Complex sentence: Japanese destroyers attacked the submarine with over 80 depth charges without success; and after damaging another merchantman on 27 December Gurnard, returned to Pearl Harbor 7 January 1944.
Split into simpler sentences: Japanese destroyers attacked the submarine with over 80 depth charges without success. After damaging another merchantman on 27 December, "Gurnard" returned to Pearl Harbor 7 January 1944.