similar questions occupying me also. I'm building a plywood 6.3 meter cabin power katamaran, started 2017. She will be driven electric, I hope mostly solar electric. So I did a lot of calculations regarding the power train and, like you, found a lot of particulars on motors and propellers are not clearly covered by public information.
Like you, my goal is to use a slowly turning propeller with large diameter because this is the efficient way. If you go electric with a small boat you have to avoid energy losses wherever. But the difficulty is to achieve a sufficient speed. If you intend to score 10 mph with the mentioned data, this will be hard.
The 1,800 rpm (no load) of your motor will under load perhaps turn into 1260 rpm (70%). A 7" pitch means a theoretical prop advance of 8,820" or 735 ft/min = 224 m/min = 13.44 km/h = 8.35 mph. If you take a propeller slip of 30 % into account it will give 5,85 mph. I don't know if the calculated losses are in the supposed area, but they comply with values I often read this ballpark in specialist literature.
Regarding cavitation you will have to test it, I think. The determining pressure distribution around the propeller blades depends on several conditions, which are not easily to find out.
(I will use brushless DC motors, therefore I'm not well informed about brushed motors, but I remember that I read about problems with the brushes, when turning backwards.
Regards, Günter
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