1961 evinrude 18hp. I want to replace the prop

I've got a 1961 18hp Evinrude model number 15034. For a motor that is 50 years old, it runs great! However, I feel like I could get better speed if I put on a different prop. Currently I have a smaller 3 blade prop on there, but what If I put on a larger 4 blade prop? I don't even know if that would help or not. If it would, how would I go about figuring out what size prop I need for my evinrude. Any advice, thoughts or concerns ya'll can give me would be most helpful.

Thanks!

Ben

you need to see how many rpms the motor is turning first, That is the key factor in proper prop picks.

How would I find out the rpm's? The manual I have for the motor isn't specific to only that motor but all evinrude motor from 1.5 to 40. Any suggestions?

You gotta put a tachometer on it. I think I'm going to buy one of these https://www.tinytach.com/tinytach/index.php.

Specs on a 61' Johnson 18 hp ( Same as Evinrude)

original cost 395.00
bore 2 1/2
stroke 2 1/4
hp 18
rpm's 4500
piston disp, 22
weight 79 lbs

Specs from a factory Johnson manual.

Now about that prop. If you'll notice, the blades come straight up off the hub of the prop. Not swept back, ( no rake), as the newer props are.
I had a situation like yours that a prop guy in Fl fixed for me. He rehubbed a Chrysler prop with an OMC hub to fit a 64' 40 hp for me.
Chrysler props were made more efficient than any omc prop of that time period. One of the good things Chrysler did.
This prop got me 10 mph more out of that set up.
A 4 blade prop won't get you more speed, just more blade slip and faster excelloration out of the hole, loosing top end speed using a similar pitch as your 3 blade prop.

If you could find a Chrysler prop with the same diameter and pitch, it would have more blade size, because of the rake, and get you more speed. Next, a prop shop that would put in an OMC pin drive hub.

Mike, at Hall's prop shop did it for me. Here's a link to some info on Hall's.
https://www.planetnautique.com/vb3/showthread.php?18175-Prop-repair-shop-in-central-Florida-experience

Thanks guys! You've been most helpful