You could go for a stick of 10 sub-c size cells running for and aft along the keel. That would be closer to the 4Ah capacity, and I would say a larger pack would be more suitable for a boat this size.
A stick of D size cells would be required if you want higher capacity whilst remaining with Ni-MH technology. I would think you will have room in that big S-class, but I could be mistaken.
Another solution would be to look into A123 lithium batteries. These offer higher capacity to size, four cells in series would give you 12.8 volts. They are a little more tricky to source in larger capacities. Suitable chargers are also a little thinner on the ground, although readily available. A123 batteries will cost more upfront, but have a much longer lifespan than competing battery technologies, so in the long term, should work out the cheapest option.
