Some things to consider regarding torpedoes:
Mk8s were WWI torps and weren't good torps either
. They required the tubes to launch them, which added a lot of weight to the boat. There was also a muzzle flash when the torp was fired.
The mk13 had several advantages: The navy had a lot of them and they didn't require the tube to launch, which ment the boats were now ligher, so they went faster.
