Why was it important to be on the surface at night?

It was about filling the boat with fresh air, charging the battery, receiving messages and updating the ship’s position – knowing the ship’s location.

The air on the submarine was only intended to last 16 to 18 hours. Since most of the crew smoked, the air was probably only good for about 16 hours. (Cigarettes were given away free to servicemen in WW2.) A primary need, therefore, was to get fresh air in the boat.

The battery also needed to be recharged. Fortunately, running the engines to recharge the battery pulls fresh air into the boat. Most of the air from the main induction goes directly to the engine rooms. However, some of the fresh air was fed to the ends of the boat and was pulled toward the engine rooms. That way, all compartments would get fresh air.

Another task to be completed was to listen for messages on the fleet broadcast. This broadcast was sent to all submarines and was repeated during the night. It may contain orders for the boat. It may also provide messages to other boats that would have been useful for your submarine to know about.

Finally, the boat would want to get a position fix. In WW2 this would have been very dependent on the weather. If it were cloudy, there were no stars to shoot, and therefore no updated position fix. Ideally, there would be a clear enough sky at dawn and dusk to see the horizon and find the brightest stars as they appear. The elevations of those stars above the horizon would have been measured with a sextant. The resulting numbers could then have been translated into your position.

Sun lines at local apparent noon could help update the boat’s position but wouldn’t usually be as accurate as a star fix. Sun lines could have been done through the periscope.