[seafarer]

Steam torpedo's used alcohol (ethanol) and compressed air and water in a combustion chamber to make steam to drive a turbine that drove the screws. The exhaust gases had to be released into the water, and the rising exhaust bubbles leave a highly visible tell-tale wake behind the torpedo.

Electric torpedo's used batteries and an electric motor to drive the screws. Since there was no large volume of exhaust gases to vent, they leave no easily visible wake (there is a slight wake from the cavitation bubbles of the screws, but it's far, far less visible then the exhaust gas wake of a steam torpedo).