Prop size question

[BCinerie]

So I have a 1981 evinrude 50hp outboard, On a 18 Lund , I bought a new prop
And it turned out to be a 1/4 inch smaller. Original was 12 by17. New one
Is 11 3/4 by 17. Do you think it will make much of a difference ?

[Poseidon]

Smaller=less drag=higher RPM's.

Do you have a tach?

[Gerry Zagorski]

1/4 inch diameter should not make that much of a difference, especially on a 50 horsepower engine. Pitch measurement is way more important.

What you will want to do is be sure the engine can get up to but not go over it's reccomended wide open throttle RPM.

For this you will need to know the engine's rated wide open throttle rating and you'll need an RPM guage.

If it's not in spec a prop shop could either add or subtract some pitch from the prop and get it in spec... You just need to tell them the reccomended RPM for the engine and what your actual rpm was.

Clipped this from another site

What is diameter?
Propeller diameter is distance across the imaginary circle that a spinning propeller makes. It can be easily determined by measuring the distance from the center of the hub to the tip of one of the blades and multiplying that number by 2.
Diameters between different propeller manufacturers may vary slightly. For example for a V6 outboard application, Turning Point uses a 14-1/4" diameter where a different manufacturer may use a 14-1/2" diameter. This small variance does not affect your performance as much as the pitch and overall design. However, larger diameter propellers - 15" or 16" diameter - are designed for larger boat applications - 23 ft +, and are not ideal for 15" - 22" foot run-a-bouts. These larger boats need more blade area to push more water.


What is pitch?
Propeller pitch is the theoretical forward movement of a propeller for one revolution, assuming that there is no prop slip. For example, a 21 pitch propeller will theoretically move 21 inches for every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. More aggressively and efficiently designed propellers will slip less.
When selecting a propeller pitch for your boat, it is important that the propeller runs at the upper end of your engines wide-open-throttle RPM range. If you want your RPM's to increase, go down in pitch. To decrease RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will change approximately 400 RPM's.
For water sports or extra people on board, you should generally drop 2" of pitch to help compensate for the added weight and drag on your boat. It makes a noticeable difference in your boat's hole shot, fuel efficiency, RPM's, and overall performance. You should ALWAYS carry a spare propeller on board, and if you're into water sports or occasionally load the boat with extra people, a spare prop with a lesser pitch is a good idea. When thinking a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.
The same pitch from different manufacturers will run slightly different RPM's due to a difference in blade design. Speed differences among the same pitch from various manufacturers will vary even more. For example a 17 pitch from one brand could run up to 7 mph faster than a 17 pitch from another brand.
Pitch is the 2nd two digits that are listed in a propeller item description (14-1/4 x 19, 19 is the pitch, 14-1/4 is the diameter)

[Capt. Debbie]

The theory is your diameter (usually unchanged) and the right pitch should make maximum RPM's a highest speed.

Too much pitch and the motor can not make designed in max rpms & Hp out. Too little pitch and it makes design rpm's & but less hp's than design ( 50 in your case) but the lack of adequate pitch means less water thrown and less speed through the water.

It's balancing act.