Well, it's too windy to test the latest model, so here's something that might go someway to answering the original question. It ought to get you in the right "ball-park" with an electrically powered scale type model or at least stop you being way off the mark.
1) Establish the speed you want the model to travel at.
2) Determine the free running (unloaded) speed (RPM) of the motor you plan to use at the Voltage you intend to apply.
3) When working in the model, the motor's RPM will be lower than this, I aim for 75-80% of the free running speed (usually a good combination of efficiency with a power reserve to cope with weeds and stuff).
4) Dividing the models speed (I usually use inches/sec) by this motor speed (revs/sec) will give a value of the propellers pitch that would match this model speed. But....!
5) This assumes there is no "slip" between the rotating propeller and the water flowing through it. Alas, this means the the propeller would generate no thrust as it fails to accelerate any water rearwards so the pitch has to be greater than this value.
6) The amount of "slip" will vary depending on the model, motor and propeller combination but 20% might be a good starting point.
7) So, multiply the pitch obtained in 4) by 125% and try a propeller with a pitch around this value.
Yes, the more mathematically perceptive amongst you might have spotter that you could have obtained much the same value for propeller pitch by ignoring slip and using the motor's free running speed. But this would have been to akin magic which is dangerous (a failure to understand the technique/technology that is being used).
As to the diameter and number of blades being used, this is where some experimentation can be called for. Also, all propellers are not the same, some have thin, correctly curved sections, whilst others have thick blunt and maybe even flat sections.
This method is something of the traditional Engineers "back of an envelope" calculation but have given me successful results many times. It did predict my latest model would be way over scale speed and, by golly, it was..!
Glynn Guest.
P.S. don't forget to allow for any gearing effects between the motor and propeller shafts.