Regen is an interesting recurring topic on this forum.
And there are various ways to look at it---find the way that suits your needs and go with it.
Just be sure to respect the laws of physics and known applicable equations and curves.
As mentioned by Chris, the return is not great and if the regen is to be had while under sail (rather than anchored in current), there’s a penalty to be paid: Speed of boat in water.
Where the regen tradeoff is not worthwhile for sailing is if you need to keep the speed up to reach the destination by a certain tide or time and if you will have power available at your destination anyway. If it costs 1knot on your speed to capture a mere 200watts of electricity, while it requires a doubing of power for the boat to increase speed by 1knot, the cost of regen can be steep if the primary need is to cover distance quickly and you expect good wind the entire way.
Where it would certainly be worthwhile is if you’re sailing and really don’t need the speed, but could or will need the electricity later.
And there’s a million other circumstances between these that could swing either way. Be conservative and have backup plans---having regen can be part of that backup plan. But you might best add a wind generator instead.
Of course the no-brainer free-energy case is where you expect to be anchoring in a high current zone----but how often do you really want to do that?
-Myles
From: electricboats@
Sent: Sunday, March 07, 2010 6:43 PM
To: electricboats@
Subject: Re: [Electric Boats] propellers
Hi Larry,
For while I had an Epod on my boat and experimented with regen. There were too many design faults with the motor to continue with it, and I eventually abandoned it in favour of a Torqeedo 4.
But I did actually see some regen in the limited opportunities I had to test the Epod. I needed around 7 knots or more of boat speed to get some useful amps - around 2 -3 amps I recall, and at around 10 or 11 knots I've seen up to 6 amps going to the batteries (thats a 48v bank).
These results are from a fast trimaran and it seemed that on days with a nice wind of say 10 to 15 knots and quite a few hours sailing, you could recover the amps from typical motor usage to get on and off the anchor or mooring. The cost was around 1 knot - that is, the prop drag when regenerating slowed the boat by about 1 knot.
From this limited experience practical regen for a keeler of less than 40 ft seems out of reach for every day sailing. The boat speeds are just not high enough on a regular basis. Especially keep in mind that regen slows the boat, so even when the boat is doing 7 knots under sail, just adding the drag of regen could slow it by one knot, and that reduction in speed reduces the regen to a barely measurable level.
As to Kevin P's idea of electro-sailing - I suspect its pie in the sky. I never saw it despite trying for it a few times. I'd set the throttle so she'd do a few knots under power - for those moments when the wind died completely, hoping that when the wind picked up, I'd see regen to top up the batteries... Well, it never happened. And she'd easily accelerate to 10 to 12 knots, and at these speeds there was no regen. To get the regen back I'd have to reset the throttle to chase the sweet spot that showed some amps flowing back to the batteries.
To answer your question about how much power should be generated with time, I would be content if I could have about a 10:1 factor for regen. That is, if 10 hours sailing could give me 1 hour cruise speed motoring, I'd be happy with that rate of return. For me, 20 amps under power gives a useful speed, and that would mean regen would need to average 2 amps. To be useful for a broad range of boats, I think regen would need to kick in around 4 to 5 knots of boatspeed. Would this be practical?
I suspect that a prop that is designed with regen in mind may have different properties to the props we have to choose from - all designed for propulsion. Perhaps in future we'll have props that are good for regen, and the price we'd have to pay is a reduction in efficiency on the propulsion side. With your expertise, maybe you can come up with prop thats good for both?
In the meantime, I'll press on with setting up a towing generator - with that I can optimise the pitch for generating. I have a variable pitch pro-pulse prop available for experiments with that.
Cheers
Chris
On 07/03/2010, at 5:16 AM, gramplarry wrote:
this discussion sounds to me that regeneration has been discussed and even tried but seems to have been less successful than everyone would like. by this i mean pure regeneration not just matching the prop to sailing speed.
what happened and what are the reasons for not being successful.
how much drag for regeneration would be acceptable?
how much power needs to be supplied by the propeller to the motor to have regeneration?
what propeller rpm and torque would start and maintain regeneration.
How much power should be regenerated with time.
what usage? house batteries and power batteries of both etc.
--- In electricboats@
>
> I believe, but can't yet prove, that the one area where "regeneration" might actually be practical is in what Kevin P calls "electrosailing" - matching the prop rotation to the boat speed so that it's regenerating about 50% of the time (very low wattage mind you) The point of this is not really regen, but rather nullifying the prop drag and getting the extra 1/2 to 3/4 knot of speed. Over a sufficient distance, this extra speed is the practical equivalent of a feathering prop (without the drawbacks) and nearly net zero energy use.
>
> I currently have a 3-blade, so the above is more necessity than choice.
>
> -K
>_,___
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