For example, I had quite lot of subs based on Soerabaja. When IJ captured that base, all subs switched their home base to SF and started to move there... Of course, very few would have had fuel for that distance. And as said, subs (and ships) start to accumulate sys damage when out of fuel. When sys damage is high enough, sub/ship sinks.
"To meaningless French Idealism, Liberty, Fraternity and Equality...we answer with German Realism, Infantry, Cavalry and Artillery" -Prince von Bülov, 1870-