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).

Kapitän zur See Eberhardt Godt
of the BdU staff (right)

Kapitän zur See Werner Hartmann (left),
Commander of U-198 after a 200-day-long patrol.
No wonder these guys are smiling to be back!