Actually you should complete your objective and then return to base as quickly as possible.
This is especially true if it's 1944 and you still have a Tambor. Here's the situation: the blind chimp chucks a dart at the magic upgrade board each time you return to port and end a mission. Upon starting the next mission you get to see what upgrades you've received.
That means that each time you end a patrol you have a CHANCE of receiving some of a list of upgrades that changes with the date. If you stretch out your cruises, you might get three chances instead of seven chances in a given amount of time.
Can you see that your sub won't be too tricked out compared to another captain who returned every chance he got? And it makes sense. Nobody can upgrade your sub at sea!
So take lots of short cruises and toss the blind chimp a free banana! He catches 'em by smell.
