The Kanalkampf comprised a series of running fights over convoys in the English Channel and occasional attacks on the convoys by Stuka dive bombers. It was launched partly because Kesselring and Sperrle were not sure about what else to do, and partly because it gave German aircrews some training and a chance to probe the British defences. In general, these battles off the coast tended to favour the Germans, whose bomber escorts massively outnumbered the convoy patrols. The need for constant patrols over the convoys put a severe strain on RAF pilots and machines, wasting fuel, engine hours and exhausting the pilots, but eventually the number of ship sinkings became so great the British Admiralty cancelled all further convoys through the Channel. However, these early combat encounters provided both sides with experience. They also gave the first indications some of the aircraft, such as the Defiant and Bf 110, were not up to the intense dog-fighting that would characterise the battle.