The Mark VIII Torpedo

The main weapon of the motor torpedo boat force in the early days of World War II was the Mark VIII torpedo; developed in 1911 by the Bliss-Leavitt Company, full-scale mass-production of the weapon began in 1913 by the Naval Torpedo Station in Newport, Rhode Island. Originally designed for destroyers, the Mark VIII was the U.S. Navy’s first 21-inch weapon, and had a long service life with several modifications, ending with Mark VIII Mod 8. While it may have been state of the art in the still-infant technology of the “automobile” torpedo in 1911, when it was issued to the PT boats thirty years later in the summer of 1941 the Mark VIII was showing its age.

To the torpedo-boat men, the Mark VIII possessed enough vices; with a top speed of 27 knots, it would take considerable skill for any PT skipper firing a Mark VIII to score a torpedo hit on any swiftly-moving enemy destroyer going at 30-plus knots. The torpedo had to be launched on an even keel from a tube; otherwise the gyroscope (which maintained the weapon's course) would tumble and could cause the torpedo to run erratically. Weighing in at At 3,150 pounds, the Mark VIII and its associated launching tube merely added more undesired weight to the PT boat, affecting the vessel’s performance.

Another drawback was the Mark VIII's small warhead; its 316 to 466 pounds of TNT (depending on mod) seemed to be no guarantor of ship-killing destruction. In addition, the warhead's simple contact exploder was defective; on occasion, it would detonate prematurely, if it bothered to explode at all. The weapon also a tendency to either run deeper than set, or to "porpoise", broaching the surface of the water during its run if it were set for anything shallower than eight feet. It was later estimated sixty-three percent of the Mark VIII’s in service had either problems in depth-keeping or defects in the exploder. Even in its early models, the reliability of this weapon may have been somewhat suspect; it seems some of the problems that beleaguered the Mark VIII during the Guadalcanal campaign just might have revealed themselves early on during firing practices from what would seem to our modern eyes, a most unlikely source--the battleships of the United States Fleet.

Torpedoes aboard a Virginia-class battleship, 1911. (Library of Congress via

In the late nineteenth and early twentieth centuries, such was the fascination amongst the world’s naval professionals with the torpedo’s capacity for underwater destruction, ship designers of the day outfitted the then new-fangled weapon to most anything that would float—and the newly-designed "dreadnoughts" with their large caliber guns were no exception. The capital ships of the U.S. Navy’s Battle Line, depending on ship class, initially had two to four internal torpedo tubes fitted laterally below the waterline port and starboard on each ship, and the battlewagons routinely joined the destroyers and submarines on torpedo firing exercises. The results apparently were far less than ideal; for example during torpedo firing practices carried out during the period 1922-23 fourteen battleships fired a total of 116 torpedoes; of those, 32 failed to run properly. Two ran around in circles, suggesting that in a real action the whole exercise could easily have been equally deadly to friends as well as to the foe. Fourteen others either ran cold (failed to launch) or did not reach the specified range, and in one moment of low comedy, one torpedo from USS North Dakota ended up scoring an unintentional hit on the target towing ship.

The big battlewagons last conducted torpedo-firing exercises in 1930-1931. Battleships Maryland, Tennessee, Colorado, West Virginia, California and Idaho each fired six Mark VIII Mod 3 or Mark VIII Mod 3B torpedoes, three to port and three to starboard, at ranges approaching 11,000 yards. West Virginia was the only battleship able to get all her torpedoes across the target line, and the umpires credited her with two full and three "partial" hits. Idaho had the worst luck with five erratic runs and lost one torpedo entirely, placing last. Maryland, with four full hits, one partial hit, and one cold shot, placed first.

As to why the Navy chose such a seemingly inferior weapon for the newly-created motor torpedo boat service, the short answer is that it was the only thing available. Of the torpedoes in service when PT boats became operational in 1940-41, the Mark VIII was the only torpedo at that time the Navy had stockpiled a sufficient number and not declared obsolescent. The Navy deemed it highly desirable in June 1940 that PT’s be equipped with four 21 inch torpedoes; of the types available (Marks VIII, XIV, and XV) the brand-new Mark XIV was destined for the Submarine Force, while the Mark XV was slated to be used by the newer (post-1934) destroyers of the Fleet. At that time, large-scale production of the new weapons had not yet begun; both types were still in very short supply. In addition, to adopt either one for PT use would have required a major redesign of PT torpedo tubes.

Firing a Mark VIII from its tube. (PT Boats, Inc.)

Another problem that revealed itself in combat was the method of launching the weapon. The Mark XVIII torpedo tubes developed for the Elco PT’s utilized a black powder impulse charge to eject the torpedo, but in the humid climate of the South Pacific, the powder would occasionally misfire, causing the “tin fish” to bang its fins on the PT’s deck as it left the tube, leaving the torpedo to run wild upon entering the water and miss the target completely. Still another danger was flare-ups—prior to installation, the torpedo and the interior of the tube was covered with a generous coating of grease and oil, in order to ensure the weapon's smooth release; but the black powder sometimes had the disconcerting habit of igniting the oil as the torpedo was ejected, causing a fire inside the tube. The resultant red-orange glow naturally revealed the boat’s position to the enemy, enabling the target to quickly side-step out of the way of the PT’s slowly-running torpedoes, while putting the boat on a bulls-eye for retaliatory gunfire. Higgins-built PT's did not have that problem--their Mark XIX tubes launched their torpedoes by compressed air.

An equally catastrophic, if not greater peril, was the “hot run”. If the impulse charge failed to launch the weapon, the torpedo would remain in its tube, but with the motor still running. There was no threat of the warhead detonating but the motor itself would continue to run on until it disintegrated and exploded, sending lethal shrapnel-like fragments across the deck unless the torpedoman could shut it down in time.

Maximum range of the Mark VIII was 10,000 yards (4.937 nautical miles), but its long range was hardly a positive attribute, for the PT boats’ torpedo fire control apparatus was not accurate enough to permit such a long-range shot. At any rate, the boat skippers learned early on to assure themselves of a hit on a target, a boat had to sneak in undetected to within 1,000 to 500 yards of the prospective victim. In contrast, the main Japanese destroyer torpedo, the 24-inch oxygen-driven monster known as the Type 93 (called the “Long Lance” after World War II) possessed a massive warhead which held 1,080 pounds of high explosive, a top speed of 45 knots, a range of up to 40,000 yards (19.749 nautical miles), and much to the misfortune of the Allied Navies, the Type 93 was equipped with exploders that worked.

It is unknown if the defects of the Mark VIII were ever corrected, or even if the weapon was ever subjected to much of the rigorous testing as the submarine-launched Mark XIV, which had many of the same flaws as the older weapon. By late 1943, after extensive field experimentation and analysis, the Submarine Force gradually eliminated many of the Mark XIV's defects. Within the motor torpedo boat squadrons, PT officers felt the boats could benefit from a better launching system, as well as a better torpedo. By August 1943, new PT’s were coming fresh out of the boatyards with the rack-mounted Mark XIII aircraft torpedo instead of the old tube-launched Mark VIII. Wholesale replacement of the Mark VIII's on boats in the forward areas soon followed; many of the older boats in both the Pacific and Mediterranean theaters were re-equipped with Mark XIII's by the late spring of 1944. An immediate benefit to the boats was an added savings in weight as well as an increase in deck space. Another advantage was in the Mark XIII itself. Developed for aircraft, the Mark XIII was smaller and lighter, and had a non-tumbling gyro, which meant the PT boats could simply drop the weapon from its rack into the sea. The new weapon ran faster (45 knots for the Mark XIII versus 27 knots for the Mark VIII), and carried a 600 warhead loaded with Torpex—a more powerful explosive composition than TNT.

Torpedo Mark VIII, Mods 3C and 3D

General Specifications:

Primary Use:   DD, DE, PT
Power:    Steam Turbine
Diameter:    21"
Length:    248.44"
Weight:    3,150 lbs (ready status)
Range:    13,500 yards (acceptance)
Speed:    27 knots


Mark:   VIII-3
Length:   27.09"
Weight (w/o exploder): 526 lbs.
Weight ofexplosive: 385 lbs. TNT

Exploder Mechanism:

Mark: III-2
Actuation: Contact
Weight: 5.25 lbs.
Mark Detonator: VI-2
Mark Booster: I

Exercise Warhead:

Mark: XVII-1
Weight, empty: 166 lbs.
Weight, ballasted: 250 lbs. (water)

Air Flask Section:

Length: 147.38"
Weight, empty: 1,540 lbs.
Weight, filled
and charged: 2,017 lbs.
Weight, air: 341 lbs. (2800 psi, 23.4 cu ft.)
Weight, alcohol: 42.5 lbs. (49 pints)
Weight, water: 93.4 lbs. (90 pints)

Afterbody and Tail:

Length: 73.97"
Weight with
Gyro and Oil: 504 lbs.
Weight, Oil: 15 lbs. (16 pints)
Mark of Gyro: XI-1, XII-0, XII-1

Sources for this article:

Bulkley, Capt. Robert J., Jr. USNR (Ret.), At Close Quarters. Government Printing Office, Washington D.C., 1962

Friedman, Norman, US Small Combatants. Naval Institute Press, Annapolis, 1987

Johnson, Frank, PT Boats of World War II. Blandford Press, Dorset, 1980

Jurens, William J., “The Evolution of Battleship Gunnery in the U.S. Navy, 1920-1945”, Part II. Originally published in Warship International No. 3, 1991. Online version courtesy of

Shireman, Douglas A., “U.S. Torpedo Troubles During World War II”. Originally published in World War II magazine, February 1998. Online version courtesy of

Copyright © 2002-2013 by Gene Kirkland