RE: Graupner Systems Cat
I know someone who has a brand-new, spare gearbox for the 606 who might be willing to part with it. I'll let you know.
I doubt the packs are connected in parallel - the minute you connect 2 packs in parallel, the pack with the higher voltage will discharge into the weaker pack and one pack could have a weak cell and if the weak cell should short out, you'd have a runaway discharge of the good pack into the bad one, and the pack could overheat and melt the hull of your boat.
I think the motors in the 606 are rated at 7.2V, so my guess is that they're wired in series (meaning that they run together at 14.4v) and that the controller connects both packs (6 or 7 cell) in series for full speed and just connects one pack for the slower speed. You would still have a separate pair of wires running from the speed control to each battery connector.
Unless of course, you see a big power resistor inside the hull which was another implememtation of a 2-speed electromechanical speed control - for "slow" speed it would switch the resistor in series with the motor to drop the voltage to the motor (generating a lot of waste heat in the process). Either of these setups is junk. So, you'd want an ESC that can handle the voltage of 14 cells or 4S LiPo, say 20 volts maximum. 40-50 amps rating would probably do.
Twice the volts at half the amps, the watts are the same, and less is wasted as heat.