Hi Walter

The tonnage, gun caliber and torpedo diameter was given by the maritime contracts between all great Navies after WW1, to made it easier to compare the arnament of the warships of all nations. In WW1 all diameters were used (in example Germany: 35, 45, 50 and 60cm). But the British 18\" torpedoes had in fact a diameter of 17.7\" and the French Navy used 55cm torpedoes.

Some details on several main torpedo types (all diameter 53.3cm or 21\"):
German G7a: also used by surface ships. Length: 7.1m
German G7e: also used by S-boats (E-boats). Length: 7.1m
British Mk.VIII: also used by MTBs. Length: 6.6m
US Mk.14: Length: 6.2m
Dutch V: Length: 7.2m
Italian W 270: Length: 6.5m-7.2m
Japanese Type 89: Length: 6.8m
Japanese Type 92: Length: 7.1m

The German torpedo G7a could not be used by the British and American submarines, because the tubes were not long enough. American subs were not able to use other torpedoes, because the tubes were too short. But the Dutch submarines used some of the 40 G7a captured by the British Navy in their boats and also some G7e after the war.
It is possible that the German U-boats were able to fire foreign submarine torpedoes, which were not longer as the own torpedoes. I think some tests were made with the British torpedoes captured on the HMS Seal. It is also possible that some tests were made in Far East to use Japanese torpedoes.. has anyone informations about that?

The Allied Navies designed own torpedo tubes for each type of vessel and the Germans used only one type of torpedo tube on their ships.

Regards
Rainer