SAN DIEGO, Feb. 21— Patrick Henry McMahon, the wounded sailor whom Lieut. (j.g.) John F. Kennedy towed to safety on his back after PT-109 was splintered by a Japanese destroyer 47 years ago, died Sunday at a convalescent home in nearby Encinitas. He was 84 years old.

The oldest member of the crew of the patrol-torpedo craft, Mr. McMahon was 37 when he enlisted in the Navy, although he was exempt from military service because of his age. He was assigned to Patrol Torpedo craft 109 as a machinist's mate first class, serving under Lieutenant Kennedy, the 26-year-old skipper.

Out of the Dark, the Enemy

On the night of Aug. 2, 1943, the torpedo boat was patrolling Blackett Strait in the Solomon Islands when a Japanese destroyer burst out of the darkness and cut the boat in two with her prow.

Two of the 13 crew members died instantly. Mr. McMahon was burned on his face, chest, arms and legs and unable to swim because of the pain, said his stepson, Kelly.

Lieutenant Kennedy took Mr. McMahon on his back and swam with the strap to Mr. McMahon's life jacket clenched in his teeth. Other sailors clung to a plank from the wreckage. Four hours later, the survivors reached Plum Pudding Island.

Finding little food and no drinking water there, they left four days later for Olasana Island, which required another four-hour swim. Lieutenant Kennedy again towed Mr. McMahon.

Help From the Islanders

A few days later, with the help of island residents, American forces took the sailors off Olasana. Mr. McMahon recovered from his burns and was discharged from the Navy in November 1945.

He later worked for the Post Office, starting as a letter carrier and then becoming postmaster in Cathedral City, Calif., near Palm Springs. He retired in 1975 and moved to Encinitas.

In addition to his stepson, Mr. McMahon is survived by his wife, Rose, and four grandsons.

Patrick McMahon (1906)