I have just aquired a Darnell/Models by Design Type VII u-boat that has bounced around six previous owners ( including me! ) in the past few years, without getting underwater once, excluding the time it sunk by accident
. It is divided into 3 watertight compartments, with the centre one reserved as a wet ballast tank, this compartment is currently sealed off completely i.e. no vent holes in the bottom of the boat.
Considering my local lake is salt water, which will restrict what I can do with the sub, I plan to alter the ballast to 'decks awash' and dive on the planes only. Hopefully a failsafe on the planes or ballast control channel will save me from going out of range.
The question is; Geared water pump, either pressurising the bolt down centre compartment or with an air reservoir, or 'old school' air pump (steam engine with buehler motor drive) with twin air bags using the centre tank as a reservoir?