Smaller the pitch, more weight it will push and get up on step. Larger the pitch is for empty boats. I run a 21 pitch on my Wooldridge for just dinging around fishing but put a 19 pitch on when hauling hunter, gear and game. In theory a 15 pitch prop will move the boat forward 15" with each revolution and a 16 pitch will move it 16". But they are so close together I don't think they will make much difference.
You can find out if you are properly propped by finding the manufactures specs for your motors. For instance WOT (Wide open throttle) for my Honda 150 is 5,000 to 6,000 RPM. If you have trouble making WOT then you are over propped, if you go over the manufactures spec for WOT you are under propped. I am running a 4 blade 15 5/8 X 15 prop and I can go over by about 400 RPM so I am going to move up to a 4 blade 16 which should give me the best possible mpg and the best possible top end speed. I currently cruise at about 25 to 26 mph at 3500 to 4000 rpm, and get about 3.5 miles per gallon. The 16 should give me a little better mpg at the same rpm. I have tried 5 different props and I really the like the 4 bladed props the best.
The lower the pitch the more down low power you will have. The higher the pitch the more top end speed. I have twin 150's and started with 15 pitch, the boat would stand on end when you gave it all the trottle from the git go. RPM's where over 6000 at WOT top speed 42 mph. Changed to 17 pitch, a little less down low, around 6000 rpm and 46 mph WOT. Changed to 19 pitch and rpm 5600 top speed 51 mph. Not nearly as much hole shot as the 15's, but a significant top end difference. Mileage gets better because you are going faster at the same rpm with higher pitch.
I've got a Yamaha 150 on a 21' North River Seahawk. A 13-3/4 by 15 prop was put on it by mistake I think and the performance was lousy. I could get on step but it took forever and my top end WOT speed was only around 35-36 MPH. My RPMs at WOT were 6300-6400 RPMs so definitely a problem.
They have since ordered me a 14-1/4 by 17 four blade prop to try out. I'll letcha all know how it performs next weekend when I get back to town.
If you want some really good simple info on props go online to www.propline.com . They have the simplest explanation I have found as well as definitions on all the prop lingo. Another avenue if you have a Yamaha is their website - find your motor on the side menu and check out the performance bulletin for you boat and motor. They have most of the popular models listed with all data on GPM, RPM, MPH, GPH, etc., etc., etc.
As I recall, you bought a 26' AK Searunner w/ twin 115s. I have the same boat, 2 years old. In search of more performance, I tried 4-blade 15-pitch props. Even tried 17-pitch, but those brought top RPM down too much and the motors tended to lug. I settled on the 4-blade 15s, without a big improvement. An option I did not try was stainless. I eventually repowered to 150s. If you are in search of more performance, I would try stainless 15s. Bob at A-1 is good about letting you swap props to find your desired performance, as long as you don't ding 'em up. I may well wind up with stainless on the new motors. Good luck.