Complex sentence: The group damaged the minesweeper "Komintern" and sank a transport ship on 21 April and by this time the Black Sea Fleets ability to supply the Soviet forces in Sevastopol was severely curtailed.
Split into simpler sentences: The group damaged the minesweeper "Komintern" and sank a transport ship. By this time the Black Sea Fleets ability to supply the Soviet forces in Sevastopol was severely curtailed.