That company says one day out of the water.
I have no idea what it costs. The batteries alone would cost a bundle. You would need 4, 12 volt batteries, or 8, 6 volt batteries.
What would be the loss of sailing speed having that prop constantly in the water? I do not imaging it spins freely like an engine with a transmission. One company charges the battery using the prop as a generator, I know that would have to slow the sailboat to some degree.
Still, a great technology in several aspects. Love the way it is so silent in the marina. Turns a noisy sailboat into a ghost ship!