Because of the way these motors make power, using the same props on them will not give optimum results. The Feigao motors, like the Hackers they are copied from, run best at an rpm reasonably close to the Kv x nominal voltage, in this case 1668 x 22.2 = 37,000 rpm. With the x442 prop this calculates to around 60 mph which is close to what you saw.
The Castles on the other hand are similar to the motors they are copied from - the Neu 1500s. These motors usually run best around 10-15% slower than the calculated rpm, or 1800 x 22.2 x 85% = 34,000 rpm. With the x445 prop this too comes out to around 60 mph, very close to what you saw. This makes sense as the Castle has more torque than the Feigao and can swing a bigger prop.
To go faster, adding diameter didn't work as you exceeded the torque capability of the Castle. Go to a prop with about the same diameter but with more pitch. Try an x642 or m545 and see if this gives you a bit more speed. You may need to re-trim the boat with the different prop, another reason for differing speeds.
BTW, a difference in GPS speed of 1 mph is seldom significant at all. Two back-to-back runs with the identical setup will often have this much difference. Why? The GPS's accuracy tolerance, water conditions, state of charge or temperature of the battery.......the list is long.
Performance isn't always about top speed either. Acceleration and efficiency matter too, as does longevity. I haven't used the Castles but IME the Feigaos have been giving good quality performance for many years. I set several IMPBA SAW records with them including the top P Hydro record of 85+ mph - on nickel cells no less.
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