What interwar ASDIC exercises taught - and distorted

Interwar Royal Navy and US Navy exercises created real ASDIC and anti-submarine experience that later carried into Second World War operations. They also revealed the utility of carriers to anti-submarine warfare, even though that value was not always carried into wartime practice. Exercise defenders had clues about the friendly submarine's capabilities and approach that would be absent against an enemy. A marker grenade or umpire judgment could also count as a kill without reproducing the sustained attack required to destroy a real submarine. Useful training lessons remain, but those artificial success conditions need to be discounted.