“Its a tossup whether you are happy to lose a knot or 2 to drag regenerating , or motorsail , and gain a knot or 2 .”
It’s not really close to a tossup. And the conditions for losing “a knot or 2” due to drag with a towed generator are pretty constrained.
Let’s see: Assuming the towed generator prop is optimized, it should be close to 70% efficient at its rated output. The generator itself might also be half decent and 90% efficient or so. More likely both of these are optimistic. The water power required to deliver the 80watts can then be estimated as:
Pwater = 80 / (0.9 * 0.7) = 127watts
If in reality, the efficiencies were 60% and 80%, respective, then we’d expect 167watts of water power required.
Let’s be conservative and say that it takes 200watts of water power (i.e. sensed as “drag” to the boat) to deliver 80watts of electricity.
Now, it’s common for our displacement craft to require doubling of the power to increase speed by 1 knot in propulsion.
With an electric boat, assuming we lose 50% battery power in delivering power to the water, then 200watts of water power equates to 400watts of battery power. Okay, so say at 6-knots, we throw the towed generator out and it’s delivering 80watts of electricity.
We know that at most it’ll be causing 200watts of drag and that this would be equivalent to sucking 400watts of power from our batteries to make up for it.
For 200watts of drag to reduce the speed to 5-knots, we’d have had to be driving the boat with only 800watts of power at 6knots prior to towing the generator.
So a boat that requires more than 800watts to go 6-knots will NOT slow down anywhere close to 1 knot by towing that 80watt generator.
This is all rough, but as you can see, it’s not that difficult to get estimates of this if you’re careful.
To note from this analysis:
· Generating 80watts of electricity cost the equivalent of 400watts of electricity --- a net loss of 320watts if the boat were driven by electric and not wind. Try it and see!
· The actual drop in speed depends on how significant 200watts of drag is to your system---e.g. my boat, at 6kts requires about 6HP. At 5kts, it requires about 3HP---so to slow the boat down to 5kts, you’d have to add 1225watts (3HP*50%) of equivalent drag power by water or air to slow down 1kt. 200watts of drag is absolutely unnoticeable at that speed and wouldn’t slow my boat down but perhaps 0.1knot. Worse, I’d lose 320watts additionally!
The above could be repeated to determine under what conditions a boat would slow down by 2kts.
In case this helps-
-Myles Twete, Portland, Or.
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Rob Johnson
Sent: Sunday, March 27, 2011 3:12 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: Experiences with Sail-powered Regeneration?
I was thinking the same thing Richard.
Its a tossup whether you are happy to lose a knot or 2 to drag regenerating , or motorsail , and gain a knot or 2 .
Lots of variables , I guess.
Rob J.
From: Richard Mair <fullkeel2000@yahoo.ca>
To: electricboats@yahoogroups.com
Sent: Mon, 28 March, 2011 9:02:05 AM
Subject: Re: [Electric Boats] Re: Experiences with Sail-powered Regeneration?
Or the extra drag may slow you down to the point you are only getting 80w or less from both depending on the start up point..
Thanks. That all make perfect sense. Given that 80W is optimal, is anybody really achieving that. Are the vendors over selling the practicality or functionality? That recharge rate will not please many owners, but it would be fine for me. Can I achieve it with a single motor or do I have to design in an auxiliary towed-type generator? |
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