On 15 February 1918 Christiansen shot down a Curtiss H12B flying boat off Felixstowe, followed by 2 more on 24 April and 25 April. In June he claimed three more Felixstowe F2As. On 6 July he surprised and damaged British submarine in the Thames estuary, killing its captain and five crewmen (the submarine reached harbour). By 11 November 1918 he raised his personal tally to 13, as well as some shared victories, which cumulatively may have brought his total to 21.