Complex sentence: Cheetham enlisted in the Mercantile Marine and was serving as Second Officer on the "S.S. Prunelle (London)" when on 22 August 1918, he was killed when the ship was torpedoed in the North Sea by a German U-boat.
Split into simpler sentences: Cheetham enlisted in the Mercantile Marine and was serving as Second Officer on the "S.S. Prunelle (London)". On 22 August 1918, he was killed when the ship was torpedoed in the North Sea by a German U-boat.