The record of the "Luftwaffe" against naval combat vessels up to that point in the war was poor. In the Norwegian campaign, despite eight weeks of continuous air supremacy, the "Luftwaffe" sank only two British warships. The German aircrews were not trained or equipped to attack fast-moving naval targets, particularly agile naval destroyers or Motor Torpedo Boats (MTB). The Luftwaffe also lacked armour-piercing bombs and their only aerial torpedo capability, essential for defeating larger warships, consisted of a small number of slow and vulnerable Heinkel He 115 floatplanes. The "Luftwaffe" made 21 deliberate attacks on small torpedo boats during the Battle of Britain, sinking none. The British had between 700 and 800 small coastal craft (MTBs, Motor Gun Boats and smaller vessels), making them a critical threat if the "Luftwaffe" could not deal with the force. Only nine MTBs were lost to air attack out of 115 sunk by various means throughout the Second World War. Only nine destroyers were sunk by air attack in 1940, out of a force of over 100 operating in British waters at the time. Only five were sunk while evacuating Dunkirk, despite large periods of German air superiority, thousands of sorties flown, and hundreds of tons of bombs dropped. The "Luftwaffe"'s record against merchant shipping was also unimpressive: it sank only one in every 100 British vessels passing through British waters in 1940, and most of this total was achieved using mines.