It is not the length but the girth in torpedoes

[Platapus]

The Germans, Americans, British, and Japanese submarines all used torpedoes that were the same size in diameter.

533 mm or 21 Inches.

This is a strange number. Not 525 mm or 550 mm but 533 mm. In imperial measurements it is still strange. Not 20 inches or 24 inches but 21 inches.

The Germans had the G7 Series
The British has the MK VIII and MK X
The Americans had the MK X, MK XIV, and MK XVIII
The Japanese had the Type 92 and the Type 95

Now all the counties’ torpedoes were different lengths but all has the same odd diameter.

Any idea why four different countries (there were actually more that used the 533 mm torpedo) who were developing individual submarine models that really did not have much in common, would all use the same odd diameter torpedo?

I don’t think there were any common components shared by these countries so that eliminates that as a reason.

Where there any shared industrial gigs or dies used by all four countries that would account for all the common submarine torpedoes to be not only the same diameter but to have the diameter to be a “strange” number 533 mm/21 Inch?

Are torpedoes 533 mm in diameter because that was the limitation of the capability to build a torpedo tube?

Or are torpedoes tubes 533 mm in diameter because that was the most efficient size for a torpedo?

Is there an optimum diameter for torpedoes?

Inquiring minds want to know