I see regen is being revisited again. I've uploaded a file on turbine efficiency entitled "Gorlov". Regarding some of the numbers being bandied around regarding potential to recapture energy via the electric propulsion hardware, I would would suggest a good goal for round trip efficiency might be 1%. Suppose you're sailing at 5 knots. Engage regen; sail 100 miles; point into wind; lower sails; and power 1 mile into wind at 5 knots on captured energy. Don't know of any vessel capable of doing this yet.
Since a prop is matched to a hull in such a way as to provide maximum thrust with minimum hp; it will produce minimum power with maximum drag when used in regen. It is the worst device imaginable for generating electricity on a moving boat. A wind generator will always be better if you compare comparable boat performance. Solar is usually more reliable than (the most common) wind gens by a factor of 5-10; and will usually produce the cheapest cost/watt over its lifetime of the green alternatives.
Getting the energy into the batteries is by far the biggest technical challenge for most sailboat conversions. It will most likely drive your choice of battery tech and BMS. You may also be obliged to size your battery bank based on its ability to absorb charge, rather than its ability to discharge. This is particularly so if a genset is involved.
There has been a lot of discussion here and on other forums about using "excess" energy when already sailing at hull speed. The idea being to use regen when the boat is over-canvassed; and that way you won't be slowed down much. This is bad economics. Good economics is to use a smaller (and cheaper) rig, and use the savings to purchase a bit more fuel as you go. To put this in perspective, a naval archetect who designs blue water cruisers reports that his boats can motor 6.5 times around the world for the price of a sailing rig for his boats.
PS. I know there are good, durable wind gens out there. They are not common, and the ones I know of start at around 5000 USD for a little one.
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