Is there an 18" version of the G7?
I thought they were a 21" torpedo
Apart from the 45cm G/125 and G/250 (Whitehead-torpedoes) there are no german 18"/45cm "G" torpedo.
The letter is part of the designation introduced by Kriegsmarine, were the first letter indicates the diameter:
G = 53,34cm/21" (including also older 50 and 52cm torpedoes used in WW2)
F = 45cm/17.7")
H = 60cm/23.7")
M = 75cm/30")
The number indicates the approximate length:
7 = 7m
6 = 6m
5 = 5m
The small letter following indicated type of propulsion and method of control. The T-number (roman) was a running number given to types as they were been introduced to operational service.
Examples:
G7a(TI) = Operational Kriegsmarine torpedo #1: 53,34cm torpedo with 7,163m length and wet-heater propulsion.
G7ef(TX) = Operational Kriegsmarine torpedo #10: 53,34cm torpedo with 7,163m length, electric propulsion and remote control.
I'm not sure the "G" in earlier torpedoes (like the G/6 and G/7) was part of a system, but I'm suspecting it also would indicate the diameter.