Targeting Doctrine?

[virtualpender]

I know at the beginning of the war that US doctrine was to fire no more than one or two torpedoes per target with the magnetic exploder set and priority went to capital warships. At some point the doctrine shifted to firing several torpedos at a target using the contact exploder and with priority given to tankers and large merchantmen. Does anyone know when the doctrine actually shifted? Any help is appreciated.

Thanks.