1984 Johnson 15hp prop

[Pussiwillow]

Hope this is the right section for this, I have a 12 foot Jon boat flat bottom,
It's got a full wood deck and what not set up nicely, boat and deck weights about 180 combined, me and my fishing buddy combined is about 350 pounds, I got a Johnson 15 hp 84' it's got a 9.25x10 prop on it, the boat accelerates like a beast! For what it is, I'd like to turn it into top speed, I'm pegged about 18-20 mph, I want a 13 pitch prop or so but the biggest I've seen yet for my motor is an 11 pitch, anybody know where I can find what I'm looking for? Or know of other years, models, brands that will fit my motor? It's a 13 spline shaft, thanks!

 

[5150abf]

Re: 1984 Johnson 15hp prop

You prop an outboard to reach a specific rpm not a speed, that is what it is once you find the correct prop.

You need to know the rpm's of the engine at wide open throttle to determine which way you need to go on a prop, a bigger prop may lug the engine which is bad.

They have 11 pitch props here at iboats but I would find out what the engine is turning now before you got one but here is whahttp://boatpropellers.iboats.com/Johnson-Outboard-Propellers/?pitch=11.00&chart=7&engine_id=27&*******=149479659t iboats has

 

[5150abf]

Re: 1984 Johnson 15hp prop

Link didn't work for some reason try this http://boatpropellers.iboats.com/Jo...=11.00&chart=7&engine_id=27&*******=149479659

 

[steelespike]

Re: 1984 Johnson 15hp prop

You should really have a tach before making a prop change.On small motors a 1" pitch change is more effective than on a larger
hp motor.Without an rpm reading is impossible to tell how well your setup is working.
By your description I'm assuming (you know what they say about ASSume)about 6000 rpm if 20 mph is accurate that indicates about 14% slip thats really high.The motor is rated for a max 7000 rpm.
At 6000 the boat should run about 22
That could indicate something is wrong with your setup.I bet the motor is too low on the transom.The antivent plate(just above the prop) should be about even with the bottom maybe up to an inch higher than the bottom.Have you tried trimming out?
Does the boat get up on the water or does it seem to lay down flat with lots of spray. What is the width of the bottom of the boat?
Is it a full 12 ft or a little short,usually indicates a smaller version.
I did a little figuring By your description I figure the decks total about 80 lbs I figure your total weight must be about 570 lbs
motor gear gas and passengers.This is very likely over the rating of ther boat which is probably under 400lbs.
Didn't figure batteries or trolling motor anchor etc.
This 570 could also account for the slow speed.

 

[Pussiwillow]

I know it's got to be overweight had no tags bit I guess rating is 350 pounds, but it sits up nice on the water and planes out very fast with minimal spray I'm going to get a tac very soon, my cav plate is about 1 inch under the boat

 

[Pussiwillow]

It is 11' 11" so it's pretty dead on with a 32 beam so a 1232 , no battery anchor or trolling motor on my boat

 

[Pussiwillow]

Also my mistake, the motor is 2 inches under bottom
Of boat, could this cause bad slippage? My motor is odd it had a 17 inch gap between mounts and cav plate my boat has a 12 inch transom I put a 3 inch jack plate on it and a piece of aluminum stock to bring it up, I'd make my own but 3m 5200 kind of bonded the plate to the boat completely and can't really move it unless I spend all day picking at it but if it's worth the hassle I'd bring it up atleast level if not inch above, what do you guys think

 

[steelespike]

Re: 1984 Johnson 15hp prop

I assume you mean the antiventplate is 2" below.
If its 2" below you may be able to raise it a total of 3" if it will work well about 1" above the bottom. This should surely help
speed some I've seen some gain a couple of mph.I'm sure at 32" wide its struggling a bit to get up on the water.This could temper
your gain but still should handle better.The motor is very likely over the rating as well.The 12" transom is an indsication as well
probably rated for 6 or 8 hp.

 

[Pussiwillow]

Hey thanks for the detailed replies, ya I'm positive my motor is way past the rating of the boat and the boat is surely overweight, but doesn't act it, I can plane at 7 mph, when I built it I made certain that I balanced it out very well, the 32 inch beam can be hurting me the most tho as it probably does sit in the water deeper then I'd like, I mean I'm happy with the speed and power and have no complaints other then I expected more but it's all making sense for sure, there is no way I can bring the motor higher that jack plate is now part of the boat haha, glued with 5200 and 6 half inch bolts, the 5200 won't budge, anyway, I always thought the very lower plate over the prop was a cavitation plate, maybe I misunderstood, the plate right over the prop is the one that's supposed to be 1 inch above the boat right? I ask because the plate above it is 1 inch higher then the bottom, thanks again this is my first boat so I'm still learning

 

[5150abf]

Re: 1984 Johnson 15hp prop

Every boat is a bit different but running the bottom plate even with or just above the bottom of the boat generally works best, I have a 6'' setback on mine and run it almost 2'' above and it works fine, of coarse I have a bigger boat and engine and it is a V hull, every boat is different and you have to try it to see what it will do but it gives you a future project.