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post #63 of Old 10-16-2012
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Re: Optimal propulsion system


Again we get to an energy density problem. Lets assume that we design a keel so that the entire mass is one big battery. And let's assume that the design will keep the same density as raw lead (we know this isn't true, but it makes the math easier).

Taking a Beneteau 31 for our comparison (because they use iron keels, so the density might be closer to correct)

1) the 31 carries 2,200lbs in ballast.
2) lead batteries have an energy density roughly equal to 1/650 of diesel.
3) converting the entire mass of the ballast gives the same energy potential as about 4 gallons of diesel.

So we suspend a huge amount of money designing the battery pack, figuring out the electrical issues, have a relatively short lifespan for the ballast, ect... And still only have 1/10th the motoring range of a comperable Diesel engine. But we can't carry jerry cans to full up on route. If we run out of power, we either have to find a marine with power, or be able to generate power on board.

So if we want to reasonably extend the range, we have to put a pretty large generator on board, which is going to require adding back the fuel tanks, thru-hulls, ect that we avoided with the electric drive in the first place. How big of a generator? Figure 15kw would be sized to the engine.

Rit now there just isn't a storage device anywhere close to the same density of diesel. For all it's problems, fossil fuels are insainly energy dense compared to anything else we have come up with, and while battery technology is getting better, there isn't anything on the drawing board that even comes close to fossil fuels.

So how much do batteries need to improve?

Diesel.......................45.4. Mj/kg
Lithium ion batteries ...0.72
Lead acid battery........0.1

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