were more economical than using destroyers, which were needed for fleet duties. Corvettes were small enough to be built in merchant shipyards and used triple expansion engines. They could be built without using up scarce turbine engines and reduction gears, thus not interfering with larger warship production. Ships that could carry aircraft, such as the CAM ships, the merchant aircraft carrier, and eventually the purpose-built escort carriers. Support groups of escort ships that could be sent to reinforce the defense of convoys under attack. Free from the obligation to remain with the convoys, support groups could continue hunting a submerged submarine until its batteries and air supplies were exhausted and it was forced to surface. Hunter-killer groups, whose job was to actively seek out enemy submarines, as opposed to waiting for the convoy to come under attack. Later hunter-killer groups were centered around escort carriers. Huge construction programmes to mass-produce the transports and replace their losses, such as the American Liberty Ships. Once shipbuilding had ramped up to full efficiency, transports could be built faster than U-boats could sink them, playing a crucial role in the Allies winning the "Tonnage war". Aircraft Air raids on the German U-boat pens at Brest and La Rochelle. Long-range aircraft patrols to close the Mid-Atlantic gap. Escort carriers to provide the convoy with air cover, as well as close the mid-Atlantic gap. High frequency direction finding (HF/DF), including shipborne sets, to pinpoint the location of an enemy submarine from its radio transmissions. The introduction of seaborne radar which could enable the detection of surfaced U-boats. Airborne radar. The Leigh light airborne searchlight, in conjunction with airborne radar to surprise and attack enemy submarines on the surface at night. Magnetic anomaly detection Diesel exhaust sniffers Sonobuoys Weaponry Depth Charges, the most used weapon, were improved during the course of the war. Starting with WW1 vintage depth charges, a version was developed. Torpex explosive, which is a 50% more powerful explosive than TNT, was introduced in 1943. Y-guns and K-guns were used to throw depth charges to the side of the escort vessel, augmenting the charges rolled off the stern and letting the escort vessel lay a pattern of depth charges The development of forward-throwing anti-submarine weapons such as Hedgehog and the Squid. This allowed the escort vessel to stay in contact with the submarine during an attack. The FIDO (Mk 24 'mine') air-dropped homing torpedo.