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Old 17-08-2014, 11:15   #1
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Cheap Electric Drive for a sailboat

saw this on the tampa florida craigslist. it's a hunter 28.5 for sale - $9500 asking - with an owner built electric drive. kind of novel and simplistic. here's the url for the ad -

1985 Hunter 28.5 Sailboat, Electric Inboard Motor, 6'2" Headroom!
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Old 21-08-2014, 19:43   #2
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Re: Cheap Electric Drive for a sailboat

18' Sailboat Electric Inboard Engine
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Old 22-08-2014, 07:46   #3
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Re: Cheap Electric Drive for a sailboat

No one ever said it was hard to build an electric boat. The problem has always been providing the power to run it. Electric motors are reliable and powerful but they need electricity.

Interestingly he only includes a vague comment comment about run time will depend on the battery bank size but never indicates the size of the current bank or the current run time. I presume this means he doesn't have the battery bank, so I'm curious how he knows it will easily reach hull speed.
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Old 22-08-2014, 08:24   #4
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Re: Cheap Electric Drive for a sailboat

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Cool little boat. It would be great with a few solar panels on the coach roof. Makes me wish I lived on lake front property.

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No one ever said it was hard to build an electric boat. The problem has always been providing the power to run it. Electric motors are reliable and powerful but they need electricity.
The same could be said for a diesel. They need fuel. Distance they go would depend on tank size vs. battery capacity.
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Old 22-08-2014, 13:25   #5
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Re: Cheap Electric Drive for a sailboat

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The same could be said for a diesel. They need fuel. Distance they go would depend on tank size vs. battery capacity.
Care to share the size of a battery bank that will replace a modest 20 gallon tank?

Sorry but at best you have a technicality that has no bearing on real life.
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Old 22-08-2014, 15:36   #6
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Re: Cheap Electric Drive for a sailboat

Interesting and answerable question. Wikipedia[1] show one gallon of diesel as having 37.95 kwh of energy and that diesels are 40% efficient. So 20 gallon tank * 37.95 = 758 kwh pack / 42% = 318 kwh pack needed to replace a 20 gallon tank. This assuming the electric motor is near 98% efficient and that the boat does not have solar or wind to "make fuel" in transit. Which could greatly lower the below tank-age needed.

[1]http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent
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Old 22-08-2014, 15:50   #7
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Re: Cheap Electric Drive for a sailboat

For comparison, I have a 10hp inboard Electric motor. It runs off of 6 ea, 6 volt golf cart batteries, in series (36 volts). At full power, I can get maybe 5 knots, for 20-30 minutes. Probably 45-60 minutes at half speed. You can probably do the math at how many extra banks you'd have to have for each extra hour to match a 20 gallon diesel tank. No realistic comparison.

Also, it takes about 6-8 hours to fully charge this bank with shore power. Never tried with solar. Thought about adding at least one more bank for island hopping, but at 500lbs per bank, not worth it. I use an outboard for long runs, and the electric for marina hops and anchoring. With solar, you could have free (and instant) power for the small stuff but not practical for long distance yet.
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Old 22-08-2014, 16:34   #8
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Re: Cheap Electric Drive for a sailboat

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Interesting and answerable question. Wikipedia[1] show one gallon of diesel as having 37.95 kwh of energy and that diesels are 40% efficient. So 20 gallon tank * 37.95 = 758 kwh pack / 42% = 318 kwh pack needed to replace a 20 gallon tank. This assuming the electric motor is near 98% efficient and that the boat does not have solar or wind to "make fuel" in transit. Which could greatly lower the below tank-age needed.

[1]http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent
Can someone tell me where my math has gone wrong here?

Taking your 318KWh:

318KWh = 318,000Wh = 26,500Ah. at 12V

Using a minimum 50% SOC for LA batteries, that means we need 53,000Ah of 12v FLA storage. That's about 235 x 225Ah batteries at around 60KG each. Or approximately 14 metric tonnes.
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Old 22-08-2014, 16:54   #9
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Re: Cheap Electric Drive for a sailboat

At 48volts that would be a 6625ah battery pack. With lithium it would weight 10,560 pounds or 5 tons. For reference a Model S pack is 90 kwh so we are talking A little over three Model S packs. Would be nice. But I would settle for one Model S pack and a boat load of solar panels.
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Old 22-08-2014, 17:41   #10
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Re: Cheap Electric Drive for a sailboat

At say $150 per 225Ah 12V FLA, that would be $35,000 (ish) in battery cost.

What would the 6625ah of 48V lithium cost? I've no experience with them, but I've seen estimates of $200 per KWh which would make it about twice that price.
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Old 22-08-2014, 23:54   #11
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Re: Cheap Electric Drive for a sailboat

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Care to share the size of a battery bank that will replace a modest 20 gallon tank?

Sorry but at best you have a technicality that has no bearing on real life.
No reason to get your nickers in a twist. I'm merely pointing out both diesel motor and electric motor operating time depend on the size of source that feed them. I'm surprised to see more and more electric around. For the time being, they serve the owners well to get in and out of the slip and down the fairway. Hope that's enough real life for you.
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Old 23-08-2014, 04:33   #12
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Re: Cheap Electric Drive for a sailboat

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No reason to get your nickers in a twist. I'm merely pointing out both diesel motor and electric motor operating time depend on the size of source that feed them. I'm surprised to see more and more electric around. For the time being, they serve the owners well to get in and out of the slip and down the fairway. Hope that's enough real life for you.
My nickers are just fine. I think everyone knew diesel motors needed fuel before you added your insightful comment.

The point still stands that to get anything even modestly close to the range of a diesel, an electric boat requires a massive battery bank that is totally impractical.

I know a few guys who are part of a steam engine club and they actually farm a small plot of land, doesn't mean they are in any way practical. They do it because they enjoy playing with steam engines. I don't confuse that with a viable alternative to farming with diesel powered tractors. So far the electric boat conversions all fall in the same category.
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Old 23-08-2014, 10:41   #13
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Re: Cheap Electric Drive for a sailboat

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My nickers are just fine. I think everyone knew diesel motors needed fuel before you added your insightful comment.

The point still stands that to get anything even modestly close to the range of a diesel, an electric boat requires a massive battery bank that is totally impractical.

I know a few guys who are part of a steam engine club and they actually farm a small plot of land, doesn't mean they are in any way practical. They do it because they enjoy playing with steam engines. I don't confuse that with a viable alternative to farming with diesel powered tractors. So far the electric boat conversions all fall in the same category.
In an earlier post you stated:
"The problem has always been providing the power to run it. Electric motors are reliable and powerful but they need electricity".
I was only stating that fuel is fuel whether it be supplied by the storage of electricity of a battery or storage of fossil fuel. Perhaps that is the insight you did not get.
In the San Francisco bay there are a handful of sailors that seem to get along just fine with their electric power. Practical?...It is for them. Not everyone wants to motor long distances.
I now see electric propulsion systems at boat shows now, so something is happening. There is no argument from me that having huge amounts of battery storage is no practical.
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Old 23-08-2014, 11:57   #14
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Re: Cheap Electric Drive for a sailboat

having converted to electric propulsion in 2008 I can say Electric Propulsion works just fine in SAILBOATS. With a lot less maintenance than diesel propulsion. As auxiliary propulsion it can be used in ways that people would not consider using a diesel to augment the sailing experience. Yes, there are limitations to a pure electric system at this point but, less than most people think and those limitations can be overcome quite easily and pretty cheaply.
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Old 23-08-2014, 12:53   #15
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Re: Cheap Electric Drive for a sailboat

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At say $150 per 225Ah 12V FLA, that would be $35,000 (ish) in battery cost.

What would the 6625ah of 48V lithium cost? I've no experience with them, but I've seen estimates of $200 per KWh which would make it about twice that price.

I paid $10,000 for a 1000ah LiFePo pack. You start to get volume discounts with bigger buys. So I would guess around $60,000. But again I think your money would be better spent on a 1000ah pack and then another $8000 in solar panels. That should get you pretty reasonable range with very low and possible zero cost in fuel. Though at $18,000 you would have to do a lot of boating to recover that cost in fuel and diesel repairs. If anyone knows the cost of fuel and repairs for a diesel per mile traveled we could easily figure out the number of miles needed for break even.
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