2015-08-09 More on the boat:: thrusters and solar power
For now, I'm looking at Blue Robotics T200 thrusters.
The thruster should reverse its sense of rotation on a regular basis.
The reason for that is the thrusters are sensitive to seaweed. The best way I can think of overcoming the problem is to have them reversing their rotation at regular intervals (say, every hour), but I've yet to figure out how to keep the boat in the right direction. May be it should just run backward one minute every hour.
Anyway, it means there must be a control (PWM) of the motor speed, while my first plan was to dumbly inject all the solar power available into the motors, without any intelligent control.
However, there's another reason why the power of the motor must be regulated: in short solar panels are constant current devices, so they have an optimum power point in term of drawn power/voltage (the more power you draw, the bigger the voltage drop, potentially keeping you away from your optimum power point). This means the MCU should monitor the output power of the solar panels, and adjust the drawn power to stay close to the panel optimum (a theoretical view that must be confirmed experimentally).
The solar panels must be assembled in parallel naturally, so the failure of one panel doesn't bring down the whole power chain.