I recently finished my first patrol of a new career and made it back to port with 250,000 tons worth of kills. (3 fuso battleships, 2 cruisers, 5 destroyers and a support merchant in 1 convoy) and another 2 european liners with my deck gun.
Anyway after I made it back to port (with only 3 fuel left) my hull was 84% damaged and I got some message about the admiral grunting as he assigned me a new ship. I was also given the option to end my career there or keep going.
I had started the career in a Tambor class, but I was assigned a Porpise class.

Why give me a lesser ship? Is there a way I can upgrade and get a better one? I got 12,000 renown for the patrol and could not possibly spend it all.
I played with everything on except outside views, so manual targeting, duds etc were all on.