Hi, Les, Good information!
the Rpm is what tells us what is happening here. A good speed to aim for (to avoid a distressed motor) is 80% of the calculated speed at that voltage (this generally puts motors with a current draw which is maneagable and an efficiency which is reasonable)
IF the Kv is really 1375 and if you are running at 12V, then the no load speed is 16,500 rpm
80% is about 14000 and you are not getting that - probably don't want it either
So if you want to use this motor - fit a smaller prop. If you want to use this prop fit a motor with a smaller kV and/or reduce the voltage. Your vessel may be quite happy at 6V!
For information - if you want to rund the same prop - try a 555motor (5 poles in the same armature length) or even more poles. But I still don't know what the boat is, so I have no direct way to help you
Subculture is exactly right
<<You need a 500/550 size motor wound to run at about 4000-5000RPM unloaded, or gear the motor you have down at least 2:1.>>
but I might have said 545/555 motor!
There are lots of 540 types which would be suitable, but thay are not found in model shops - they are found in printers and lorry accessories and have lots of fine wire and low kVs
andrew