I cant ask him but i can guess this from what my dad has said (he is an engineer and electrician heavy plant)
The diesels would use gears to regulate the speed of the shaft which is connected to a generator which then in turn charges batterys, so to regulate speed push a button and it will up the out put.
The diesels do not acctualy drive the shafts its electric motors and they are probly govened by the output of the battery's so if i used 7 volts then my speed wouldnt be as high as me putting 24 volts into the motor.
Whats more on the oberon they have 3/4 tonne batterys and they only produce 2volts each and theres what 150 of them on board, i just wonder how long it took for them to discover duracell
.
Those little penlite batters are 4 and half times more power in them than the batterys on obies