The First Watch Officer, or in German, ‘Erster Wach-Offizier’ abbreviated to IWO, did exactly that: supervised the first watch of eight out of twenty-four hours where the first watch was on the bridge. Their watches were the same as the US Navy, four hours on, eight off, four on, eight off. The first watch began at 0400 and lasted until 0800. In the sixteen hours not spent on bridge watch, however, the men had other duties to perform according to their rate and speciality which took up a large amount of time. The average ‘workday’ aboard a U-Boat on war patrol was a minimum of 12 to 14 hours. In the handful of hours remaining, the men had to eat and sleep.
The specific and most important duty of the First Watch Officer was to ensure the operability and readiness of the torpedoes, guns mounted on deck, and the fire control systems which supported those weapons including the hydrophones et al. This took time especially when it came to torpedoes. While torpedoes look very sturdy when we see them in movies,
they were quite finicky in real life. Their internal mechanisms were very delicate and they had to undergo constant maintenance so they would actually work when they were fired.
The First Watch Officer had another key responsibility: aiming and firing the torpedoes in a surface attack. German U-Boat doctrine called for night attacks from the surface against Allied ships. So First Watch Officers had a critical responsibility when the boat attacked and it was the IWO who gave the order, ‘los’ or ‘fire.’ If you watch Das Boot you will see how this system functioned.