In its day I believe the Shackleton was as good as any ASW aicraft. ASV21, with a good operator, was an effective search and location radar against snorting diesel submarines. It also had great deterrance value. Only a brave (or foolish) submariner would continue to snort if he detected the prescence of an MPA. If he couldn't snort he couldn't chage his batteries. Without charged batteries he couldn't make progress submerged. Radar was always supplemented by visual lookouts who often spottted submarines. If a datum was established from a visual sighting or disappearing radar contact (sub going deep) a sub could be tracked and attacked using the Mk1c Sonics system. The advent of Minuturised lofar (Jezebel) buoys about the time that the Nimrod/P3 and Aurora came into service allowed passive (unalerted) detection of diesel and early (noisy) nuclear subs. All of these systems were dependant on the skill of their operators. Shackleton crews were widely acknowledged to be amongst the best in the world.