New propeller for my 15 hp 1980 johnson

[Mike Redmond]

Ordered a new prop for my old motor,original was a 9 1/2 by 10 new one that came in is a 9 1/4 by 10 blades are quite wider also,guy at the shop says he ordered original size but thats what came in,,will this propeller work just as fine without messing up my motor or should I refuse and wait for quite a bit again since I live out in the back of beyond
Mikey

 

[kstrayhorn]

The blade faces will have more surface area. I've replaced my original prop both with a new OMC/BRP one and a Solas one and both have larger faces than the original. 9 1/4 is just going to be 1/2" smaller diameter than 9 1/2. Mine ('86 Evinrude 15hp) had a 9.25x8 on it so that's just what I replaced it with. In any case, as long as you have the right spline count that one should work. There will likely be some performance differences with a smaller prop (If I'm right, less face area -> less water moving -> decreased acceleration / less resistance -> higher RPMs -> increased top speed) but with a 1/2" difference in diameter I'd think they'd be nominal.

 

[Pappy]

Well....to begin with....the difference between 9 1/4 and 9 1/2 is not 1/2. It is 1/4 or 1/8 per side, take your pick.
Secondly it all comes down to RPM. Without knowing what boat and what RPM the engine is turning the question is impossible to try to answer accurately.
We do not know whether this engine is trying to push the Queen Mary or an 8' hydro. We also do not know what the average load is or where it is placed or if there are any issues the owner is trying to get around.
Mikey, more information!!

 

[kstrayhorn]

Wow, you're right. I guess my math gets fuzzy that late at night. I was really just trying to let him know that my comparable motor came with and still runs the prop diameter he is asking about. I didn't intend at all to pretend to be an expert on the matter and I thought the wording made my uncertainty quite clear. Was I wrong to say that he would be fine?
From his question I gathered that the situation has become a hassle, and he wants something that will run. I didn't claim that there would be no effects, but I tried to let him know what I thought he might see. We don't have enough information, but even without details a general performance differential relative to what he had before should be able to be drawn.
If I am incorrect in any of this, please let me know! I absolutely don't want to continue spreading bad information.

 

[Pappy]

Sometimes it's better to not answer a question rather than try to sell a guess as fact. This is even more important when your answer can affect the person in the wallet!
At this point neither one of you knows what RPM your engine is/was running with propeller A or what changed in RPM with propeller B. Do you have the same prop? You sure? Same manufacturer? Same boat? Same load? Same elevation as the OP?, same engine manufacturer? Same horsepower? Same engine height? Same trim rod position? Same weight placement in the boat?
Even if you (we) would have had simple information like propeller A is Part number xxxxxx and he bought propeller B, part number xxxxxx and asked for a generalized answer you could have made a great answer if you had those same propellers. Again zero information in the question to help us help him.

Now....I am not coming down on you personally. I am trying to make this into a learning opportunity for anyone who may read it. It is a bunch easier to ask for more information about a question before risking an answer that may cause someone to spend money needlessly.
Please stick around and contribute, its all about helping.

Cheers!

 

[kstrayhorn]

No, you're perfectly correct. I guess I just didn't realize what I didn't know. I guess commenting when you shouldn't is easy to do when one is ignorant of his own ignorance. I just figured holding all variables constant other than diameter, it was a safe less-than-educated guess, especially since he already had the wrong part in hand. Thanks for steering me right though.

Hopefully OP can find his way back from "the back of beyond" and we can get some info to figure this out. I had considered moving up from 9.25 to 9.5 myself after I had seen someone else do it, so I'm curious to see if I made the right call by sticking with the OEM specs. But I guess you can never go wrong by not erring from what it was engineered and designed for...

 

[Mike Redmond]

Heres more info,the boat is a hand made by me,14 ft 6: weight when new was 350 lbs,it a john boat 4 ft on bottom 5 ft at the top,al built with 1/2 mdo plywood this why its so heavy lolll but man o man is it ever stable on the water,original prop on that motor was 9 1/2 by 10 new prop is 9 1/4 by 10 I weigh 200 lbs my fishing buddy weighs 240 lbs,add a 5 gal gas tank, ice box, fishing tacle, oars and the odd time a case of 24 beers(shhhhh dont tell the fishing cops loll)beign a John its a flat bottom boat when I am alone in it,it gets up on plane but with my buddy,,,,no way doesnt matter how I try to spread the weight it wont plane with both of us guys in there,,according to the guy behind the counter it will have more push at the start but at the end it should have a little less speed,,,but you all know how salesmen are most of em would sell their Moms to make a nickel.... that new blade has more surface then the old tough and its an original BRP new in the box prop hoping this is enough info ,,my fear was to over rev that old motor talked to a few old timers over here was told to stop worriying mabe they are right then again maybe not,this why I posted here quite a few guys on this site have a lot of knowledge about boats motors and fixer upper tricks,,so thanks to anyone whos got info on the subject,, Mikey
PS if I could I d post pictures but Im more in the old dinosores when it comes to the computer world,,, so have a magical day everyone ehhhhh. Mikey

 

[Pappy]

MIke, know your transom angle on the boat? Where are you located? Flat bottom instead of a semi-V?
Boat is 350, 6 gal fuel is 36 plus tank & line weight, lets say 44, cooler at 25? persons weight at 440, safety gear at 15, tackle at 15, anchor at 8. Engine at 86.
Comes out around 983# static weight on a 4' wide bottom. Keep filling in the blanks and we can go from there.
Since you built the boat yourself (way to go, by the way!) the transom angle will be all important to know.

 

[Pappy]

An easy way to figure any transom angle...........Transom angle is all important to being able to plane a boat easily. Need that number.

Place a carpenter's square with the long side ( 22 1/2" ) against the keel. When top edge of the short side ( 14" ) comes in contact with the transom, take the measurement between the inside corner of the square and the boat bottom. Each 1/4" is equal to 1 degree in slope of the transom.

 

[Mike Redmond]

When I built it, the sides were 18 inches from the top part of the 18 in to the bottom 5 inches were cut to meet the top, that would be the angle ,when built I chopped off an extra 2 inches on the transom for a short leg engine(16 inches left back there),will have to wait till the snow melts(yup still got snow out here)but its melting away another 2 weeks n the yard will be dry,,n the water on the lakes is still hard at this time of the year loll.But its a fantastic area for trout fishing Mikey

 

[Pappy]

Mike....give me the number of degrees of transom angle. Don't understand a thing about what you just wrote!!

 

[Mike Redmond]

I will find out whats the exact angle of that transom,gonna dig up an angle inspection tool I have some where in the house,,,what I meant was for example take a piece of plywood 18by 18,,mark 5 inches off on one corner,then draw aline straight up thats all I had on the plans,,but I will measure them degrees pretty soon were supposed to get rain for next two days,just hoping that the weatherman is wrong,,, Thanks for your assistance Mikey

 

[Mike Redmond]

72 degrees

 

[Mike Redmond]

Or if you prefer 18 deg past 90

 

[Pappy]

Thanks Mike!
Manufacturers normally use around 15 degrees or so. You are fine with that angle....actually better for your application.
Now, You may end up with a prop with less pitch for your normal two person application. May not be what you wanted to hear but something like what kstrayhorn is running may be what you need with that load. At any rate I do not think another 10" pitch prop will do the trick for you in your situation.
Your transom angle is working in your favor but am taking for granted you are running the engine tucked in all the way and still will not plane with the second person. If not that needs to be done and evaluated.

 

[Mike Redmond]

Never tried with motor close to the transom, always did my best to keep it at a 90 deg drop but will give it a shot as soon as the water softens up(lakes are still frozen out here yet,maybe in a week or two

 

[Pappy]

Good deal, Mike. You will notice quite a difference and will not need your other prop unless you want a spare.
Start all the way in and work your way out until it will not plane that two person load. Use different pin locations for your
varying onboard loads.

 

[kstrayhorn]

So you want it as far up/out as possible without preventing plane (find where it doesn't plane, then go back down one setting)? Recently put my outboard on a new boat and am having to revisit the issue.

If you're interested, I have some input on those props. Nothing beats the original factory one, but good luck finding one of those. I got up in a dry creekbed during duck season and spun mine. Replaced with a ~$100 OMC/BRP replacement that didn't last 6 months. Granted, I probably wasn't as easy on it as I should have been, but I was not at all impressed with the quality. Recently went with the cheaper ~$50 Solas, and haven't had any complaints, but it's been in the water once. At least the powdercoating seems to be holding up better though, and it runs like it never has before. Just my experience, take it as you wish.