18-inch Mk.7 was an American lightweight torpedo developed by Bliss-Leavitt in 1911 as the first American torpedo used on submarines and surface vessels alike. By 1917 an airborne Mk.7 Type D variant was developed, but it has never seen serial production. Mk. 7 remained in service until the end of the World War 2, primarily on the O-type submarines.
| Pros | Cons |
|---|---|
| Low mass | Low maximum range |
| Relatively low speed | |
| Small explosive filler |
Because of its short range and relatively low speed, the torpedo is best used against barges and large ships rather than other boats.
General info
Effective damage
With a 93 kg warhead the Mk. 7 can destroy any coastal vessel and a majority of a low-BR destroyers, larger vessels will require a hit directly on the magazines to achieve a kill, though ships equipped with a torpedo bulges might be completely immune to the weapon.
Comparison with analogues
The closest peer among US torpedoes is the Mk.8-3 C/D, a torpedo with nearly double the explosive mass, 15% slower speed (50 km/h instead of 59 km/h) and over triple the range, making it overall by far more versatile weapon.
The closest foreign equivalent would be the Japanese Type 44 No.2 which has a similar mass and same calibre, but a better speed, higher explosive mass and a slightly longer travel distance.
Usage in battles
Much like all lightweight torpedoes, it's best used against large, slow targets with no torpedo defense. When using it, one must remember about the minimum arming distance, which is 50 m, or triple the full length of the PT-3. When faced with multiple fast coastal vessels and no alternative targets, it's worth to just launch torpedoes in a general direction of the enemy vessels — sometimes lucky hits are the best one can hope for.
History
The Mk. 7 was the first American steam-driven torpedo that could be used by both submarines and surface vessels alike. Designed in 1911, it was also the first American wet heater torpedo.
The exact calibre of the torpedo was 17.7 inch, or 450 mm, though in the US it was typically rounded to "18 inch". It carried the Mk 7 Mod 5 warhead with 326 pounds of either TNT or TPX and a Mk 3 Mod 1 contact fuse. Torpedo had two modes, a low-pressure for 3,500 yards (3.2 km) travel distance and a high-pressure for 6,000 yards (5.5 km).
