20HP Johnson prop

[tacoma_2002]

I've decided to keep my '92 15' polarkraft (not sure of model) until I find a larger boat that I'm dead set on, so I'm in the process of fixing her up a bit.

I've got several good chunks out of my "stock" ( I assume) prop over the past 10 years, so I'm looking to replace it and perhaps upgrade a bit. I've done alot of research and I know that I want Aluminum since I fish alot of rivers and smaller lakes, but I need a bit more "umph" out of the old 20. I do not have a way to measure my RPM's, but they seem to be relatively low at WOT and it takes a good 30 seconds to get on plane with my boat loaded with gear and a buddy in the boat with me. The current prop size is a 10x13. I was thinking of going with a 12 or even 11" pitch prop.

I'm running a semi-v all aluminum, that with me a buddy, boat, and all my gear, probably weigh about 1500+ lbs. By GPS she runs 15-20 depending on load and conditions. My thinking is that if I can get my boat on plane quicker and get a few more RPMs out of the engine I can run faster and actually put less load on the engine itself. Maybe even save a little fuel.

Alone with no gear, battery or trolling motor the thing jumps on plane in less than 10 seconds and screams, but loaded I'm struggling a bit....not to mention sucking fuel like a dragster.

I guess my questions are

What is stock?
What are you running/recommend?


I'm going to reconfigure my boat also (I'll show you in a upcoming thread) and add another battery and another (6gal) fuel tank along with another seat, so I'll be carrying another 100 +/- lbs by spring.

Thanks in advance!

 

[steelespike]

Re: 20HP Johnson prop

Welcome,You will very likely need a tach before we are through; but we can get started if you get me some more info.
Can you tell me the speed when she's empty and "screaming"?Also need the year of the motor?

 

[tacoma_2002]

Re: 20HP Johnson prop

Its a 1990.

I haven't GPS'd it fully unloaded. Estimated guess is probably 23mph? However, it acheives plane in about 5 seconds and hits top speed in probably 30 seconds.

The avg speed I achieve is around 18 +/- with a passenger and unloaded.

Thanks for the welcome!

I've lurked here for a while, buying odd and end boat parts from iboats, and I've always loved boating! I was a Ford tech for 4 years, and I've always loved working on things (ATV's, vehicles, etc)...however....I've only done a few minor things to my boats/engines (cleaned carbs, etc), so this is a whole new world to me!

I recently moved to Lake Village, AR and live alongside lake Chicot, so I've been on the water everyday I possibly can. The last time I went out I burnt 6gals of fuel and only covered less than half the lake (about 15-20 miles)...then again this was at WOT in fairly choppy conditions (6" wake or so).

 

[steelespike]

Re: 20HP Johnson prop

If we're going to be working without a tach we really need more info still need the year of the motor.
And it would be really helpful to know the dry weight of the boat or at least the style ie: a bare bones
aluminum utility with a tiller or a full blown model with floors live well steerring etc.. Describe your typical light load
tanks,batteries trolling motor etc.your weight. The performance,mileage and health of the motor depends on the right prop
We need 5 parameters to fully evaluate your setup. Your only able to give us one so I think you can see we,re scrambling for info.

 

[tacoma_2002]

Re: 20HP Johnson prop

The engine is a 1990 J20CRESB

This is a "light" load. 6gal fuel cell, deep cycle batt, live wells and cooler are empty/dry, and 8lb anchor. I'm 6'4" 230.

The bottom and sides of the boat are all thin gauge aluminum (along with the "flotation humps" in the back). I have the boat in my shop now tearing it down about to recarpet. All flotation/styrofoam is completely dry.

Engine holds a consistant 140lbs of compression on each. Boat hits on both cylinders and runs like a top (recent tune up). I've experimented with the trim a bit and I found if I lower the engine it struggles more to plane, and if I raise the engine it gets the bow up quicker, but when it achieves full plane it runs into an "air pocket" and over revs for a second then takes back off. I have it set at optimum trim (planes best and doesn't hit the "air pocket").

Prop is a bit chewed, but not terrible. I'd planned on replacing it before spring anyway, but I'm trying to figure out if I need to change pitch/diameter.

I also looked into upgrading the motor entirely with a larger size. Unfortunately, my rating plate is missing and I do NOT want to over power this thing.

 

[steelespike]

Re: 20HP Johnson prop

We're really up against it no really accurate speed and the prop size as the only accurate
detail and the boat laid up so no test runs loaded or unloaded.The apparent brisk performance lightly loaded
worries me if you go to a lighter prop you could easily over rev with a light load.
I think the best way out is to get the prop repaired and do some test runs lihght and loaded in the spring.
It would be very good the have a Tinytach so we can get gps speed,rpm and the prop size we know as 13.
I can get the gear ratio off the internet.If your boat is factory equiped and you have a name or model we may be able to find the weight.If a prop change is in order you can keep the old one for a spare.
By the way your motor should use about 1.8 gph(gallons per hour @ wot.whether loaded or un loaded.
2 gph wouldn't be the end of the world.
1.8 is based on about 9% of the rated hp. We usually use 10% to figure in your head but the 20 hp is easy to multiply.Was your trip 20 miles round trip or one way?
What was the total time at wot? and how much putting around?

 

[Mi duckdown]

Re: 20HP Johnson prop

have that prop rebuilt. chunks? without tach, and speed/ gps no one can answer this for you.

 

[Dirtscooter]

Re: 20HP Johnson prop

I've decided to keep my '92 15' polarkraft (not sure of model) until I find a larger boat that I'm dead set on, so I'm in the process of fixing her up a bit.

I've got several good chunks out of my "stock" ( I assume) prop over the past 10 years, so I'm looking to replace it and perhaps upgrade a bit. I've done alot of research and I know that I want Aluminum since I fish alot of rivers and smaller lakes, but I need a bit more "umph" out of the old 20. I do not have a way to measure my RPM's, but they seem to be relatively low at WOT and it takes a good 30 seconds to get on plane with my boat loaded with gear and a buddy in the boat with me. The current prop size is a 10x13. I was thinking of going with a 12 or even 11" pitch prop.

I'm running a semi-v all aluminum, that with me a buddy, boat, and all my gear, probably weigh about 1500+ lbs. By GPS she runs 15-20 depending on load and conditions. My thinking is that if I can get my boat on plane quicker and get a few more RPMs out of the engine I can run faster and actually put less load on the engine itself. Maybe even save a little fuel.

Alone with no gear, battery or trolling motor the thing jumps on plane in less than 10 seconds and screams, but loaded I'm struggling a bit....not to mention sucking fuel like a dragster.

I guess my questions are

What is stock?
What are you running/recommend?

<><><> It sounds to me like you have a good feel of your boat as it is set up now, and the difference in it's performance according to how it's loaded. We recently dropped from the stock 13 pitch prop in about the same condition as yours to a new 11 pitch and took the strain off off our 28 Johnson. It's now running about 6MPH faster and the RPM is about right according to the sound. I believe if you listen to automotive engines for enough years your ear tach is fairly reliable. You can do as we did and wait for a prop deal on Ebay If nothing else it's almost a sure thing that an 11 pitch would be a good thing to keep onboard for when you have a heavy load.. Is the 20 old enough to be a shear pin prop? at/SIZE]

 

[steelespike]

Re: 20HP Johnson prop

Not sure whats going on here. We seem to have resurrected a 6 month old post And included a possible solution.
You really can't prop correctly without a tach. A 2 stroke doesn't have the same feel or sound that a 4 stroke especially compareing an automotive sound to a 2 stroke outboard sound.
There are those out here that can't tell a 2 cylinder outboard is running on one cylinder except for the lack of power.
you gotta have a tach and gps for speed to get it right.
Prop is a thru hub 14 spline.