The best off-the-shelf option I have seen for a HP is from Krautler.  They offer submersible motors that can easily be incorporated into a swing up drive off the side of the hulls.  These pods work almost as well in reverse as forward:

These pod drives are propped tp maximise bollard pull rather than best motoring efficiency.  My experience suggests that is a good way to go although it reduces the battery range by maybe 20% or so; not an issue if you have diesel back up. 

I know Robin Ward was considering Krautler through all4solar but I do not know what was eventually installed:

The cost estimates I saw from all4solar were lower than what Oceanvolt are offering.  

The units installed on the 18m proa used DC motors rated at 9kW on 48V driving a gearbox and large plastic prop.  They each have 5kWh batteries.  The total cost was almost recovered from the sale of the 60HP outboard.  We managed to cover 14nm at 4kts in dead calm with some current assist with that arrangement.  The air cooled diesel was around $2000 landed.  The new 52HP diesel cost a packet and is still being sorted.  The one aspect we did get right was the water pick up.  The main flaw was sizing the drive belts for the generator rating rather than the motor peak torque.  If the diesel is started in gear the governor is full opened and the motor achieves peak torque until the govenor settles down.  The dampening plate on the output shaft is not sufficient to prevent the belts from breaking.  

With the diesel, the generators can be spun up to rated voltage of 90V so each generator output is 18kW, meaning two are close to the rating of the diesel.   

The other point worth mentioning is that the battery systems and the diesel generator system are totally independent apart from the drive legs on the 18m proa.  There is a single changeover switch that enables either mode.  That is easy to achieve with DC motors/generators.  In fact the generators are identical to the motors.  It is a bit more complicated with induction motors because you need to be able to source reactive power from an overexcited synchronous generator or fit capacitors with an induction generator. 

If you have reliable controllers then you could just stick with charging the batteries from the generator.  All this can be done with off-the-shelf equipment.  The size of the generator would be selected in conjunction with the size of the batteries.  I would set the limits at (a) the generator being able to supply continuous power to hold 4kts in dead calm conditions and (b) the combination of battery and generator being able to sustain  4 hours at 5kts in 25-30kts of wind.   

Rick