a circular or ladder-like pattern. When fired at a convoy, this increased the probability of a hit if the weapon missed its primary target. U-boat developments During World War II, the Kriegsmarine produced many different types of U-boats as technology evolved. Most notable is the Type VII, known as the "workhorse" of the fleet, which was by far the most-produced type, and the Type IX boats, an enlarged VII designed for long-range patrols, some traveling as far as Japan and the East Coast of the United States. With the increasing sophistication of Allied detection and subsequent losses, German designers began to fully realise the potential for a truly submerged boat. The Type XXI "Elektroboot" was designed to favor submerged performance, both for combat effectiveness and survival. It was the first true submersible. The Type XXI featured an evolutionary design that combined several different strands of the U-boat development program, most notably from the Walter U-boats, the Type XVII, which featured an unsuccessful yet revolutionary hydrogen peroxide air-independent propellant system. These boats featured a streamlined hull design, which formed the basis of the later nuclear submarine, and was adapted for use with more conventional propulsion systems. The larger hull design allowed for a greatly increased battery capacity, which enabled the XXI to cruise submerged for longer periods and reach unprecedented submerged speeds for the time. Waste disposal was a problem when the U-boats spent extended periods without surfacing, as it is today. Throughout the war, an arms race evolved between the Allies and the Kriegsmarine, especially in detection and counterdetection. Sonar (ASDIC in Britain) allowed Allied warships to detect submerged U-boats (and vice versa) beyond visual range, but was not effective against a surfaced vessel; thus, early in the war, a U-boat at night or in bad weather was actually safer on the surface. Advancements in radar became particularly deadly for the U-boat crews, especially once aircraft-mounted units were developed. As a countermeasure, U-boats were fitted with radar warning receivers, to give them ample time to dive before the enemy closed in, as well as more antiaircraft guns, but by early to mid-1943, the Allies switched to centimetric radar (unknown to Germany), which rendered the radar detectors ineffective. U-boat radar systems were also developed, but many captains chose not to use them for fear of broadcasting their position to enemy patrols and lack of sufficient electronic countermeasures. Early on, the Germans