World War II torpedoes were straight runners, as opposed to the homing and pattern-running torpedoes that became available later in the war. They were fitted with one of two types of pistol triggers — impact, which detonated the warhead upon contact with a solid object, and magnetic, which detonated upon sensing a change in the magnetic field within a few meters. One of the most effective uses of magnetic pistols would be to set the torpedo's depth to just beneath the keel of the target. The explosion under the target's keel would create a detonation shock wave, which could cause a ship's hull to rupture under the concussive water pressure. In this way, even large or heavily armored ships could be sunk or disabled with a single, well-placed hit. Initially, the depth-keeping equipment and magnetic and contact exploders were notoriously unreliable. During the first eight months of the war, torpedoes often ran at an improper depth, detonated prematurely, or failed to explode altogether—sometimes bouncing harmlessly off the hull of the target ship. This was most evident in Operation Weserübung, the invasion of Norway, where various skilled U-boat commanders failed to inflict damage on British transports and warships because of faulty torpedoes. The faults were largely due to a lack of testing. The magnetic detonator was sensitive to mechanical oscillations during the torpedo run, and to fluctuations in the Earth's magnetic field at high latitudes. These early magnetic detonators were eventually phased out, and the depth-keeping problem was solved by early 1942 with improved technology. Later in the war, Germany developed an acoustic homing torpedo, the G7/T5. It was primarily designed to combat convoy escorts. The acoustic torpedo was designed to run straight to an arming distance of 400 m and then turn toward the loudest noise detected. This sometimes ended up being the U-boat; at least two submarines may have been sunk by their own homing torpedoes. Additionally, these torpedoes were found to be only effective against ships moving at greater than . The Allies countered acoustic torpedoes with noisemaker decoys such as Foxer, FXR, CAT, and Fanfare. The Germans, in turn, countered this by introducing newer and upgraded versions of the acoustic torpedoes, such as the late-war G7es, and the T11, but the T11 did not see active service. U-boats also adopted several types of "pattern-running" torpedoes that ran straight out to a preset distance, then traveled in either