Need recommendation for prop on 1980 15 hp evinrude please

I hope this is in the right place...ill try to include as much info as I can, im new to boats. My boat is an alumacraft 1442 flat bottom riveted jon, I have a 1980 evinrude 15 hp in good condition, but needs a prop. The boat is 230 lbs, so wieght is between 930 lbs max and probably 550 minimum for boat, motor, gear and people, I use the boat around 5000-7000 feet above sea level. It currently has a badly worn and chinked 9.5x10 prop, and with a total weight of 570 about, the best speed I got was 16-17 mph, bumping up to 18 once or twice on my gps, and with about 680 and with wieght shifted around for best speed we got up to 13 mph, and with a max load, we were down to about 6-7 mph, and I dont have a tachometer (need to get one) What I would like is a good estimation on size/pitch/3 or 4 blade, as to have a good prop mostly for higher speeds, and being able to reach the best top speed with the heavier loads. I dont go out much alone, so generally it will be in the mid to upper load levels, with seldom use in the lower end. I REALLY appreciate any information, this is a fun project my father and I are going in on. thanks again

Knowing your RPM's is important. I experimented from the standard 9.25x10 prop to a 9.25x8 prop on a 70's Johnson 15 hp mounted on a heavy boat rated for a 40 hp (I fish restricted horsepower lakes). The boat with a 9.9 carb ran 10 mph. With a 15 hp carb it runs 13 mph (a little faster right after ice out from the denser water). Switching to a 9.25x8 prop increased the mph to 15, but the increase in noise made me switch back to the 9.25x10 prop. I do not remember the exact rpms but the 9.25x10 was around 5200 rpm and the 9.25x8 was about 6800 rpm. All this information was done on a fully loaded 15 foot 1956 Arkansas Traveler which was converted to a bass boat layout.

FYI - a 1974 Johnson 6hp pushed the boat at 6 mph.

The point is the increase I experienced was marginal and the tradeoff in noise was not worth it.