Hi Gerard, although Kentrat served as you state his longevity would a lot be by his commands. His early success on U74 were all in 1941, before the Allied anti-U-boat took the incentive away from the U-boats. There followed while at a shore posting from March until September 1942, which was followed by being appointed C/O of U196, which saw his achieve his last successes in 1943/44, not in the killing grounds of the North Atlantic but down in the Indian Ocean while en-route to Japan where took up another shore posting until interned by the Japanese after the German surrender.
As for Cremer this commander certainly led a charmed life throughout his career. All his successes however were within three patrols during 1942, following severe wounds received on patrol in October 1942 he left U333, when recovered returned to her in May 1943. By this time patrols were generally a matter of survival which he managed in his last five, two of which U333 aborted due to war damage, there were alas no further successes. Regards John