There had been a number of projects to develop an electric torpedo even before WW2. The obvious advantages of such an arrangement, an electric propulsion, would be the lack of traceable wakes on the surface. This, it was thought, would give priceless advantages to submarines attacking ships.

    The inter-war Mk22 never reached a stage in which its use may have been possible. The war, however, provided the U.S. and Royal Navies with a completely developed electric torpedo. Germany had build on during the inter-war years, and the Royal Navy, upon capturing U-570 in the Atlantic, found several of those weapons in the submarine. 
Both the U.S. and Royal Navies adopted the new weapon with slight modifications. The Mk 18 was, at first, not trusted entirely by the submariners. It came in two variants, Mod-1 and Mod-2, of which Mod-1 arrived much earlier and was slower. This slow speed was not liked by the submariners. However, the use of the torpedo increased quickly and by 1945 (having been introduced in late 1943), 65% of all torpedo attacks were made by the newer weapon - although Mod-1 had been largely replaced by Mod-2.

    Post-war reviews, however, proved that despite its stealth, Mk 18 did not score better than Mk 14, partially because neither escorts nor freighters saw incoming torpedoes too frequently (merchants only in 10% of all cases), partially because Mk 14 was of higher speed and made evasion more difficult.

    Both a passive and an active homing variant were considered to be build, but neither found its way to the fleet. 
Initial problems with Mk 18 were hull wrapping, excessive noise and the disturbingly dangerous batteries, which, if not checked for their hydrogen gas every once in a while, could develop serious fires.