Submarines were a special type of warship that was usually submerged into the water of the seas during naval combat. They averaged a slow speed of 3 to 4 knots while submerged. Submarines also could not be detected with a conventional radar, but instead had to be detected with a sonar, thus making cruisers and battleships on their own very vulnerable to these warships. Submarines were not armored and very vulnerable to gunfire, and only had one deck gun themselves to use when on the surface. This deck gun was not bigger than 5', and usually was not very effective. The submarines main way of attacking was using torpedoes to attack, making them deadly when the submarine was close to another warship. They were also very vulnerable in the fact that anti submarine combat was very effective in taking down these submarines. If a submarine was damaged and needed a way of escape, it would usually dive 50-200 meters and then try to escape from enemy warships that way. The most example of submarines being usefully was Germany's U-Boat program during World War II, as they easily attacked Allied shipping over the Atlantic Ocean without any contest early in the war. They were eventually countered later in the war by the british, in which less Allied merchant ships were sunk and the British ended up having enough supplies for an invasion of Germany-controlled France.