The Royal Navy used three different diameters of torpedoes during WWII. The 18in torpedo was used by aircraft, but also by some motor torpedo boats (MTBs). Despite being designated as an 18in torpedo, it was actually a 17.7in (45cm) weapon. The 21in (533mm) torpedo was the standard torpedo used by surface sips, submarines and MTBs. There was also a 24.5in torpedo, used in the torpedo tubes of the two Nelson-class battleships. Each of these had various Mark numbers, indicating differing engines or warheads, but the

Of these torpedoes, the 18in had the longest pedigree. The first Whitehead torpedoes purchased by the RN had this diameter. As engine and warhead technology progressed, the RN wanted to be able to continue using the ships and equipment designed to use earlier torpedoes. As a result, the 18in torpedo became a standard. However, as larger torpedoes became available, the 18in was relegated to roles requiring a lighter torpedo - primarily aircraft-launching. The RN's first 21in torpedo was designed in 1908, and it would enter service in 1910. Development was driven by the desire to have a torpedo with a larger warhead. The contemporary 18in variant had a warhead of ~150 lbs of wet gun-cotton. Going to 21in allowed for a warhead of 200 lbs to be carried. Once again, as new warheads and new engines became available, the 21in became a standard heavy-weight torpedo.