[peggus]

With the right motor you can control it with just a potentiometer and a contactor. Using a sepex motor you can control the field with a potentiometer and just connect the armature to the pack, via contactor.

The field power is just a few percent of the propulsion power so the losses in the pot are acceptable, you still need a sizable pot though, (rheostat).

With full field power (low pot resistance) the motor will spin very slowly, to speed it up- reduce the field power.

With a series motor, the power losses in the pot would be too great to deal with.

 

[peggus]

As bad of an idea as the giant pot is, you can take comfort in the fact that someone came up with a worse idea and actually tried it.

Scroll to the bottom of the page for the salt water immersion controller.
http://www.poormansev.com/id24.html

 

[peggus]

A contactor controller may be an option for you, the better way to do them is to series/parallel the batteries so you get even discharge across the pack at all speeds. Some googling should get you started.