To counter Allied air power, "UbW" increased the anti-aircraft armament of U-boats, and introduced specially-equipped ""flak" boats", which were to stay surfaced and engage in combat with attacking planes, rather than diving and evading. These developments initially caught RAF pilots by surprise. However, a U-boat that remained surfaced increased the risk of its pressure hull being punctured, making it unable to submerge, while attacking pilots often called in surface ships if they met too much resistance, orbiting out of range of the U-boat's guns to maintain contact. Should the U-boat dive, the aircraft would attack. Immediate diving remained a U-boat's best survival tactic when encountering aircraft. According to German sources, only six aircraft were shot down by "U-flak"s in six missions (three by "U-441", one each by "U-256", "U-621" and "U-953").