The Mk 27 was a development of the airborne Mk 24, using guide rails to fit the torpedo tubes of the submarines. It was to be used passively, without use of any means of detection beyond passive sonar, against Japanese convoy escorts, from the rear tubes to put the risk of it striking the launching submarine. It would swim out of a flooded tube in order not to confuse it through the sound of compressed air swooshing around it. It was extremely successful. Used in the last month of 1944 and onwards, it destroyed 24 escorts and damaged nine, with 106 torpedos expended. This success convinced the USN of the usefullness of the new concept (which had already been tried, successfully, by the Germans, though Allied countermeasures made it more difficult in the Atlantic), and the discarded fleet submarine project of 1945 carried two external torpedo tubes specifically for the anti-escort torpedo.  
   The Mk 27 was used postwar until replaced by Mk 37.