Rank question
 

I have been playing for some days and finished 3 patrols with the IXD2. I have noticed that one major difference between uboat mission and us carrer is that I could request other combat units.
However, how can I get a better rank in order to command more units, like German Cruisers? Shall I get more tonnage?

If you click on the folder next to the captains log, you can see what ranks gain you. The tabs at the top select the given rank,informs you of what is required to gain the rank, and the left side notepad tells you what captains abilities you get for the rank.