Hi,
Friedrich Christiansen of the Hornets of Zeebrugge was an accomplished seaplane ace. However, determining his score is very much a work in progress. He's been variously credited with as few as a handful and as many as 27 victories though German lists generally credit him with 21 as his final score.
Neal O'Connor devoted a good section of his Volume VII in the Aviation Awards series to Christiansen and his victory score. He poses the very real possibility of victories gained in conjunction with German submarines and other naval units - as leader of the flight he was credited alone (like in the case of the submarine C.25 where he and 4 or 5 others attacked but only he was credited). His score also includes steamers and a Dutch schooner according to Neal who arrives at 19 as a final tally.
According to
ATL the victory list shows: 13 victories including a submarine (C.25) and a coastal airship (C.27).
The ATL authors conclude much the same as O'Connor about how this seaplane pilot got his victories. Christiansen's name shows up very little on contemporary victory lists like the Militär Nachrichtenblatt.
Further troubling is a statement in Volume VII saying that of the seaplanes in Christiansen's score, none were acknowledged in
The Sky Their Battlefield, but I do not have that book so cannot confirm.
Christiansen's score is very difficult to comprise. As a matter of fact, his Pour le Merite was not granted for his aerial victories (as his later "official" tally would suggest) but for his stellar performance as a naval flier and leader.
To really get into this I'd advise a look through the Christiansen entries in both books mentioned above and perhaps a look at the British seaplane losses in TSTB.
Kind regards,
Reinout