Hi Scott. It's not a problem to use 12 volt motors. MinnKota has some
smaller powered ones available.
If you are using an interior motor with the shaft going through the hull,
there are 12 volt motors available too. It's just more efficient to use 36 volt
motors. A sub with only one 12 volt battery won't go very far or very
fast.
A small 12 volt MinnKota trolling motor like one used to push a small
fishing boat will run about two or three hours on a fully charged 12 volt marine
deep cycle battery. ( at half speed ) It won't push very hard though.
A submarine has a LOT more drag than a little 12 foot boat on the
surface. If there is any water current at all the small 12 volt motor won't be
able to counteract the forces applied to the submerged vessel.
If the sub was small enough and there was no current it could be useful to
"putz" around near shore in a lake but you wouldn't be able to move very far or
go down very deep before you ran out of power. It's just more efficient to use
higher voltage motors. Higher voltage with lower amp draw for the same
horsepower. Lower voltage with higher amp draw means pushing the motor at max
and abusing the batteries. You would save a little money by buying a cheaper (
smaller ) motor but the motor will wear out faster and the batteries will take a
beating too.
Batteries cost more than the motors.
An underpowered sub could be quite dangerous as well. Any water
current at all or maybe a little wave action could easily push an underpowered
sub into the rocks, drive it down, or into all sorts of trouble.
There have been subs built with NO motors. Karl Stanley's first sub "C-Bug"
had no motors. It used variable ballast to move through the water. It worked
quite well but it had limitations in maneuverability. His new sub has 4 X 36
volt motors.
I have 4 X 36 volt motors on my little sub, but in reality, I will be
running them at 1/4 speed most of the time.
No strain on the motors or batteries, so life span of the system should be
good. The power is there if I need it for short bursts of speed or to fight
against a current but a full power burst puts a big strain on the battery
system.
A 12 volt motor is so small that running it a full bore all the time would
cause over heating of the whole system.
Frank D.