The Mark 15 torpedo, the standard American destroyer-launched torpedo of World War II, was very similar in design to the Mark 14 torpedo except that it was longer, heavier, and had greater range and a larger warhead. It was developed by the Naval Torpedo Station Newport concurrently with the Mark 14 and was first deployed in 1938. It replaced the Mark 8 torpedo on surface ships with tubes that could accommodate the longer Mark 15; this primarily included destroyers built after 1930. Older destroyers, primarily the Wickes and Clemson classes, continued to use the Mark 8, as did PT boats early in World War II. During the war 9,700 were produced at Newport and at the Naval Ordnance Station Forest Park, Illinois.
The Mark 15 had the same basic design problems that plagued the Mark 14 for the first 20 months following U.S. entry into the war, though this was not realized nearly as quickly by the destroyer crews as it was by the submariners. One major shared deficiency was the Mark 6 exploder, which usually caused duds. Surface-combatant torpedo attacks very often included confusing splashes from gunnery and aerial bombs, obscuring smoke screens, and quick maneuvering to evade counterattack. Rarely was a destroyer given a chance for a slow, careful surprise attack. Torpedo results were difficult to estimate under these circumstances. The correction of the Mark 15's problems would depend on the submariners solving theirs.
The Battle of Vella Gulf on the night of August 6–7, 1943, was the first in which a surprise torpedo attack by U.S. gave the Americans an overwhelming advantage in the following gun battle, though one Japanese warship was hit by a dud torpedo and escaped. By September 1943, effective methods of torpedo deployment were beginning to be distributed to all U.S. destroyers.
|Mark 15 torpedo|
Mk 15 torpedoes aboard USS O'Brien (DD-725)
|Type||Anti-surface ship torpedo|
|Place of origin||United States|
|Used by||United States Navy|
|Wars||World War II|
|Designer||Naval Torpedo Station|
Naval Torpedo Station|
Naval Ordnance Station Forest Park
|Effective firing range||6,000 yards at 45 knots (5,500 meters at 83 km/h)|
|Maximum firing range||15,000 yards at 26.5 knots (13,500 meters at 49 km/h)|
|Warhead weight||825 lb (375 kg)|
|Mk 6 Mod 13 contact exploder|
|Engine||Wet-heater combustion / steam turbine with compressed air tank|
|Speed||26.5 - 45 knots|
Anticipating the possibility of war with Japan, the United States planned to move their battleships across the Pacific with the fleet train. Cruisers and destroyers would be responsible for defending this large formation at night. Fleet exercises held during the 1930s revealed the confusing nature of close range engagements during hours of darkness. In 1932, during Fleet Problem XIII, "attacking" destroyers closed to within 500 yd (460 m) of USS Saratoga before being detected. Fleet Problem XV in 1934 placed the destroyer screen 7 nmi (13 km; 8 mi) beyond the battleship formation, but the battleships were unable to differentiate "friend" from "foe" at that distance. Screening destroyers were subsequently stationed at effective searchlight illumination range, 3 nmi (5.6 km; 3.5 mi). Recognition improved at that distance, but torpedo hit probability increased as evasive maneuvering of the large, compact force was restricted within the closer screen.
United States Navy War Instructions (FTP 143) published in 1934 remained in effect through the initial 1942 engagements in the Solomon Islands. The instructions emphasized defense to avoid the attrition objective of Japanese planning:
Searchlight illumination range effectively covered launch positions of United States torpedoes, but not the Japanese Type 93 torpedo. Japanese ships could remain outside of illumination range, launching torpedoes at American ships revealing their position with gunfire and searchlights.