That had to do with the number of crew members.
On the minesweeper, we actually could do three watches, but since we were only two navigators, the two of us made six hours watches, if such happened.
And on the landing craft, I've been on as a reserve officer, there was one navigator. We had two watches only rarely (= always when I've been on board). Sometimes, the officer of the watch was doing the navigational stuff as well ...