[Fr. Frank]

Originally Posted by Bushwacker

... If you don’t know what your WOT rpm is, any prop testing you do is a complete waste of time!

That's almost Gospel, Denny.

In 2002, I re-powered a previous 20' Seafari from a 120hp Chrysler motor to a re-manufactured XR4 2.4L making about 230 hp on the dyno, after balancing and blueprinting the motor. I tried a lot of different props. I was running a lower unit off a 225 hp with a 1.65 gear ratio, so pitch was an issue.
I got a Solas 3B stainless 17P to get me 6000 rpms, but it was only giving me 38-39 kts trimmed out at WOT.
I next tried a Mercury Offshore 4B at 15P, but I only got 5600 rpms at WOT, producing about 37 kts.
I tried a Laser II 17P 3B, and got 5900 rpms, but it was a dog out of the hole and only produced 35 kts. (Lots of slip with that one).
I next tried several props in succession, finally settling on the Quicksilver Mirage 17P 3B, which got me 5850 rpms, and 40 kts, and still had decent handling and acceleration. I ran that for a couple of months, but still didn't think I was getting all the performance I could from the motor.
After talking with Jerry Schiaccitano of AAA Outboard Machine Shop in Crystal River, who raced classic Chrysler Outboards, I got in touch with an engineer at PowerTech. After sending him the specs of the Seafari and on my motor, including photos of the hull, they designed and made for me a custom 15.25" X 16.5" three blade propeller based op their OFS design.
Max RPM was 6150 rpms trimmed out at 43 kts. Hole shot was just under 4 seconds, and while maneuverability was slightly down from the Offshore 4B, I could maintain plane down to 13 kts with the trim tabs. The prop did, however, cost me $735.


The point of this story is show that performance can be tweaked, and sometimes come from unexpected experimentation.