After further consideration of designing the planetary gearing system, it has been concluded that it may be susceptible to clogging from debris in the water environment.
To avoid this a new brushless motor with a slower RPM and higher torque has been selected.
It has a given Kv of 380. This can be used to calculate the theoretical torque of the motor using the formula T=Kt*I (eric, 2015), where T is torque, Kt is the torque constant and I is the applied current. Kt can be calculated by Kt=1/Kv. (eric, 2015)
If our current supply in around 9A then our theoretical torque will be, T=(1/380)*9=0.02368Nm.
Since our propellers are 50mm in diameter this torque should sufficient enough to allow the propellers to turn through the resistance of the water.
The Kv is also a good indicator of the RPM/volt applied tot he motor (in air). So out of the water we expect the motor to turn at 4560 RPM, as we are using a 12v supply.
These torque calculations seem to be too low however so a simple test will have to be done to confirm the torque.
eric, 29 july 2015. meaning of motor kv. [online]. Learning RC [accessed: 01/02/2017]. available from: http://learningrc.com/motor-kv/