Re: 15hp evinrude, need help choosing new prop.
Without knowing your current pitch or RPMs, any guess is as good as another...
If I were you I'd get an inductive tach or a tiny tach, and figure out what my RPM is right now at your top speed. Proper range for that motor is 5500 to 6500, and to get it to plane out you want to be closer to the higher #. If you RPMS are too low indeed you need less pitch. If you're already spinning over 6500 RPMS, you might need more pitch.
Also if you have a factory prop, it is clearly labeled with diameter and pitch. Remove it from the motor and look it over. Sometime #'s on the face of the hub are hidden when it is installed, but a true OEM Evinrude/Johnson prop I believe has diameter & pitch stamped right on the side of the barrel between blades somewhere. You should be able to find this info.
Also, have you tried different trim positions and/or motor heights? It is EXTREMELY common for people to mount these motors too low in the water creating extra drag...might try laying a board on top of the transom to raise the motor up a couple inches. Also pay close attention to the angle of the motor...set your A/V plate about even and level with the bottom of the boat. Small angle changes make a noticable difference...you have 4 holes to choose from for tilt position. Even if it looks about right, try all 4 different positions and a couple different motor heights anyway, and try to figure out which works best. And while you're out there, try a 1" thick board, 2" thick board, and 3" thick board on top of the transom to try your motor in 1" increments of height positioning. Again you just want to pick the one that performs best.
I have the same motor as a kicker on my 17 ft Landau (aluminum Bass Tracker type of boat with full floors, swivel seats, steering console, full livewell, 50 hp motor, 20 gallons of fuel, 2 adults, and a huge amount of gear...presumably much heavier than yours.) While it does not plane out, it comes really close. When I had that same motor as a short shaft on my 14 ft V-bottom rowboat with about 650 pounds of people/motor/gear not only planed out, but ran darn close to 30 mph...while turning about 6600 rpm. That motor loves to spin and scream...
I learned that a cheap inductive tach off Ebay (about $20 shipped) works real easy on that motor with a piece of reflective tape on the flywheel. You need to do it on a cloudy day though...direct sun screws up the reading.
Not sure how heavy your boat is, but I believe if not overly loaded, and motor positioned properly, your boat should plane relatively easily and run a solid 20 mph or better.
You absolutely certain you have a 15hp carb, and not a 9.9 carb? In 1974, the carb was the ONLY difference between those 2 motors, and if someone in history put a 9.9 carb on it (much much much more common) as part of a repair, you may actually have a 9.9 motor and not even realize it. See Leroy's Ramblings website (google it) for details on how to ID the difference...they look awful similar.