Jak,
You already have reasonable experience with regen while sailing. If you stop and think about your towed generator, it is optimized for power generation and it produces about 80W (13.5V x 6A) at 6kts, probably considerably less at 5kts. I would not plan on doing any better than that with a system that is designed for propulsion, the regen has to be a secondary consideration.
So you need to ask yourself, how much of the time do you expect your 26' boat to be able to sail at 6kts? We'll assume that your 7kW motor is driven with 48V, that means that your max power will be around 145A. If your drive is somehow as efficient as your towed generator, then your max regen will be 1.5A at 6kts. I think that this is optimistic, but we'll go with that number for now.
We have no idea where on earth you sail, but let's assume that you'll be satisfied with a capacity of only 1 hour at 4kts under power or a practical range of 4nm. You'll probably need about 2kWh of usable energy to cover this distance. So how long will it take to recharge 2000Wh with an 80W charger? The answer is 25 hours of sailing at 6kts to recharge from 1 hour powering at 4kts. That assumes no other electrical load during the recharging period, house loads will make the recharging process take longer.
So far we've been estimating with best case scenarios, in practice, your results will probably be somewhat less due to prop inefficiencies, drive train friction, controller losses and charging losses going into the batteries.
Larger (faster) boats can do more with regen, there are a number of 40'+ catamarans that are pretty successful at collecting power at sailing speeds around 8kts and faster, but your 26' boat won't be able to do that. 8kts is hull speed for a 45' waterline length in a displacement keelboat.
So I would guess that Niki was getting as much regen as the laws of physics allow for, which is nowhere near the expectations that drive manufacturers set when they say "you can recharge your batteries while you're sailing". I find electric drive manufacturers to be disingenuous when they pitch regen in small (less than 30') sailboats
Therefore, I think that it is pretty safe to say that 2kWh of regen is a theoretical maximum for 24 hours of 6kt speeds, regardless of the size of the boat. In practice, the results will probably be somewhat less.
I want regen as much as you do (maybe more), so please, somebody, prove me wrong.
Fair winds,
Eric
Marina del Rey, CA
1964 Cheoy Lee Bermuda 30 ketch
5.5kW Propulsion Marine drive, 8kWh LiFePO4 batteries
--- In electricboats@yahoogroups.com, "Jak Mang" <jak.mang@...> wrote:
>
> I am planning on building a 26 foot sailboat with a small (7kw) electric motor. I am hoping to get regeneration to work under sail. I have not heard a lot of success stories. I do have a towed generator on another boat that will produce 6 amps at 6 knots. The tow is sometimes difficult to handle, so I would like an integrated system. I am aiming for a long distance racer that would not motor except for in and out of marinas. Basically an engine-less boat with a little help for maneuvering. The regen would also supply the house needs for navigation and lights. Solar panels are great, but take up space and increase windage. The batteries will be designed in as part of the ballast package.
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> It sounds like the correct combination of prop, gear reduction, motor and boat is difficult to arrive at. Niki Perryman of Classic Boat in the UK is removing her Thoosa because they could not get regen to work properly. One problem with tuning the system is probably the cost of switching components that are not initially sized properly.
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> I am also considering using two props, one for the drive motor and another for a small towed-type generator permanently mounted inboard. I know that this will work, but it increases complexity, number of holes in the hull, etc.
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> I would appreciate hearing about any experiences that you have had with regeneration under sail.
>
> Thanks,
> -jak
>
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