[Platapus]

I think I may have found half the answer.

My original question was actually two separate but related questions

1. Why did Japan, Britain, Germany and United States all use torpedoes with the same diameter (533mm/21”)?

2. Why was 533mm/21” chosen?

The answer for the first question is that Japan, Britain, United States, Germany and Italy were all in cahoots with Robert Whitehead’s through his and other related companies.

Stabilimento Tecnico di Fiume (and its many incarnations)
Schwartzkopff (later known as Berliner. Maschineubau A.G.)

both were developers of Whitehood’s designs. They all had dealings with Japan, Britain, Germany and Italy. Representatives from all countries interacted with each other.

In 1908 as Whitehead moved toward 533mm/21” torpedoes, it is understandable that the other distributors would follow suite.

The Whitehead Mk VIII was the first UK developed 533mm/21” submarine torpedo. Its design dates back to 1925.

Germany’s G7a was their first 533mm/21” submarine torpedo designed in 1930. The G7a is an oddity as it was a follow on to the ship launched G7. The G7 was 500mm/19.7” in diameter. Why the G7a needed the extra 1.3” is still a puzzler.

Italy developed the W270/533.4x7.2 Velosa in 1935. Japan was especially interested in the design of the Italian W270/533.4x7.2 Velosa torpedo and copied some of the components.

Japan was using Whitehead designs for their 21” Type 43 ship launched torpedo in 1910. In October 1917, the Japanese Navy decided to move toward the 533mm/21” design for their submarines. The 533mm Type 92 was designed in 1934 and used designed from the G7a.

The United States is an oddity. The relationship between Whitehead and the United States Navy was stormy at best.

Even though the Whitehead torpedoes were successful and widely sold, from 1870-1900, the United States Navy concentrated on other non-whitehead designs.

Lay, Barber, Ericsson, Lay-Haight, Sims-Edison, Cunningham, Howell were all designs evaluated and used by the United States Navy with the Howell design having the most usage.

In 1891, the United States Navy contracted with the Bliss corporation to build the Whitehead Mk 1 ship launched torpedo. This started the short relationship between the United States Navy and Whitehead. This also started a longer relationship between Bliss and Whitehead in the joint development/improvement of torpedo designs.
By 1901, the last Whitehead torpedo, the Mk 5 was contracted for use in the United States Navy. It was an 18” ship launched torpedo.

In 1904, the United States contracted with the Bliss-Leavitt corporation to build their Mk 1 21” ship launched torpedo. This started a long history of USN - Bliss-Leavitt contracts.

Starting with the Mk-X 21” torpedo in 1917, the design and production of future was under the auspices of the United States Naval Torpedo Station in Newport with few exceptions.

The United States Navy decided on 21” as a standard diameter for ship launched and submarine launched torpedoes in 1917 with the commissioning of the USS Caldwell (DD-69) that was designed to fire the new Bliss-Leavitt Mk-8 21” torpedo. One of the few exceptions to this was the Mk-27 19” torpedo developed in 1943.

So we have somewhat of and explanation as to why the United States used 21” torpedoes. Command decision in 1917.

We also have an explanation as to why Britain, Japan, Italy, and Germany used torpedoes of the same diameter -- they shared designs and variations in designs with the Whitehead designs.

What is still unanswered is why did Whitehead, Bliss-Leavitt, and Schwartzkopff all decide around 1920 to change to the 533mm/21” design in the first place?

There had to be a reason.