Vintage Mercury Prop Swap 7.5 -> 9.9?

tamathumper

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Jun 15, 2010
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I have the two motors shown below. The 7.5 has a three-blade prop, and the 9.8 has a two-blade prop. I'm parting out the 7.5 and wondering if it would behoove me to put its three-blade prop on the 9.8 which I intend to keep and use on my fishing boat. Would it fit, and would it be an improvement?

Mercury 1975 "75" 7.5 hp Outboard Motor
Serial #: 4090234

Mercury 1966 "110" 9.8 hp Outboard Motor
Serial #: 1913579
 

steelespike

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Apr 26, 2002
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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Try it for fit.Try to find the size on the barrel or under the prop nut.
Generally a 3 blade will work better than a 2 pushing a load.
Assuming it fits the question is if the pitch is close enough.
I would assume the 7.5 would have less pitch. If your boat is heavy or loaded that may be an advantage.You shouldn't change props without a tach. A tinytach is about $40 and easy to install.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

I've never taken one of these things off, but I've got the service manual, so I can give it a try.

I was hoping someone could say "yea or nay" before I broke something.
 

dazk14

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

I've swapped them back to 1969. I'm guessing it will fit.

You'll know it when you try. Don't forget the spacer and re-torque after an ~hr of running.

A Merc stock 2 blade came standard on the 9.8hp and there was one stock 2 blade even larger. I believe they were 10/11 pitch.

The 7.5 hp came stock with a merc 3 blade and all 3 blades were lower than the 2 blades. and I'm guessing yours is 9" pitch.

Depending upon the boat/load the 3 blade will probably work best. As Spike said above, a tach is the ideal way to set it up.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

I've got a portable tach and feet-per-minute gauge with a selection of tips/probes. Can I use that against the flywheel to measure the RPM?

Am I measuring at WOT for a particular value?
 

steelespike

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Your looking for a wot(wide open throttle) rpm of 5,000 to 5,400.Would be nice to have a gps speed to go with it. If it over revs you just throttle back a little.
With a partner and some gear it isn't likely.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

GPS speed is entirely doable! Thanks for the suggestion.

The boat is an aluminum 12-foot fishing boat, deep-vee and very heavy duty.

Usually has me and my two kids in it with fishing gear. The 9.8 just barely gets it up on plane after about 10 seconds of WOT.
 

dazk14

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Well, that makes it easy. Go with the lower pitched 3 blade.

Merc was pretty good years ago. They knew no one had a tach and few had a speedo (although no good speeds like a GPS), so their "Quicksilver prop catalog" had speed and boat size to help selection.

Pretty funny, back in the early 70" Consumerreports actually tested some outboards and posted speeds. Very unusual.

They actually warned people of the power of a 7.5HP Merc. (unexpectedly strong).
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Unfortunately this one is unexpectedly not starting, so I'll be parting it out, I just figured I might be able to make good use of the prop on my 9.8 if it made any sense.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Dang, they're not even close. The 9.8 has a very slim shaft with a 1/2" nut, and the 7.5 has a much larger shaft with a 5/8" nut.

I thought I might be able to get the center "splines" out of each prop and swap them but they won't budge.
 

dazk14

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Have you checked for water in the lower units. That's the weakest link.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

I'm not sure how that's related to my original post about swapping the props?
 

dazk14

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

It's only related to which engine you keep, while you still have the option.
 

tamathumper

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Re: Vintage Mercury Prop Swap 7.5 -> 9.9?

Not an option unfortunately. The 7.5 is effectively dead and I don't have the energy or time to figure it out.
 
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