What is the best prop for a 1975 25hp evinrude?

[calvinyonce]

I have a 1975 25hp Evinrude mounted on my 74 Sears jon boat. The boat is lightweight aluminum and is sixteen feet in length. What kind of prop can I use on this motor to improve performance such as speed? The boat with just me in goes about 25mph. I rarely have a heavy load in the boat at most two people and some gear. Maybe four hundred pounds total. I do have a plywood floor and deck in the boat that weighs about 75lbs. Right now it has the correct stock omc prop that I believe is a 9x10 pitch. Would it even help to get a newer prop?

 

[Silvertip]

Re: What is the best prop for a 1975 25hp evinrude?

Our crystal ball is currently out of order so we can't help much since we have no idea what RPM your engine is currently turning. When selecting a prop you have one choice -- select one that allows the engine to rev at or very near the upper end of the engine manufacturers recommended wide open throttle RPM band (usually in the 5000 - 5500 rpm range). Performance will be whatever that prop provides. Overpropping lugs the engine, slows you down and is harmful to the engine. Underpropping provides good hole shot but causes the engine to over rev, use more fuel and is also harmful to the engine. Unless you know what your engine is actually turning at WOT we have no idea whether the current prop is correct, under propped or overpropped. You need a tach to determine WOT rpm.

 

[steelespike]

Re: What is the best prop for a 1975 25hp evinrude?

The best way to do this would be to get something like a Tinytach $47 and a gps and test it for wot rpm and speed.
Guessing the slip and using your"about" 25 mph The calculator says your rpm is about 5100.
The very bottom of the rating hp is rated at 5500 rpm range is 5000 to 6000.
some things to check;be sure the throttle opens all the way and that then link & sync is right.
also the anti vent plate (just above the prop) should be about even with the bottom of the transom..
Is the bottom clean and smooth?Have you adjusted the trim for best speed?
Jon boats usually get up on the waster fairly easy but the bottoms are full if rivets and it probably has 3 or more keels
and sometimes they lay down kind of flat..
The 25 is rated for over 30 mph on a light reasonably fast boat.
If the prop looks original and kind of tired a new prop might help both the condition and technology.
But without the hard facts of wot rpm and gps speed there is no way to know which prop for your setup. Some cells have gps apps.

 

[calvinyonce]

Re: What is the best prop for a 1975 25hp evinrude?

Thanks for the info.