Would Britain need 20-knot escort carriers to counter a diesel-powered German fleet?
Short answerNot for convoy escort: escort carriers need to stay with the convoy, while faster fleet carriers would be required to search for and strike raiding task forces.
NotesThe two carrier missions remain separate. An escort carrier only needs enough speed to stay with its convoy while launching anti-submarine and protective aircraft. Chasing a German surface group would pull it away from the merchants it is meant to cover and expose a slow conversion to heavy gunfire. Faster light fleet or fleet carriers are the appropriate ships for searching, shadowing, and striking long-range raiders. Another Ark Royal or additional Illustrious-class carriers might therefore be ordered before the war. The propulsion counterfactual changes fleet-carrier demand more than escort-carrier speed.