Hi,

The motor choice depends on several factors: the sub's size, battery, the torque, etc. Several manufacturers such as Graupner, Robbe, Mabuchi, Johnson, and Pittman make electric motors. I've never been familiar with brushless motors however.

A servo is a device attached to the receiver unit which does the physical work inside an RC vehicle. It is "the little black box" that has all the linkages and control rods hooked onto it. It has an electric motor and a gearbox which allows it to control all your control surfaces (e.g: rudder, diving planes, ballast tank valves, etc).

As for ballast tanks, several manufacturers make those, check these links out:

Pump-ballast system made by OTW:

http://www.otwdesigns.com/DiveSystems/divesystems.htm

Norbert Bruggen:

http://modelluboot.de/

Engel piston tank:

http://www.engel-modellbau.de/catalo...49a1b11b39c858

Some people also like to make their own ballast tanks, for several reasons: compatibility issues, price...... or just for the fun of out it!!!