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 theoretical problems.

  1. Charge monitoring
  2. Overcharging.
  3. Current and voltage limiting

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 would need to monitor the state of charge of the battery, and use a signal from this monitoring system to control the controller.


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 higher than the battery. This could damage an unprotected controller!

However our controllers have over voltage limiting that causes the controller to speed up the motor when overvoltage occurs, so the braking effectively ceases (so does drag from the propeller). However this over voltage is often at the peak handling voltage of components such as the main capacitors and these may well overheat and vent out.

It is therefore necessary to either, feather the blades if possible, mechanically brake the propeller, or (probably easier) electrically disconnect the motor from the controller.

The solution

We have outlined the problems above: 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.

The full solution is not a simple one and will need some intelligence or interlocks built in, possibly via a microcontroller.

It may well be easier to use the speed controller only when driving under electrical power and to use a totally separate circuit to control charging.

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