Ok DM. Found it. You are quite correct in all you say about lead acid (gel being the worst for dropoff at anything more than a few amps). Cyclone's are the exception.
The remark about ballast was valid though. The Faireys, although fast, were heavy boats for their size for good reason. Their seekeeping was a product of that weight.
Most of the video I've seen of huntsman models show them behaving quite unrealistically because they are too light. Usually IC, I have to admit.
I've estimated that my model could be around 8Kg, or slightly more, displacement on the correct draft marks. Anyone actually knows, give me a call.
The boat (I've added a lot of extra wood in its construction) comes in at 4Kg.
Motors, shafts, rudders, ESCs are just over a Kilo.
Leaves 3Kg for batteries.
Alan