Regeneration

Regeneration refers to the _potential_ ability of sailing vessels to extract some of the energy available while sailing by letting the prop freewheel and using a properly configured motor/controller combination which supports regen to feed elecrticity back into the batteries.

While a tantalizing prospect, and proven for land-based electric vehicles (or even hybrids such as the Prius), a quite different set of conditions are present on a boat. In a land-based vehicle, nearly all of the vehicles potential energy is available for capture during braking. On a boat, only that energy which can be recovered by the prop is available, and that is a small fraction of the total (water being free to bypass the prop unlike road under a tire)

A post on the ElectricBoats Yahoo! group explains it best

On the Yahoo group Electricboats I attempted to determine whether anyone has actually demonstrated practical regeneration (search "mythbusters"). I defined practical as a 10:1 ratio - e.g. 10 hours of sailing to provide 1 hour of motoring. My challenge basically went un-answered, so I set out to do some measurements of my own.

My findings are that under optimal conditions on my vessel, I could expect to recover about 50-100 watts of power (including about 50watts lost to the controller), and then only in a very steady breeze. As my system nominally uses 1500 watts for propulsion (4kts) and this represents a ratio of > 15:1 I declared the myth busted. Note that we're not even factoring in the house load which on most boats is going to approach 100 watt-hours.

A counterpoint to my skepticism includes comments by Nigel Calder, a renowned author and expert on boat systems :

"One thing we already know from the Lagoon is the tremendous regenerative capabilities of these systems – at times this boat was producing 3 kW under sail from each of the electric motors (one in each hull)." (from http://www.maloyachts.se/Blog/tabid/195/EntryID/16/Default.aspx)

But here's a contrary report on the Lagoon 420 using a similar rig.

And here's another, slightly more detailed report of practical regen - albeit on a very sophisticated, and lightweight boat. Nigel Calder references this boat in his very detailed article in Boatbuilder Magazine but suggests it is under-powered for heavy conditions.

As with all scientific progress the proof will be in the reproduction, and until we get some data of comparable installations producing comparable results, I think skepticism should be encouraged. I tend to drive this point home a bit, in part because I was so disappointed to see the actuality so different from the hype. More research is required to demonstrate actual numbers across a range of vessels.

However, I do believe a system could be designed to meet the challenge - it would just be expensive and include some combination of things like:

    • Fancy controller electronics which can dynamically feather the prop rotation to boat speed.
    • Large efficient prop, facing the right way and pitched/shaped to optimize regen
    • A Kort or Rice nozzle to concentrate the potential energy around the prop
    • Direct drive to reduce transmission losses.
    • Super-capacitors to swallow the input efficiently

*"Feathering" or "electro-sailing". This involves setting the prop to freewheel sufficient to neutralize the drag of the prop through the water. This can be considered a form of regeneration. Let's say, for example by feathering the prop, you can gain .5kts on a 20nm trip - so instead of going 4kts, you're going 4.5. That saves about 15 minutes on the trip - not a lot, but on my boat that would be about 10AH saved. However, a good folding prop might net you the same gain and not require the added expense of a system with regenerative capability.