When electric power is fitted to a sail boat, it is attractive to think you can use the sail power to recharge the batteries. Indeed: the propeller is driven when sailing and will regenerate a lot of power so this seems like a sensible thing to do.
However - there are several theoretical problems.
Motor speed controllers are designed to control the speed of the motor. Yes, they can and do recharge the battery during braking - but charging is not monitored or controlled. It is simply an energy dump of braking energy into the battery and the fact that this happens to charge the battery is really serendipitous.
To charge under sail you would then have to monitor the battery state of charge and use a signal from this monitoring circuit this to control the controller. Relatively easy to do!
When the battery becomes fully charged: what do you do then? 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 no battery connected.
It is necessary to either mechanically brake the prop so it cannot rotate or (probably easier) electrically disconnect the motor from the controller. But the monitoring circuit must then make sure that it is not re-connected again while it is still generating.
Under sail, the boats speed could (and often will) be far higher than when under motor. Although there is 'slip' between water sped and propeller speed, depending on the propeller's efficiency the voltage generated by the motor could be well in excess of the battery voltage.
Under these conditions, no speed controller known can retain control: the motor is effectively coupled through and uncontrollable MOSFET arrangement (probably a full bridge) direct to the battery. Current will be uncontrollable and could well destroy the controller.
So when under sail, you will probably have to change mechanical gearing so that the prop shaft speed can never exceed the full speed when under drive. Alternatively a variable mechanical speed control (variator) might be an option. Or mechanical braking could be applied to the prop shaft.
We have outlined the problems above: clearly there are solutions but the solution is on overall system design thing: it must involve mechanical and electrical components. These must
Clearly the solution is not a simple one will need significant intelligence so will require control by a computer/microcontroller. If you are a programmer capable of making and programmming such a controller then we may be interested in helping you!
It may well be easier to use the speed controller only when under electrical power and to use a totally separate electronic circuit to control charging: after all, you are likely to be under sail more time than under electric power, so current required for charging will likely be lower than for driving.