I followed David’s advice and googled “Evo” for information on an outboard which claims to provide this feature– and Howard is right (up to a point) there are outboards which can do this.

I had thought that any exceptions to what seems to be a 5 knot threshold are probably only those simple systems where there is a direct drive (no reduction gearing)  between the propeller and the motor/generator, which would normally exclude outboard motors. But the “Evo Navy” outboards are in fact exactly just that exception – pods - with the engine in the water directly driving a propeller. So, I can believe it. 

Here is what E-Propulsion claims for its Evo Navy series of outboards:

At 5 knots boat speed, the 3kw outboard can generate about 100 watts, and the 6kw outboard can generate perhaps 150 watts. This would not be insignificant – if it could be fed into the vessel’s 12V house battery, you would be seeing a good little trickle of at least (8 – 10 amps?) which would be very nice, and I would be happy on a beam reach to leave the leg down and lose a little bit of sailing speed to get that.

However, there may be a catch.  According to the E-Propulsion  manual, two conditions have to be met. 1: You can only use regen when the battery is less than 90% charged - that’s not an issue.  But 2: You can only use regen when charging the E-Propulsion 48V battery. I take this to mean, even if you are not actually limited to using E-Propulsion’s own 48V battery, you would not anyway be able to simply divert the charge to where it would have the most noticeable effect (your 12V house battery.) There’s probably a way around that, and I am sure someone wil be able to tell us what it is.  Anyway, in the big scheme of things, in terms of a 48V battery bank, 100 watts is not much, though certainly better than nothing.

Without a special propeller I still doubt that regen is likely to contribute much to an electrified sailboat's battery bank. Swapping propellers backwards and forwards isn't really very appealing. I think I would rather try playing around with some sort of towable or retractable screw, of optimum dimensions, connected to its own generator.

Or approach my bank manager for permission to look at the following:

This is a sail-drive unit with a large diameter electric motor and a controllable pitch propeller, with intelligence built into the system. The pitch is continually optimised for all requirements in forward, reverse or regen mode. Of course, the software also enables feathering for pure sailing mode. I am guessing, but I suspect the large diameter of the housing bespeaks a low-revving very efficient motor.

The propeller diameter is 17", its a 48 volt system with options of 10 or 15 kw motor.

Here is what is claimed in the way of hydrogeneration:

0.5 kw at 5 knots. A full kw at 7 knots if your sailboat is powerful enough.

Now, wouldn't that be nice? It shows what a difference the right propeller can make, and I suppose that applies equally to propulsion mode.