The main issue I had with higher speed regeneration related to the high rpm of a fixed pitch prop.  If you have a variable pitch prop then the pitch can be adjusted to limit the rpm per the Oceanvolt system.  

Power is a function of force times speed.  So you are correct in your assessment that it takes less force at speed to produce  given power.  Regenerating at say 2kW on a boat doing 20kts (10m/s) requires a force of 200N.  That may result in 19kts instead of 20kts AND not much change in apparent wind speed.  Trying to get 2kW at 6kts requires a force of almost 700N.  That would probably halve the speed on a HP.  So the faster a boat moves the less extra drag associated with regenerating a given power level.

In my view the main weaknesses with an electric saildrive is the inability to beach without protection for the saildrive.  From a pure sailing perspective it is just added drag so reduces pointing ability and top speed.  The 18m proa has been accidentally beached 3 times and partially grounded once and purposely grounded numerous times in a tidal creek once the new rudders were fitted.  The partial grounding resulted in bent ruder stocks.  The second grounding resulted in destroyed rudders. The 3rd and 4th groundings were no issue because the new rudders were designed to support the weight of the hull.  Usually sail drives on a monohull are protected by heavy appendages that do not exist on a HP.  Cruising cats often have strong rudder skegs and mini  keels that allow the boat to safely beach. Any added appendage is going to increase drag.

For a HP - If you are competent at fabricating then I would opt for drive units like used on the 18m proa.  For a smaller proa look at what is commercially available in electric thrusters.  If you have the money then my recommendation would be the Krautler fixed prop with ability to lower and raise.  I would aim for battery capacity of minimum 2 hours at 5 knots and up to 4 hours.  I would have a small 240V diesel generator and at least 1kW charger for the batteries plus as much solar panel as I could fit. I may play around with regen as I already have but would not rely on it for charging the batteries.  

I consider the outboard on the tender as an interim solution and would aim to avoid circumstances where I could be motoring in adverse conditions.  Hard to beat a small inflatable tender with a small outboard that can be easily lifted onto the deck and lashed down. I would be contemplating strong points for mounting swing up drives even if the tender was the starting option.

If you are in tropics or subtropics, I expect an easily driven boat with heaps of solar panels would cover quite a lot of ground on solar alone.  Essentially it has infinite range.  It possibly makes the doldrums an attractive proposition for an ocean crossing.  

Any good sailing boat should be safer sailing rather than motoring and your list is fundamental.  The schooner rig HP appears to be right in concept.  I personally like the small fixed, strong rudders near the ends and the big dagger board in the middle as I see these as easier to manage the forces than two big rudders near the beams.  On the 18m proa we have carried its modest rig to about 27kts when reaching and about 22kts when hard on the wind.  We have had up to 30kts running just off dead square to avoid the mast flogging.  With the Aerorig, the easy reef is to drop the main into tightly set lazyjacks and running backstays then just sail on the jib sheeted from the leading end of the boom to pull it into the wind while the sheet on the trailing end acts as a safety to limit boom range.  It has never been out in winds more than 35 knots although it did drag a 3 tonne mooring about 300m in 50kts of wind when the jib unfurled while moored.  In winds over 35kts, it is beyond the ability of two old men to handle without everything being powered.  With a schooner rig, first reef would be easy enough to drop the trailing sail although it would be tedious to lift and lower the sails each shunt.  Sailing on the trailing sail would produce a lot of weather helm and make rounding up likely.  One of the issues with a HP is accessing the trailing end of the boom when it is hanging over the water.  So if reef lines are not in place then you need to completely drop the sails and get the boom inboard to access the ends.  On the 18m proa the trailing end of the boom collides with the cabin roof if it is allowed to travel that far.  It is to high to reacxh from the deck.  It has damaged solar panels when someone unfamiliar with the boat was “trying things out”.  It would be wise to make certain you can get the boom on deck where it is easily and safely accessed.

The jib on the 18m proa has furling but the furling drum is small and makes furling difficult.  I have doubts about furling the jib in winds over 30kts but it is only 26sq.m and we have carried it to about 35kts through gusts but not tried to reef it in those circumstances.   

One other detail I would take great care with on a HP is working on keeping the windage as low as possible.  On my pedal boats, I have determined that half of my energy is going into air when pushing into just 10kt wind; about 15kt apparent.  Point is, it does not take much power to move a slender hull through the water.  It is almost certain that the prime design consideration for a propulsion system on a HP will be motoring against the wind.   Look very closely at what is presented to the wind and how well it is faired.  The cabin of the 18m proa has 2m of headroom over the deck and bout 3m over the ww hull.  The only place it really needs that much is the walkway from the cockpit across the cabin to the ww hull.  The ends and portion over the ww hull could have considerably more slope without impinging much on headroom.  The large windows allow a lot of light in and it gets hot inside the cabin in summer - nice in winter though.   Rooftop solar panels in Australia are mandated to be mounted with a slope so they self-clean and reduce direct impact from hail.  The panels on the 18m proa cabin are almost flat and are not self-cleaning so sloping cabin top is better in that regard.  The beams are chamfered but they could be even more streamline.  The portion of the cabin over the bunks is not very useful.  It slopes slightly and not the safest place to walk.  It is area that could be covered in panels.  There should be a flatish walkway with grab rail or safety lines to get to the ww side of the ww hull.

Rick

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