Kapitän zur See
No exact rules regarding service length for promotion from Fregattenkapitän.
Outstanding was the promotion of Korvkpt. Wolfgang Lüth, who got in 18 months the rank of a Fregattenkapitän and a Kapitän zur See. (The promotion to the Fregattenkapitän was antedated for a month.)
The Kapitän zur See wore the braid shoulder-strap with two stars, the Kapitän zur See (Ing.) has a cog-wheel on the shoulder-strap.
They had a broader stripe on the shade of cap. On the arm they wore four broad strips and the star (engineer officers also wore the cog-wheel).