H21-class submarine
H21-class submarines statistics:
| Number completed before 1919 | 10 |
| Number completed after 1919 | 14 |
| Lost in action | 2 (1939-45) |
| Lost accidentally | 3 |
| Prototype date | 1918 |
| Surface tonnage | 440 |
| Submerged tonnage | 500 |
| Length | 171 feet |
| Engine | Diesel |
| Horsepower | 480 |
| Screws | 2 |
| Surface speed | 13 knots |
| Submerged speed | 10 knots |
| Torpedo tubes | 21 inches |
| Number of torpedoes tubes | 4 |
| Guns | 1 |
| Crew | 22 |
