The first SDV to be operationally deployed was the Mark VII, which entered service in June 1972 after being tested between 1967 and 1972. It could carry three SEALs plus a pilot sitting in compartments fore and aft. It had a hull made from fiberglass and non-ferrous metals to hinder detection and was powered by a silver-zinc battery attached to an electric motor. The Mark VIII SDV, the model that is still in use today, began to supplant the Mark VII starting in 1983.