Just a few experiences with TIIs and the -88 in RFB+OpMonsun:
Small merchant, ~1500 tons
2 torpedos fired, one hit under the smokestack and one dud. Surfaced and used 10 rounds HE to finish him off.
Medium sized merchant, ~4500 tons
2 torpedoes fired, one hit under the rear crane and one just forwards of the bridge. Surfaced and used 17 rounds HE to finish him off.
Medium sized merchant, ~5000 tons
2 torpedoes fired, one hit under the rear crane and one just forwards of the bridge. Used 13 rounds HE to finish him off.
All the targets went down under 30 mins from the first torpedo strike. Had I waited longer I probably wouldn't have had to use that much 88-rounds, but I decided to give some practice to the gun crew while I still can(September 39).