Complex sentence: In 1937 he joined the Cunard White Star Line and his first liner, the 16,243 ton liner RMS "Lancastria" and when the war started he was Senior Third Officer in the 26,943 ton RMS "Britannic".
Split into simpler sentences: In 1937 he joined the Cunard White Star Line and his first liner, the 16,243 ton liner RMS "Lancastria". When the war started he was Senior Third Officer in the 26,943 ton RMS "Britannic".