Vice Chief of the boat?

I was wondering, for those, WWII sub buffs out there, when the skipper was releaved because of some badly needed sack time or whatever, it was generally the XO. I say generally, because as someone pointed out to me, the skipper can appoint just about anyone he thought competent to man the helm. My question -- the chief, the engineer, who in Das Boot, stands behind the plainsmen, who fills in for him? Was there a 'second' chief aboard? Was there a protocol for this?

The answer is the watch system. You have designated watch officers, who would generally (I think) do 2 4-hour shifts per day as Officer of the Watch (OOW). On a sub, this might be the Captain, XO & Nav officer.

/bux

PS Its been awhile since a) I've read "Iron Coffins" I was in the Navy and c) played Enigma, so my facts like my hull may be a little rusty