Hamburg, Blohm und Voss shipyard
U-boats are in fact surface ships with the ability to sail under water. Sixty meters long and equipped with 2 diesel engines it reaches over 17 knots on the surface. Under water the boat uses 2 electric engines and batteries and can reach 8 knots maximum. The underwater range in ‘ stealth mode’ of 4 knots is just 80 miles. After that the boat has to surface to recharge the batteries with the diesel engines and take in oxygen for its crew of fifty. In practice a U-boat, especially at this stage of the war, sails on the surface most of the time.
The main weapon of the U-boat is the torpedo. A type VIIc has 14 torpedoes that can be launched from four bow tubes and one stern tube. A torpedo is actually a small unmanned submarine with its own engine (often on compressed air) and an explosive charge which is large enough to sink most ships with one hit. The German torpedoes explode when they hit something (a ship). There are also magnetic detonators that rely on changes in the magnetic field (near a ship). However, this was counteracted by degaussing (demagnitize) the ship. Abbekerk was degaussed in 1940 already.
Furthermore, a U-boat has a deck gun and two anti-aircraft guns which they can use only above water.
The power of a U-boat is invisibility en surprise. Once it is attacked itself a U-boat is extremely vulnerable. Little is needed to damage the pressure hull and make it impossible to go under water. Therefore, when discovered, a U-boat will seldom enter combat on the surface, even with a lightly armed merchant ship like Abbekerk. When a U-boat is discovered and attacked her only defense is to dive and try to escape under water.
U-604 is commanded by Kptlt. Horst Höltring and the coming months will face a training / work-up period in the Baltic Sea. In this stage of the war, the training is still very extensive and heavy. Only August 1, 1942 she will be ready to go on her first patrol.