Complex sentence: Three crewmen-the engineering-officer and two ratings-lost their lives; the engineering officer re-entered the sinking U-boat and perished while opening the galley hatch, to hasten the boat's sinking and prevent the British from boarding it.
Split into simpler sentences: Three crewmen-the engineering-officer and two ratings-lost their lives. The engineering officer re-entered the sinking U-boat and perished while opening the galley hatch, to hasten the boat's sinking and prevent the British from boarding it.