Hi there If it's any help to you i just completed a 1/12th scale model of a Brede class RNLI lifeboat totally from scrap materials.
It should have two props but i decided on one 40mm prop run by an old 540 motor.
Actually the model is 3 inch under scale as the real thing was 33 feet so 1/12th scale would be 33 inches, my boat is 30 inches.
As luck would have it that 540 motor, and the 40 mm prop give it a fair turn of speed, pretty good scale wise, so i reckon that 2 props of 45 to 50 mm will do fine for your model, though i don't know how heavy it will be.
Here are 3 pics to give you (I hope) some idea)