An apparent mismatch in torpedo measurements comes from British service terminology. From roughly 1890 onward, British weapons conventionally called 18-inch torpedoes were actually 17.72 inches, or 450 mm, in diameter. The Italian figure is therefore given as 17.7 inches and the British one as 18 inches even though both correspond to about 450 mm. The distinction separates a weapon's nominal service label from its exact measurement.
Notable Point · 16:09-16:33
Why British 18-inch torpedoes measure 17.72 inches
A short technical aside explains why nominal British torpedo terminology and exact metric conversion appear to disagree.