Prop pitch for 15hp converted Johnson

[Toolbagtyler]

I have a 1992 Johnson 9.9 that I installed a 15 hp carb on. It’s on a 92 Tracker guide v16 with a small front deck. Gained about 4-5 mph on average with different weights in the boat. Currently have a 9 pitch prop on and got 14mph at 5650 rpm with two guys, two 29 group batteries, two trolling motors, 6gal tank and gear. Got 17 mph at 6500 ish rpm alone. If I put an 8 pitch prop on do you think I would gain more speed when loaded? Would I over rev alone? I feel like I’m right on the edge of plane by myself but just can’t quite get there.

 

[Weldorthemagnificent]

That's a lot of boat for that motor. Have you considered a hydrofoil for a bit more lift?

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[Toolbagtyler]

That's a lot of boat for that motor. Have you considered a hydrofoil for a bit more lift?

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Yes I have one on there. Yeah I know it’s not the ideal motor. I’m just trying to get the most out of it. Most lakes I fish are troller only or 20hp limit. I was gonna try to get a 20 but it wash much cheaper to try the carb swap. I’m happy with the results for what I need, like I said just trying to get as much as I can out of it.

 

[Pappy]

when you get down into the lower pitch propellers you will normally find that their overall profile is not designed for speed. They are designed to plane larger loads.
The 9" pitch prop you currently have is the one to stay with. 6500 with one person is in the ballpark. That engine can turn more RPM but an 8" pitch propeller is almost like a fan blade instead of a propeller!!

 

[Weldorthemagnificent]

20 can be souped up to 25 or 30...just sayin. Lol. I have a similar sized/equipped 16' and I run a 35 johnnyrude. A 15 would plane it with no batteries and deck etc... Around here a good 9.9 goes for as much as a 20Hp.

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[Toolbagtyler]

when you get down into the lower pitch propellers you will normally find that their overall profile is not designed for speed. They are designed to plane larger loads.
The 9" pitch prop you currently have is the one to stay with. 6500 with one person is in the ballpark. That engine can turn more RPM but an 8" pitch propeller is almost like a fan blade instead of a propeller!!

Yeah I just wasn’t sure if I should waste the time and money to see if the 8 pitch would plane. Cause I’d imagine if I got on plane I would gain a little speed due to not plowing. Guess there is only one way to know for sure haha

 

[GYPSY400]

I'd keep what you have for now, but try and lighten the load as you have a lot of boat for that motor. When money allows ( and covid prices calm down) look into a 20 or 25hp. If you get a 25 or larger, you can buy a decal kit for the cowling to make it look like a 20hp.

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[Weldorthemagnificent]

The difference between a 20 and 25 johnnyrude is simply a restriction plate between the carb and motor. To go to 30 HP it's a carb swap. Most game wardens know to check the model number on the clamp frame despite what the decal on the cowl says. So if fishing 20 HP restricted lakes, I'd go to a 20 and remove the restrictor. I'd imagine you'd be happy with the gain to 25 HP. The 9.9/15's are great motors on a 12' or light 14 but you're asking a lot from it on a 16 equipped like yours.

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[Pappy]

when you get down into the lower pitch propellers you will normally find that their overall profile is not designed for speed. They are designed to plane larger loads.
The 9" pitch prop you currently have is the one to stay with. 6500 with one person is in the ballpark. That engine can turn more RPM but an 8" pitch propeller is almost like a fan blade instead of a propeller!!

Yeah I just wasn’t sure if I should waste the time and money to see if the 8 pitch would plane. Cause I’d imagine if I got on plane I would gain a little speed due to not plowing. Guess there is only one way to know for sure haha

A prop change is not done to stop "Plowing".
That is accomplished by moving weight around and trim pin position on the engine. With the boat/engine combination you are running this has to happen more often than if you went to a larger engine.
With the boat in my avatar I change the trim pin Position depending on whether I am running a one or two person load and that is with a 35hp engine on a light 16'.
There is more than one way to know for sure and you are in the right place to learn this .....or you can waste your money..."haha"

 

[turbotodd]

been through this.

motor is too small for that hull. I wouldn't use less than a 20hp.

I also understand some waters are 10hp max, as was the local lake for eons. They lifted the hp restriction finally. If that is your case, you have no other choice.

When you put the 15hp carb on it, you're not 15hp. Did you change the tuner? Reed spacer? If not, you have maybe 12hp. If you do the tuner and everything else, which requires removal of the powerhead, there is a sizeable gain particularly from 5000 rpm and up. Max rpm on a true 15hp is 7000.

if you don't have a max hp on the waters you are using, look at going to at least a 20hp. You don't have to run full throttle all the time, you can back off after you are on plane, and your fuel usage and noise will decrease drastically.

I find it dangerous sometimes to be on open water with a rig that is underpowered, in that if/when a storm blows in, you need to be off the water asap. Plowing water isn't gonna get you off the water very quickly. And, it certainly won't let you skim the top of the chop. At that point you are a displacement hull and subject to riding the chop, peak and trough, which can get real rough and leads to capsizing if you can't handle it.

 

[Toolbagtyler]

been through this.

motor is too small for that hull. I wouldn't use less than a 20hp.

I also understand some waters are 10hp max, as was the local lake for eons. They lifted the hp restriction finally. If that is your case, you have no other choice.

When you put the 15hp carb on it, you're not 15hp. Did you change the tuner? Reed spacer? If not, you have maybe 12hp. If you do the tuner and everything else, which requires removal of the powerhead, there is a sizeable gain particularly from 5000 rpm and up. Max rpm on a true 15hp is 7000.

if you don't have a max hp on the waters you are using, look at going to at least a 20hp. You don't have to run full throttle all the time, you can back off after you are on plane, and your fuel usage and noise will decrease drastically.

I find it dangerous sometimes to be on open water with a rig that is underpowered, in that if/when a storm blows in, you need to be off the water asap. Plowing water isn't gonna get you off the water very quickly. And, it certainly won't let you skim the top of the chop. At that point you are a displacement hull and subject to riding the chop, peak and trough, which can get real rough and leads to capsizing if you can't handle it.

From the research I have done the 92 9.9 adopted all the change(tuners, reeds etc) and the only difference was the carb. My local lakes are a 20hp limit or trolling motor only so I’m just trying to make sure with what I have until I can upgrade all together. I’m pleased which the increases I made and was just looking to see if I can get any better.

 

[turbotodd]

when u decide you're tired of it, you can get a 25 yamaha, and order a set of 20hp decals for the cowling. Or just put a 20 cowling on it. F20 models from 2019 and newer are the exact same motor as the 25, ecu is different and I think that is the only difference (but I haven't looked that closely). Or a 20hp Johnnyrude, with 30 mods.

 

[Crazyboat]

Do a search for PROPGODS online, the guy will ask load, boat, condition questions and get you the prop you need to optimize your ride. The advise is free, you don't have to buy a prop from him, but if you need one it would be nice.

 

[Grumman338]

I have a 1992 Johnson 9.9 that I installed a 15 hp carb on. It’s on a 92 Tracker guide v16 with a small front deck. Gained about 4-5 mph on average with different weights in the boat. Currently have a 9 pitch prop on and got 14mph at 5650 rpm with two guys, two 29 group batteries, two trolling motors, 6gal tank and gear. Got 17 mph at 6500 ish rpm alone. If I put an 8 pitch prop on do you think I would gain more speed when loaded? Would I over rev alone? I feel like I’m right on the edge of plane by myself but just can’t quite get there.

 

[Grumman338]

9.5x10 prop was standard on 15Hp Johnson

 

[airshot]

6500 rpm...that motor won't run long! I am thinking 5500 rpm max for that motor, I had a 1994 converted. Your pushing a lot of boat and weight for only 15 hp. I would move to the 10" pitch to save your engine, or better yet...get a bigger motor!! You need it for he weight your pushing !

 

[wmk0002]

6500 rpm...that motor won't run long! I am thinking 5500 rpm max for that motor, I had a 1994 converted. Your pushing a lot of boat and weight for only 15 hp. I would move to the 10" pitch to save your engine, or better yet...get a bigger motor!! You need it for he weight your pushing !

I have the same motor and the factory service manual. It lists the max rpm range as 5500-7000 for the 15 hp model and 5000-6000 for the 9.9hp. I was skeptical at first too but when I bought it, it came with a 8" pitch prop and was turning pretty close to 7000 rpms. I don't use it much but I've never had an issue and it sounds fine at WOT at those revs.

 

[wmk0002]

I have the same model motor with the same upgrade, done by a previous owner. When I bought the motor, it came with a 8" pitch 3 blade aluminum prop and was turning 6900-7000 rpms with me and a light load in my 1648 flatbottom. If memory serves, that was pushing me at about 19mph....maybe bumping 20 which was pretty good for that hull imo. The 8" prop had really good holeshot too. I tried a factory 10" prop next and it maybe gained 1 mph more max but really hurt it out of the hole and the rpms dropped of a ton. I later bought a 9" pitch 4 blade aluminum prop and it has been what I mainly use with it ever since on that hull. It's a little faster than the 8 and still has good holeshot. Main thing I like about it is that I like where the rpms are at when I'm solo or with a partner so I can just leave it on as an all purpose prop vs changing them out to match the load and performance stays pretty similar.

I have had a Yamaha 25 on it for a few years now so the 9.9 is now primarily used on my 1440. Still using the 9" 4 blade but probably could go to the 3 blade 10" one

 

[airshot]

I am gonna bet there were some misprints on those service manuals, looked up my old omc info and the factory states maximum sustained rpm is 6000, max overall at a nonsustained rpm 6250. Recomended max rpm was 5500-6000. Your motor, you can run it however you want. Been a boat owner for over 60 years, mostly outboards, I can assure you your motor will be short lived at 7000 rpm, but, that is your choice, good luck.

 

[Grumman338]

Hey You all !
Dont worry I am never going to do 7000 RPM regardless I still do 55MPH in a 65MPH lane in the canoe.