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Recharging Under Sail

When an electric motor is fitted to a sailing boat, it opens up the possibility of recharging under sail by using the water flow over the propeller to turn the motor to act as a generator.

However – there are several related issues that must be addressed.

Battery charge monitoring

Motor speed controllers are designed to control the speed of the motor. Yes, they can and do recharge the battery during braking, but that charging is not monitored or controlled. The controller will route all the energy developed straight into the battery. To recharge safely under sail you need to monitor the state of charge of the battery, and stop charging at an appropriate point..


The next question is what happens when the battery becomes fully charged?  Switching off the controller might seem an option: but the propeller will still be generating voltage and you must consider what the controller will do with a generating motor and an inactive controller. Disconnecting the battery is one option you might consider – but under sail the motor can be generating a voltage high enough to damage the controller.

Another option is to use the over voltage limiting of a controller to cause the controller to speed up the motor when overvoltage occurs, so the braking effectively ceases (so does drag from the propeller).

A final option is to use the controller to short circuit the motor, this will provide a strong braking force for the propeller, but will also cause a significant current to circulate internally within the controller. This would result in heating and possibly worse.

The solution

Any solution would need an overall system design involving mechanical and electrical components that would:-

  • Ensure that the generated motor voltage can never exceed the maximum battery charging voltage.
  • Mechanically or electrically disconnect or brake the motor when charging is complete or when maximum voltage is reached.

We are looking at developing software for our Pro-360 controllers that will manage the points above correctly, watch this space…..