Good points as usual Eric.
From: ewdysar@yahoo.com [electricboats] <electricboats@yahoogroups.com>
To: electricboats <electricboats@yahoogroups.com>
Sent: Tue, May 23, 2017 10:46 pm
Subject: [Electric Boats] Re: PROPELLER REGENERATION WITH HPEVs MOTOR UNDER SAIL?
That's not exactly what I said. What I said is that nobody seems to be able to generate more than 100W at 6kts. At higher speeds, much more power is available. I'm guessing that most streams flow at rates higher than 6kts, certainly so if one uses a collector and develops system head (fall of water) to accelerate the flow over the impeller or prop.
If your boat can sail at sustained speeds of 7 or 8kts or more, regen can become a significant source of energy. But for sailboats in the 30' range (the sweet spot for electric drive systems with 5-6kW of power and battery packs around 10kWh or 500 lbs.), hull speed is about 6.5kts and 6kts is about all you can average in winds less than Beaufort 6. So the energy from regeneration on most of our boats is limited.
Here's what I posted last September (post #19662)
"Since you brought up regen, I'll say it again. I haven't seen any substantiated report of 80W @ 5kts. Push the boat up to 6kts and I would be very surprised to see 120W of regen. I've seen brief moments of 60-70W while sailing my boat at 6kts but nothing consistent. Once a boat hits a consistent 8kts+ (big boats and multihulls), then regen starts to cover more than conservative house loads.
It's the data that nobody likes to hear, but if one motors at 4kts, you typically have to sail 40 times as long at 5kts to recover all the used energy. That means 15 minutes motoring at 4kts will take 10 hours at 5 knots to recharge the traction bank, longer if you're using any of the energy for house loads like lights, radio, or an autopilot. At 6kts, the time to recharge 15 minutes of 4kts motoring drops to 6.7 hours. Again, adding in any house loads extends the recharge time. But on a long passage, 100W for 24 hours does come to 2.4kWh. If you can keep your house usage to half of that, then 1.2kWh could end up back in your batteries. It will rarely be of any significant value during coastal hopping or gunkholing. I still want somebody to prove me wrong, I've been waiting for a while now...."
Mark from Santa Cruz just posted some preliminary figures after his recent prop change. He observed 1.8-2.2A (we'll call it 2A average) at 50-51V (we'll call it 51V) at 6kts. And the results are (drum roll please)........ 102W at 6kts. Hmmmm. I guess that I'm still waiting....
The bottom line is that I'm OK with these numbers. But this is not the impression that potential electric converts get when they read "Generates power while under sail" or "Most boats will start regenerating at 3.5 – 4 knots boat speed. When at, or near hull speed, the charging current should be around 4A – 6A." in an electric drive product description. While these figures might be true for some boats (as discussed earlier) it's not really true for most of the boats that these systems are sized for. Another vendor states "Not all boats will be able to achieve regeneration. The amount of power generated should be considered supplemental and comparable to what you can achieve through wind and solar chargers." This sounds closer to my personal experience.
The point is that I'm trying to share my practical experience to help people set realistic expectations of their electric conversions. We had another thread last year that discussed the article "Ditching Electric" from the magazine Classic Boat. The owners of the boat "Siandra" were removing their electric drive after three years. It appears that their biggest complaint was that their system did not regenerate as much energy as they expected and therefore their electric drive did not support their particular use case. Perhaps if they had been told how much power they would really get from regeneration, they would have found other ways to mitigate their energy deficit. As it was, they believed their salesperson, spent considerable money converting to electric, wrote a supportive article about electric conversion in 2008, got dissatisfied, spent considerable money converting back to diesel and wrote a less supportive article in 2011. I'm sure that their experience and exposure to their story hurt the electric boat community more than it helped. All stemming from unrealistic expectations.
Fair winds,
Eric
Marina del Rey, CA
I still have yet to see anyone back up a claim of more than 100W at 6kts, even from a $10,000+ computer-controlled, variable-pitch, transom-mounted, in-water generator.
Fair winds,
Eric
Marina del Rey, CA
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> Well - I'd certainly agree if you referring to a fixed pitch propulsion propeller being used for re-gen, but there are other ways to skin this cat. Prop's and generators optimised for the purpose can do better.
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> http://www.cruisingworld.com/gear/electronics/green-power-on-passage
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