A small Navy submarine installing listening devices on the ocean floor near San Clemente Island became entangled in a cable, stranding three crewmen 1,270 feet under water several hours before it broke free, officials said. Two rescue vehicles were en route to the area in Wilson Cove when the 26-foot submarine Turtle broke free, the Navy said. Although the submarine had been stranded at 1,270 feet, crew members were in no immediate danger because the vehicle had a three-day air supply, said Navy Lt. Sonja Hedley of the San Diego Submarine Base. The 24-ton vessel was designed to search, locate and recover objects on the ocean floor at depths to 10,000 feet, the Navy said. The submarine became entangled in a line while installing underwater hydrophones used to listen to submarines or other operations in the area. Officials said it was not immediately known how the submarine became entangled.