11 and 11X9. I suppose I could buy an aftermarket prop. Any recommendation on whether I should go with stainless or aluminum. I am running in a deep lake and have very little chance of hitting something.

 

[hwsiii]

Re: Prop help for a 1989 20hp Johnson

You an buy a stainless but I don"t really feel you will gain much speed doing this. You can buy an inexpensive 10" pitch aluminum prop here.
http://boatpropellers.iboats.com/Johnson/20_HP_(1984-1997)_10.00-Pitch_Propellers/36/?**********=341917132&*******=215886655


H

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks. I checked my OMC manual and it said the 20hp Johnson had a gear ratio of 13:28 (0.465). If I divide 1 by 2.15, I do get 0.465, so I assume my gear ratio is 2.15:1.

Do you think an OMC 11x9 prop would work as well as a Vortex 10.5x10? I was thinking I would like to stay with an OMC prop since I do not know much about the Vortex props.

 

[junior1113]

Re: Prop help for a 1989 20hp Johnson

eigther one is going to put you real close to where u need to be. but hwsii has a program that picked a 10" with that gear ratio. so i would agree more with the 10". the 10 will have more top end power. the 9 would have more low end power. eigther prop will out perform that 13 your running now

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

I am going to order a prop today and was wondering whether I should go with the 10.5x10 or the 10.5x11. I am currently running the original 10x13 as stated above and get about 4600-4700 rpm's at WOT.

My OMC manual says I should get a prop that would run at the midpoint of the motors rated motor rpm which is between 4500-5500 rpm. Before I order, I was curious as to whether anyone knows an estimate of the rpms I would be turning with a 10" pitch and 11" pitch. Also, I will be increasing the prop diameter by 1/2". Is there a rule of thumb as to how much that would slow down the rpms?

 

[steelespike]

Re: Prop help for a 1989 20hp Johnson

Generally diameter is simply a function of the design and doesn't directly affect rpm.If your getting a prop ;check out the turning point rascal in a 10"
H usually gets prop selection right on.

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks. I just want to be careful with this old motor. It has probably never turned more than 4700 rpm. I suppose running close to 5500 rpm would not hurt. Would a rule of thumb for pitch on this size boat be 1"=175rpm's. With a 3 inch pitch decrease, I would then be turning 525 extra rpms ( 5225 tot) and a 2 inch pitch decrease, would be 350 extra or 5050 rpm's total.

 

[hwsiii]

Re: Prop help for a 1989 20hp Johnson

Psteurer, I just ran some numbers instread of just seeing what pitch prop to change to and I find that even with two people in the boat if that motor is working right you should be able to turn a 12" pitch prop, so don't buy anything yet. I will publish the numbers as soon as Photobucket is back online, so in my opinion you need to check the motor first. Check the butterfly in the carburetor and make sure it is all the way open and entered in the throat. hek plugas and compression.


H

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks. I bought the boat/motro 6 months ago and did quite a bit of work. This is an old style 1963 runabout boat with a 1989 motor. Compression is 118/120 on the two cylinders, new plugs, carb rebuild, link/synch which included the throttle plate in the carb completely open at WOT. I am running 2oz Sea Foam per gallon of mix. The motor runs great now. I just bought a new Teleflex tach and noticed the low rpm's at WOT. With 2 people in the boat, I run great (21mph). With 3 people (extra 150 pounds) in the boat I can just barely get on plane.

Here are some pictures if you want to see it running at WOT: http://rides.webshots.com/album/570373113GtbioD

I appreciate all the help from this forum. I am thinking a lower pitch will help with the speed and hole shot. Just don't know how low to go.

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Forgot to mention that I am running in a lake at 3000 feet above sea level.

 

[hwsiii]

Re: Prop help for a 1989 20hp Johnson

After looking at the pictures and seeing the hull design and how well you fixed it up, i would reommend you go with the 10.5 x 11 OMC prop and you will be happy. That prop will work excellent with that boat and motor.

H

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks very much for the advice. Would you think the Michigan Wheel brand prop would work as well as the OMC? I like supporting iboats whenever I need to buy parts.

 

[hwsiii]

Re: Prop help for a 1989 20hp Johnson

I had to go look and see whih model it was and it is their Vortex and it will work fine.


H

 

[countvlad]

Re: Prop help for a 1989 20hp Johnson

Thanks very much for the advice. Would you think the Michigan Wheel brand prop would work as well as the OMC? I like supporting iboats whenever I need to buy parts.

keep us updated we would like to know how things worked for you..

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Well, I got the 10.5x11 Michigan Wheel prop. What a disappointment. It did not fit. The blades of the prop would just barely hit the keel of the motor when I turned it by hand. I rechecked the spacer to ensure I had it on right. It was. I also checked to ensure I had the right hub kit. It was 118 which and said it was for my 1989 20hp Johnson. I am surprised it did not fit but will return the prop and probably go with an OMC 10.5x11.

I am turning a little better with my old 10x13 prop. It was a cool weekend with temperatures in the 50's and 60's. I increased my rpm to 4800 (from 4600) and also increased my speed to 22.5mph from (21 mph). It might have been the cooler weather or maybe using 2 oz per gallon of Sea Foam is blowing some carbon out of my motor. But I have picked up some rpms. I think I still need a little less pitch.

 

[steelespike]

Re: Prop help for a 1989 20hp Johnson

You might check your motor height.The antivent(cavitation) plate just above the prop even with trhe bottom of the boat is considered a starting point.
Raise it about 1/4" at a time to a point just before venting becomes a preoblem.Just add 1/4" sticks on top of the transom and be sure clamps have a good grip and it pumps water.Could get you a little better overall performance.
Careful placement and elimination of extra weight.And careful motor trim
can help.The turning point Rascal will deliver close to ss performance at aluminum prices.

 

[psteurer]

Re: Prop help for a 1989 20hp Johnson

Thanks. My motor is bolted to the transom so it would a chore to lift it. The cavitation plate is level with the keel so I guess I will just leave it there. I understand what you are saying though.

I just went to the Michigan Wheel website and saw this note for my prop:

REQUIRES USE OF XHS HUB KIT 118 - 3-1/4" GEARCASE - MAY INTERFERE WITH SKEG ON ANY MODEL

The Michigan Wheel does interfere with the skeg for my motor. It's funny but I searched there entire catalog and my motor series is the only one that has this note. What a bummer. It looked like a nice well made prop.

I went to the Turning Point website. I could not find a prop for my application which is a 20hp 1989 Johnson.