Re-propping early 80's Johnson 25hp 2 stroke

[KC8QVO]

I am curious what you all think here about this one.

We have a 16ft Lund, sorta like a current model SSV, with a 25hp motor, both early 80's. The motor runs like a top, but likes to drink fuel. I was out with it this summer with 2 people, a spare motor (9.9hp Johnson from the late 90's/early 2000's), and a few tanks of fuel. That was all the boat wanted to run.

I would like to get some more thrust in the water. What is the likelihood of re-propping the motor to get it to push a bit harder? My ideal situation is to get the boat to break on to a plane with 4 people and gear, top end speed at that point doesn't matter - I just want to get it out of the water.

Then again it may be all together a loosing battle. I know I can't make more hp, but just thinking about the differences between a standard motor and a kicker motor made me think about the prop. A kicker is meant to push a big boat with a little motor, not fast, and in my situation I'm trying to push the same size boat just with more "push".

Another boat I know of recently was a 16' alumacraft with a 40hp. With 4 people loaded up with hunting gear it would still get up and go around 30mph. Big difference in HP, I know. All I want to do is get our boat on plane.

 

[KC8QVO]

Re: Re-propping early 80's Johnson 25hp 2 stroke

You know what, I'm not sure what year the motor is. I looked online and it looks like it might be early 90's. Here is a picture. Maybe we can dial in what year or range it is. Some other notes is it is electric start only (unless you take the shroud off I believe there is a way to rope start it) and the exhaust is thru-prop.

 

[SeaKaye12]

Re: Re-propping early 80's Johnson 25hp 2 stroke

The year can be determined by looking up the model number on the transom bracket. Just post it here if you don't know how to look it up. From looking at your picture...your shift lever is on the front of the motor...which indicates it's a 1990 or newer.

Many of the 25 HP Johnson/Evinrude motors can be brought up to 35 HP by either swapping out the carburetor with a carb from a 35....or in the case of the newer 25's...I've been told that it's just a matter of swapping out the main jet in the carburetor. Once you know which model you have...you can consider doing that.

As far as prop changes go...the only way to do it accurately is to use a tachometer. Take a look on the existing prop and see what size (pitch) it is...and we can go from there.

Chuck

 

[KC8QVO]

Re: Re-propping early 80's Johnson 25hp 2 stroke

AH. Now we're talking.... I like the 35hp idea. I'll have to try it.

There is a maintenance crew up there so I'll see if I can't get one of them to check the motor for me. I need the size/pitch of the prop too. Though, I know it is a stock prop. Whether or not it is the original one I don't know, but it would be pretty much what came with it spec-wise.

 

[steelespike]

Re: Re-propping early 80's Johnson 25hp 2 stroke

That looks like a pretty good sized 16. If its a bare bones boat,no floor,live well or other hindrances it should plane with a 25.
We had a 18ft bare Crestliner with a old Gale 25. It planed easily with 6 adults aboard.I think your 16 should do well with 4 aboard.
Un fortunately I don't know what size prop we had.
Weight placement and motor trim are important.Where were these 3 tanks of fuel?
Your 25 should use about 2.5 gallons per hour at wot. regardless if the boat is empty or full.
If use is higher you may have a leaky fuel pump diaphram or other fuel distribution problem.
As far as making it a 35 someone should be along with some info. It's surely not just a jet change
And I'm pretty sure not just a carb change.
Were talking about a 40% hp increase
I believe displacement is the same but I think the similarity ends there.

 

[KC8QVO]

Re: Re-propping early 80's Johnson 25hp 2 stroke

Thanks for the info. The boat will plane with the load we had, but barely. It wont take any more people though. The 3 tanks we had were 6gal each. There was one in the rear on the motor and the others were right in front of the rear bench. The spare motor we had was a johnson 9.9 2 stroke and rode in the bow, my brother was on the rear center bench. He fished off the bow and sat on the front center bench a couple times moving spots and that killed the plane-ability. It also shot water up over the boat, so I know what you're saying about weight distribution.

The boat has 2 floor panels - between the rear banch/rear center bench & between the two center benches, no panel for the bow. Aside from that it is a bare-bones boat.

As to the fuel consumption - just from memory I'd say we used more than 2.5gal/hr. I'll have to look at the map again but it seems to me we got about 15 miles on a tank/not quite 3mpg. At ~18mph that is 6gal/hr. Hmm that doesnt sound right.

 

[Willyclay]

Re: Re-propping early 80's Johnson 25hp 2 stroke

AH. Now we're talking.... I like the 35hp idea. I'll have to try it.

Before you make any decisions, you might want to read this thread first. Good luck!

http://forums.iboats.com/johnson-evinrude-outboards/converting-20-hp-johnson-35-hp-274566.html

 

[KC8QVO]

Re: Re-propping early 80's Johnson 25hp 2 stroke

Thanks for the link Willyclay!

It may be a while before I can get my numbers on the motor. I can't get in touch with them yet - left a message. This is sounding pretty exciting.

Just curious - was there a standard prop that was used on that generation 25hp motor?

 

[Willyclay]

Re: Re-propping early 80's Johnson 25hp 2 stroke

Glad to help. I have a 1985 Johnson 25HP and, as far as I know, there was no "standard" prop. You have to match prop size/pitch with boat weight, hull design, load and primary use. IMHO, the single best thing you can do to answer your prop questions is to obtain a tachometer to determine WOT engine RPM. Once known, that will give you a baseline for making decisions about the performance of your motor and/or prop swaps. Good luck!

EDIT: Looking at the picture you posted, that motor appears to be sitting low on a cut-down transom and also trimmed "IN". Mounting height of the motor is critical due to lower unit drag and that may be a long-shaft motor on a transom intended for short-shaft motors. A picture or two of the motor and transom from outside looking forward would help answer those questions. The trim angle issue on any motor without PT&T is always a compromise. You just have to find the sweet spot that works best for that boat through trial and error.

 

[steelespike]

Re: Re-propping early 80's Johnson 25hp 2 stroke

Before any soup up you may want to figurte out why it is using about 6 gph instead of 2.5.
Keep in mind that 2.5 gph. at wot is the same whether pushing a big pontoon or a little speed boat.
The 35 will use about 3.5 gph. but that 3.5 will go farther because you can go faster.

 

[KC8QVO]

Re: Re-propping early 80's Johnson 25hp 2 stroke

Thanks for the info. The motor/boat/etc is about 1000 miles away from me so I can't just run out to the dock, try something, and make a quick change, or take a picture for that matter I will try and get the numbers off the motor from a guy there but I don't think pictures are going to happen until I'm up there again. Next summer will be the closest chance to experiment with more. If I get the prop size I can call the local marina(s) and see if I can round up some used props in other sizes around that to try before making a decision. That may happen sooner - I could spare an hour of prop swap time on a short trip. The marinas may have a tach I can use too.

I don't know that when that motor was paired with the boat there was much "tuning" done, honestly.

 

[Willyclay]

Re: Re-propping early 80's Johnson 25hp 2 stroke

The motor/boat/etc is about 1000 miles away from me so I can't just run out to the dock,....

Wow, you have some extra challenges. Good luck!

 

[KC8QVO]

No Title

I got to checking the motor the other day. The prop that is on it is a 10x13. See the pictures below for detail. It does not look like it is in terribly good shape, either. Do you think that could be a cause for some (if not all) the power issues? Would simply replacing it with a like-prop be enough? Or would playing with the size/pitch do more for what I am after?

 

[jestor68]

The 13 pitch is listed as a "cruising" prop. That's next to the largest pitch(15) available. These are "light load" props.

The two "heavy load" props listed are the 9 and 10 pitch.

I believe that your boat, with all the stuff you describe in it, qualifies as a "heavy load" for a 25 HP motor.

 

[KC8QVO]

Is my logic correct in that a smaller pitch prop, same diameter, is going to push the same amount of water, just slower/more force? Would it not be a good idea, then, to bump the diameter up also - that way the prop is slicing through "more" water? I guess my thought is a smaller pitch is like more torque on a slim tire. If you boost the torque with the same tire you slip more, boost the torque and boost the surface area of the tire (bigger tire) you boost the available power to the ground, or in this case, the prop to the water?

 

[KC8QVO]

No Title

Here is a better shot at one of the blade edges on the prop - you can see how it is worn. How bad will that affect performance? Would filing the burrs off and balancing the prop be of an merit? Or is this one trash?

 

[KC8QVO]

Here is the model number of the motor:
J25TEETS

If I am looking at things right, that comes back to a 1993 model. I can't imagine it is that new, maybe it was an upgrade.

 

[steelespike]

As I've said many times he rte a boat like your 16' bare untiliy will do better with a load than a full dress 16.
Years ago our 25 Gale on a bare 15 ft lLone star utility would struggle with 4 people and gear. We put it on a bare Crestliner 18ft utility.
It planed easily with 6 adults and 2 children not fast but planed about 20 mph.
You do need to change to a 9 or 10" prop Probably a 9 because your better off under propped if it turns out to be wrong.
Check that it is actually on 2 cylinders. You wouldn't be the first not to realize you were on one cylinder except for the lack of power.
Cleaning up the prop will help but isn't likely to make much difference.
Empty the boat do a test run with you alone.A healthy 25 should run at 25 to 30 mph by gps.
As I said last year a 25 should use about 2.5 gph at wot.If yours is more you may have a leaky fuel pump diaphram.

 

[KC8QVO]

The clearance on the lower unit is 5-3/4" from the center of the prop shaft. So if I am doing my math right here the stock prop is a 10" so going to an 11" will still allow for 1/4" clearance between the prop blades and the guard (5-1/2" X 2 = 11", 5-3/4" - 5-1/2" = 1/4").

So what I am thinking is trying an 11x9. Hows that sound? A little wider and a decent cut on the pitch.

The prop is a 14 spline thru-hub exhaust. Is there anything else I need to know regarding the shaft specs?

 

[KC8QVO]

I talked to a guy at a boat store here and they can get props in for me, but they will not accept returns. That is a bummer because I'd like to try a few different sizes to see how the performance changes. He cautioned me on a 9 pitch prop saying that would be more for a pontoon boat and would not work for my application. He suggested dropping 2 gear inches on the pitch to an 11 (from the stock 13). If I understood his logic there is a 5% increase in power with every drop in gear inches = a 2 GI drop is 10% increase. Does that make sense?