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I think you're off by about a factor of 10.40 NiMH batteries (2.5 AH, 1.2V) arranged in parallel is equivalent to 1 battery, 1.2V, 100AH.
Going with D cells, you would need about 1/4 that number, or 100 batteries or so, which at least will make it easier to wire up. Perhaps even consider larger batteries such as F size.
Oh, one note about running 12V car batteries in parallel. Kill one cell, and it will quickly kill the whole set. I don't know if NiMH batteries suffer with the same low cell problems that lead acid batteries have.
Hi David, Before you undertake any electrical work, try to take a close look at the ballast arrangements in your craft, (Can some ballast be replaced by lead acid batteries ?) and consider the fixed installation of a solar panel for charging purposes.
In as far as the choice of batteries goes, I would go for lead acid batteries in any case, but please stay clear of Lithium batteries. As Lithium batteries and water are two incompatible friends and have a potential to cause severe fires in your craft.
There are some large format NiMH batteries. As mentioned, the Prius batteries are NiMH, I think, as well as the batteries in some of the early electric vehicles.
However, there was a patent on the NiMH batteries which has very much damaged the production of large scale NiMH batteries. Most of the patents are due to expire sometime this year (2014). It is hard to say if there will be a resurgence of the technology once the patent encumbrance is lifted.
As far as using Amps as a rating on batteries. As you know, Watts=Amps X Volts.
Likewise, the discharge current is rated in either Amps, or percent of "capacity" in AH, so a 100AH battery (independent of voltage) might be rated for a 0.3C discharge, or 30 Amp continuous discharge.
How is longevity rated on your little AA batteries?
Another option to consider is battery packs already designed for various hand tools. They should be already setup with necessary chargers, monitoring equipment, and etc.
Beware of assembling large stacks of cells that aren't designed for such assembly! You can get away with 10 x AA NiMH cells in series because in the event of one failing open-circuit the voltage across it will only be 10.8V. But if you put 40 cells in parallel and one fails short-circuit, the prospective fault current through the dud cell is several hundred amps and more than likely to burn a hole in your boat.
Face it, lead-acid batteries are crude, oldfashioned, heavy, but extremely robust, tolerant and reliable for high current demand. There's a good reason why they are the only approved technology for most small planes, and judging from the problems Boeing are having with Dreamliner batteries, I'd rather have a PbAc in a boat than anything else right now.
I wouldn't consider a capacitor as giving a boost for more than a few seconds. And, really any "boost" would be due to an over-voltage which you might be able to wire directly with your batteries, assuming the batteries are capable of putting out the amps you require. Of course the over-voltage does put your motor at somewhat of a risk. I suppose the super cap may also help a bit with peak current draw, if you normally use far below what your batteries can put out, but occasionally hit higher current loads (many batteries have peak, and continuous current ratings). Can you lug your boat motor?
You're talking about doing rowing, sailing, and whatever. Everything depends on the size of your boat.
I've driven an Electric Ford Ranger using lead acid batteries. They are HEAVY for pretty marginal range. I'm considering an electric pedal assist vehicle, but will choose a design that is much lighter.
Likewise, a boat is somewhat more forgiving with a bit of extra weight, but the weight is not entirely free. Add 1000 lbs of batteries, and it will be a pain to row by hand upstream.
Here is a list of energy densities. The attraction of gasoline is 50 or so times the energy density of the Lithium Ion batteries.
Anyway, there is an advantage of light, high energy density batteries, even if you would benefit from some ballast. But, other than the advantage of being able to easily shift a water ballast, there isn't a lot of sense carrying around a boat load of water either.
I visited a small museum at Windermere that had a collection of electrically powered boats that were very popular in Edwardian times although that said it was rare to find any surviving craft because of the destructive effect of leaking acid.This is something to consider if using Lead acid batteries.