sry, it isn't my mother tongue... :-)
Maybe I made a mistake:
The early PTs used 21" dia torpedoes, equ. ~ 533 mm. (also shown on: www.pt-boats.com)
The OD of the Launch tubes must be really bigger, nearby 600 mm, In 1/20 scale rundabout 30 mm.
Note: IMHO the Dumas 1/20 PT 109 doesn't fit any scale claim,