RE: Electric power system questions for a large PBY-5a
First thing is, don't get too hung up on the exact prop size. If you end up with that exact prop, you just made a lucky guess, but the final prop size will depend on quite a few factors, the most important of which is how much clearance you have on that PBY. Because the engine nacelles are so close together and right over the fuselage, you're limited as to how big a prop you can turn. Is 13" the maximum? If not, what is? With electric, you want to spin the largest diameter prop you can to get the best efficiency out of the power system.
If the airframe is going to weight 10lbs, then the whole plane is probably going to weight close to 20lbs. What do the plans say about final weight? What have other builders of this plane said about how much theirs weigh?
Going with the 20lb target weight, and typical warbird-heavy wing loading, I would recommend a minimum of 2000 Watts split equally between the two motors.
Going with my favorite go-to motor brand, AXi, it looks like your 13x8 3-bladed guess was pretty darned close to right-on. Using a 7S LiPoly pack and drawing 41 Amps, the AXi 4130/16 will spin a 14x8 2-bladed prop at about 8500 RPM for an input of just over 1000 Watts. The rule of thumb to go from 2-blade to 3-blade is to reduce the diameter by an inch, so the motor should perform similarly on a 13x8 3-blader! If you need more power, you can always go to a 13x10.
The Horizon Eflite Power 60 motor will produce similar results, as it is uncannily similar in design and specifications to the AXi 4130/16.
One pack of 20C rated LiPolys of around 3000-3500mAh capacity per motor, and you're set.
Sounds like this plane was designed around a pair of .91 4-strokes?