Complex sentence: On 16 March, she sank the cargo ship "Kofuku Maru"; but, since the remainder of the patrol was void of worthwhile targets, the submarine returned to Fremantle on 8 April.
Split into simpler sentences: On 16 March, she sank the cargo ship "Kofuku Maru". The remainder of the patrol was void of worthwhile targets, so the submarine returned to Fremantle on 8 April.