Hi Tim,
Thank you for continuing the discussion, developing this answer led me to some data that I hadn't seen before.
Many people smarter than me have been trying to towards this goal for a long time. If you believe that there is a better way to collect hydro-power on a boat, let's talk specifics.
The Ampair UW100 hydro generator (permanently mounted) generates 50W or 80W at 6kts, depending on the model.
http://www.ampair.com/downloads/UW_100_manual_rev_Feb_2007.pdf
The Aquair 100 using a "towed turbine" generates 30W or 55W at 6kts depending on the turbine.
http://www.ampair.com/downloads/Aquair_100_manual_rev_Feb_2007.pdf
The Ferris Waterpower 200 claims 144W at 6kts.
http://www.hamiltonferris.com/water-power.html
And the high tech leaders, Watt & Sea (they're the expensive ones that keeps popping up in discussions) have a cruising version that they claim produces 120W at 5kts and 500W at 8kts. I'll estimate that their graph shows 180W at 6kts, certainly the best claim that I've seen.
http://www.wattandsea.com/en/cruising-hydrogenerator
http://www.wattandsea.com/sites/default/files/W%26S-cruising-GB.pdf
So if we assume that Watt & Sea is doing it right, at least better than anyone else, let's do the math.
I don't know the watts to knots performance of your boat, but we can use mine for an example. If we compare 5kts motoring (a little slow for most recreational boaters) and 5kts sailing (an easy day sail). Motoring = 2500W and regen = 120W or 4.8% total system efficiency (your term) or TSE for 5/5 operation. If we say that 4kts motoring is fast enough, my load drops to 1100W. Now motoring = 1100W and regen is 120W. or 10.9% TSE for 4/5 operation. Now let's kick the regen speed up to 6kts. Motoring at 4kts = 1100W and regen at 6kts = 180W or 16.3% TSE for 4/6 operation, hey, we're pretty close to your 20% target.
All of this assumes a 28'-32' sailing auxilary. So what does it take to get there. Many of these sailboats are in the 4-6 ton range so the 5-6kW drives that are available from the list's vendors are just about right sized and should deliver motoring performance close to what I've quoted. These drives cost about $5000 without batteries, chargers, extra instrumentation, new props, etc. I put my installation with Lithium batteries at somewhere around around $10,000, not including some of the general boat repair and upgrades that I did at the same time. But as I've stated before, I've only seen 70W at 6kts (for a variety of reasons) or 6.3% TSE for 4/6 operataion. The Watt & Sea lists for $10,000, but there are reports of "boat show prices" out there around $7200. They don't have a 48V model, but perhaps they could program one (might cost more, I'll say $10,000 for the 48V version). So I'll say that by doubling my conversion cost, my boat should operate at the previously listed 16.3% TSE for 4/6 operation.
Would it be worth the money? Let's consider my case. To cover the case that I might need to charge my boat away from the slip, I bought a Yamaha 2000W portable generator for $1000. It will power my shore power charger and provides 28A @ 54V or 1500W. So far, I've never needed to charge my batteries underway, although I know that I can from a one hour test. So for my use case, the extra cost isn't really justified.
At this time, I don't know if it is possible to get closer to your 20% TSE target. But if drives get more efficient, or someone does better than the Watt & Sea product, your proposition could become reality. For now, it might actually be too optimistic.
Good question and thank you for encouraging me to do the analysis.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Tim" <haywardt@...> wrote:
>
> Eric,
> I guess I should have made myself more clear.
> Streams don't flow at anywhere near 6 knots and the impeller I am thinking of is in the stream channel and has no penstock or dam. There are also tidal generators that use screw impellers in a tidal channel and extract energy with good efficiency at much lower current....
>
> --
> My contention is this:
> A stream of water 18" in diameter moving at 4 knots contains a significant amount of energy, doesn't it? If we aren't able to extract that energy we are doing it wrong.
>
> Personal Conjecture:
> From a marketing perspective it seems that if, for every five hours of sailing, you could motor for an hour, electric drive would be very attractive to a large number of sailors. A 20% total system efficiency doesn't seem to optimistic. A package that ran on regen would be compact, simple, and mass-marketable. Sailors could get back to sailing, rather than engine management, and still have the ability to maneuver and dock under power.
>
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