correct prop size for a 40 horse johnson

[brehlers69]

My brother runs a 1975 40 horse johnson on a 16 foot semi v flat bottom jon boat and doesnt think he has the proper prop on. We run on the missouri river up here in north east nebraska duck hunting and fishing. Can you give me your expert opinion for what size he should have on. thanks

 

[Mark42]

Re: correct prop size for a 40 horse johnson

Why does he think its the wrong prop? What is the RPM at WOT?<br /><br />I can't tell you what is good for your brothers boat. That depends on a bunch of factors, such as weight, hull shape, HP, profile, etc.<br /><br />But I can give you an idea of how my 40hp is set up. My MFG is 14' 6", weighs about 900 lbs, (motor, boat and accessories) and is a planing hull. The motor is a freshly rebuilt '93 40hp. I started with an older 13 pitch OMC non-cupped prop and it was easy to over rev the motor to the point of hitting the built in rev limiter at 5800 rpm, even with a full load. I switched up to a 15 pitch cupped prop (Turning Point Hustler) and the tach now reads 5000rpm wot even with a load. This is a good setup for my boat.<br /><br />So your brothers boat being 1.5' longer, probably a few hundred pounds heavier, might need the 13 pitch. At least it is a starting point.<br /><br />Hope that helps some.<br /><br />Mark.

 

[brehlers69]

Re: correct prop size for a 40 horse johnson

he has an aluminum 16 foot semi v john boat. I dont think it weighs over a couple hundred pounds with the motor on the back. He has a 40 horse short shaft 1975 johnson. We run it on the river and Im sure he doesnt have a tac on it. I looked at the prop and all i got was 012022 and the brand Michigan. He wanted to get a new prop but doesnt know what this one is. Just wanted to know what size is best for his new purchase.

 

[NathanY]

Re: correct prop size for a 40 horse johnson

012022 is a 10 3/8 x 12.5 The OEM number for that prop is 378580. Also you did not say that was a pin-drive motor, so props might be harder to find unless you buy brand new. Get a TinyTach and see what the actual RPM's are.

 

[Paul Moir]

Re: correct prop size for a 40 horse johnson

What he's got on there now is a 12.5" prop (10-3/8 x 12-1/2 to be precise). That does seem a little light for your rig or better suited to a heavier rig. But without a tach, this is all just guessing.<br /><br />A Tiny Tach might make sense: http://www.tinytach.com/ <br />EDIT: Nate posted while I was typing - looks like we're thinking alike... Just to add - brand new you have a choise between OMC (BRP now) and Michigan Wheel for props. No real difference.

 

[Mark42]

Re: correct prop size for a 40 horse johnson

If you think it's really light and want to take a shot in the dark, switch from the 12.5 pitch to a 15 pitch. Remember that the motor weighs in around 180lbs all by itself. 6 gallon gas can, add 50lbs, the rest you can figure. Maybe the weight of the boat is on the hull plate.<br /><br />The boat will go faster at idle with a larger pitch. It's the first thing I noticed with the new prop while idling through the marina. Went from a stroll, to a fast walk.

 

[Paul Moir]

Re: correct prop size for a 40 horse johnson

Mark, don't forget the '75 40hp has a gearcase ratio of 12:21, while I belive yours has a ratio of 12:29. IE, at 5000 rpm your propshaft is spinning 2070rpm, while his is spinning over 2800rpm. <br />So an inch of pitch on his prop is different from an inch of pitch on your prop by about 35%. Now that's a theoretical comparison and reality is going to be different, but you see what I'm saying?

 

[Mark42]

Re: correct prop size for a 40 horse johnson

Paul,<br /><br />I wasn't aware of gear ratio differences. I can see how that will change things a bit. So he should be running a lower pitch for the same conditions, right?

 

[Paul Moir]

Re: correct prop size for a 40 horse johnson

Mark42: exactly. I figured you had just overlooked the fact that he has a 1950s technology gearcase. <br /><br />I figure he might end up with a 13" or *just* maybe even a 14" prop, but without the tach we won't know. In fact, I don't think they (OMC) ever made a pin-drive prop steeper than 14" to fit the 40hp.

 

[jimmbo]

Re: correct prop size for a 40 horse johnson

Most of these old 40hp had props in the 11 to 12 inch pitch range and these were on little 14 ft fiberglass hulls. Also the rpm range is 4000 to 5000.