daddle, I can think of three or four reasons for going with larger wire size inside the
battery box than that of the wire leading away from the
battery box.
One would be in case you ever want to upgrade the
wiring outside the box. Another would be if you ever want to use the house bank to jump the
engine starter battery. A third would be that you want to reduce the internal resistance of your battery bank as much as possible to supply the best voltage that you can. Finally, these wires are not fused and while you wouldn't expect currents larger than those drawn from the bank, if a cell ever shorts out or otherwise becomes unequal to those of the other batteries in the bank, there could be some high internal currents.
I agree with you though that, although the batteries should be secure inside the battery box, a flexible interconnection would be better than rigid bars. The box allows a bit of room for the batteries to expand and
contract with temperature, etc..., so there is, literally, some wiggle room.
witzgall, that's my plan -- connect to the bank at opposite corners, as it were.
Bill, I've been through the calculation a few times, and I believe the #6 to be good. It's a simple
boat, with not much more than a few lights and some sailing
instruments to
power
Thanks all for the input!