- Second World War (1939-1945)
- Oberleutnant zur See (1st Lieutenant at Sea)
- Kommandant "Vp. 203" / 2.Vorpostenflotille
- Awarded on:
- June 29th, 1944
On the night of the 27-28.06.1944 Martienssen and two other R-Boats engaged a British destroyer force south of the island of Jersey. Ultimately only Martienssen’s R-Boat succeeded in escaping destruction, but in the battle the British destroyer HMS Eskimo was badly damaged by the R-Boats as well.
Up until this time Martienssen had shot down at least 8 enemy aircraft, and participated in the destruction of further enemy aircraft and light ships. He also distinguished himself in multiple minesweeping operations. For the sum of his successes he would be decorated with the Knight’s Cross.