A modern prop for an older Johnson?

[jrs_diesel]

I have a Johnson 6 HP, that is nearly 40 years old (it's a 72) with it's original 3 blade aluminum prop.

I also have a new 6 HP Mercury, and looking at the 2 props side by side, I am amazed at how far prop technology has come.

Anyway, I'd like to get a newer, more efficient prop for that old Johnson. Does anyone know of a company that makes a modern prop for older outboards?

 

[Bondo]

Re: A modern prop for an older Johnson?

I also have a new 6 HP Mercury, and looking at the 2 props side by side, I am amazed at how far prop technology has come.

Huh,..??..??

 

[jrs_diesel]

Re: A modern prop for an older Johnson?

Bond-o, The two motors I own sit side by side on the same sawhorse in my garage. The difference in the prop design between the two is remarkable. Which is also why I am interested to see if there is a more modern design available for the older motor.

 

[Bondo]

Re: A modern prop for an older Johnson?

Ayuh,... Got a Picture,..??
I'm curious as to what Modern Design means,..??

There's Alotta different Styles of props, for many different reasons...

That Johnnyrude prop has been workin' pretty good for the last 40 years ain't it,..??
Newer ain't always Better...

 

[steelespike]

Re: A modern prop for an older Johnson?

There are new Michigan props available here at Iboats.Can't tell you if they are "modern" technolgy.But they would be the latest production methods.
Its likely that you copuldn't tell thr difference between an new prop and an excellent original
prop.On a carefully set up boat intended for that size motor speeds could be about 12-14 mph

 

[jrs_diesel]

Re: A modern prop for an older Johnson?

Here's some pictures to illustrate what I'm referring to.

The first two shots are of the propeller on my 2007 Mercury 6 HP (which is a longshaft that's used on my sailboat). The last two shots are of the 72 Johnson 6 HP prop.

The Johnson prop has thicker blades, and the blades appear to have more skew. Plus it looks like the blades flatten out towards the tips and have no cup.

The Mercury prop in comparison looks more sleek and efficient. That's where I got the idea that there might be something better for my old motor.

Steelespike, thanks for the head up on the Michigan Wheel props. I'll give them a look.

 

[steelespike]

Re: A modern prop for an older Johnson?

Based on your description and the pics it does seem that the Merc is a more sofisticated design.
It appears the merc prop size is possibly 7.9 X 8 If thats the case it may be a little too much prop for the application.If its a 4 stroke.May be ok if a 2 stroke.Depends on the size of the sailboat.The motor needs to be able to rev.

 

[Frank Acampora]

Re: A modern prop for an older Johnson?

It is also quite possible that the Merc style prop with more rake will not fit between the gearcase and exhaust snout. Older engines did not have a lot of room there.

Your six is not a performance engine. It was designed to move a tin boat at relativly low speeds. (although as kids on a lake with horsepower restrictions we used to get up to about 20 on 12 foot aluminum hulls) The stock prop will do quite well. Anyway, at that low horsepower, prop changes other than pitch usually have very little effect. Save your money and stay with what came on the engine.

Technically speaking, prop blades are a section of a helix--think spiral conveyor. If you look at your Johnson--prop, that is--you will see that while the blades appear to be bent back at the tips, they still describe a helix. Speed will be determined by the pitch of the helix. Rake and skew affect other performace issues. Besides, your Johnny is a better engine than the Merc. LOL Start some controversy here.

 

[Silvertip]

Re: A modern prop for an older Johnson?

What two props look like is of very little consequence, especially when the props are from two different engines. I suppose a "cleaver" propeller in your view would be classified "modern" yet they've been available for decades. Since the Merc is a "sail drive" it very likely has a high ratio (low gear) in the lower unit. That allows the engine to twist a larger diameter prop with a little more pitch than an engine with a conventional lower unit intended to push a small V-hull. Just like the Merc Big Foot lower unit uses the lower unit from a 75/90 HP three cylinder (2.34:1) vs the 1.83:1 ratio lower unit on the conventional 40/50/60 HP engines. The Big Foot motor is intended for use on work boats and pontoons and swings a 14 inch prop. The conventional Merc spins a 10 inch prop. The visual difference in these props is very dramatic. Each is designed for a specific purpose. Props are designed to work with a specific engine/gear ratio. While prop manufacturers blow their horn about hole shot, top speed, cruise, durability, fuel economy and a host of other factors, every one of those is a compromise. That's why prop selection is such a crap shoot. There simply is no "one size fits all" propeller for any engine. Think how simple prop selection would be if all we had to do is look at them. You can't even buy a pair of shoes just by looking at them. You need to try them on to see if they fit and that they serve the intended purpose.

 

[jrs_diesel]

Re: A modern prop for an older Johnson?

Based on your description and the pics it does seem that the Merc is a more sofisticated design.
It appears the merc prop size is possibly 7.9 X 8 If thats the case it may be a little too much prop for the application.If its a 4 stroke.May be ok if a 2 stroke.Depends on the size of the sailboat.The motor needs to be able to rev.

It is indeed a 4 stroke. And yes it is over propped for the sailboat. I am buying a high thrust sail prop for it this spring.

Thanks for the advice everyone. There's only one other prop pitch available for this motor (the Johnson), so I will just see how this one runs, and if it needs more pitch, then I'll get the other one.

Thanks again

 

[steelespike]

Re: A modern prop for an older Johnson?

There are 3 sizes listed here at Iboats.
I didn't check to see if actually available.

 

[jrs_diesel]

Re: A modern prop for an older Johnson?

Thank you for pointing that out Steelespike. I just went off of the Michigan Wheel site. They had two listed, one of which I already have (same diameter and pitch).

I am rebuilding the powerhead, and once I get it broken in I'll get the speed and RPM readings. From that I'll decide whether or not to get a new prop.

 

[steelespike]

Re: A modern prop for an older Johnson?

These little motor usually need something like a tiller extension to get your weight farther forward. If the boat is designed to plane 12 to 14 mph may be possible.