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Old 04-27-2008
brak brak is offline
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Let's look at this from another direction - weight vs range.
So, my boat currently has an approx. 50hp (50kW) engine and a 50-60 gallon fuel tank (it's inside the keel, and I don't know for sure as I try not to go on empty, the fuel on the bottom is pretty nasty). 50-60 gallons of diesel (at 7 lbs a gallon) weight something like 350-420 pounds.

Now, the engine from E-POD system (they had a spec) provides an equivalent of 7-9 hp outboard (yikes) and has a max. continuous amperage of 170 amp (and recommended cruise amperage of 50 amp, which clearly would provde about 30% of max power, or 2-3 hp). It is also 24-48v, which of course makes a lot of difference, but let's assume 24v somehow (24v * 50amp = 1200 watt, or 1.2 kw or about 1hp, at 48v it's 2 hp - but hey, 1 hp is aplenty for a boat, right?).

Ok, so 50 gallons of fuel last me from 50 to 75 hours. Let's go with 50, just to give electric a headstart. We'll need 50*50 = 2500 watt-hours of 24v electricity to run this thing. Now, modern battery amp hour ratings are given at 20 amp draw (and go down with higher draw) but we'll assume the best. So, our favorite 6v golfcart dekas are 215 amp hours each. We would need 4 times each for 24v and then 12 times groups of 4 for 2500 watt hours. Total of 48 batteries. At 65lbs each, thats about 3000 lbs. I suppose if you replace the ballast with batteries that might just work 3000lbs may be a bit difficult to stow though, what with carrying each of the 48 batteries. Of course since you can't discharge the batteries all the way (unlike the fuel tank), you need twice as much - 100 batteries, 6000 lbs

Of course that just gave us a system that is 50 times less powerful (1hp vs 50). For more power, don't forget to multiply the battery count and weight times 50

With current lead-acid battery technology, electric propulsion is just not there. Anyone trying to sell it is either a crook or a nutcase.
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