Operational 1/20th scale US Navy 21" Mk-8 Torpedo

The modelling objective is an operational 1/20th scale US Navy Mk-8 (Mk-VIII) Torpedo. This was the torpedo originally used in WW1 and on US Torpedo Boats and other similar vintage, early war boats and submarines prior to the availability of the much improved Mk-XIV (Mk-14) torpedo's during WW2.

Mk-8 on board PT-21 - Reference from: http://pt-king.gdinc.com/torpedo.html/torpedo.html

The main weapon of the motor torpedo boat force in the early days of World War II was the Mark-8 torpedo, developed in 1911, prior to WW1, by the Bliss-Leavitt Company. Full-scale mass-production of the torpedo began in 1913 by the Naval Torpedo Station in Newport, Rhode Island, originally designed for WW1 destroyers, the Mark-8 was the U.S. Navy’s first 21-inch (533.4mm) diameter torpedo, and had a long service life with several modifications, ending with Mark-8, Mod-8. While it may have been state of the art in 1911, when it was issued to the PT boats some thirty years later in 1941 the Mark-8 was certainly showing its age.

To the torpedo boat crews, the Mark-8 possessed a number of serious vices, with a top speed of 27 knots, it would take considerable skill for any PT skipper firing a Mark-8 to score a hit on any swiftly moving enemy destroyer travelling at 30-plus knots. The torpedo had to be launched on an even keel from a tube, otherwise the gyroscope (which maintained it's course) could tumble and could cause the torpedo to run erratically. Weighing in at around 3,150 pounds (1,430 Kg), the Mark-8 and its associated launching tube added a great deal of weight to the PT boat, badly affecting the vessel’s performance.

Another drawback of the Mark-8 was it's small warhead, only containing 316 to 466 pounds (140 - 210 Kg) of TNT (depending on the model) which seemed to be no guarantee of inflicting sufficient damage to WW2 enemy vessels. The warhead's simple contact exploder was defective and on occasion it would detonate prematurely, if it managed to explode at all! It also had 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 around eight feet (2.5Mtrs). It was estimated that some sixty-three percent of the Mark-8’s in service had either, problems in depth-keeping, or defects in the exploder.

Torpedo Mark-8, Mods 3C and 3D

General Specifications:

Power:    Steam Turbine
Diameter:    21"                     (533.4mm)
Length:       248.44"                (6,310mm)
Weight:       3,150 lbs (ready)   (1,430Kg)
Range:        13,500 yards         (12,350Mtrs)
Speed:        27 knots

Warhead:

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

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)

Reference: From torpedo boat manual 1st February 1943.