Excerpt from the book The Attack on Taranto, by V-Adm. B.B. Schofield.
The Attack on Taranto, by V-Adm. B.B. Schofield, [RN], © B.B. Schofield 1973, Ian Allan, Shepperton, Surrey 1973. Published and distributed in
the United States of America by the Naval Institute Press.
The following paragraph is about the attack on Taranto.
p. 36-7
The average depth of the anchorage where the battleships lay was 49 feet (15 m) and the torpedoes
which the Swordfish were to be armed and on which the success of the attack depended, were the
standard 18 inch (457 mm) Mark XII set to run at 27 knots at a depth of 33 feet (10 m). They were
fitted with Duplex pistols which the Illustrious had brought from England and which had only come
into service shortly before the outbreak of the war. They differed from the ordinary contact pistols
in that they included a device actuated by an enemy ship's magnetic field as the torpedo passed
underneath her and which fired the primer and exploded the warhead. Since the bottom of a ship
is more vulnerable, being less well protected, than the sides, the use of these pistols, it was
hoped, would increase the effectiveness of the attack. However, they still had some faults which
had not been eliminated when they were brought into service, one of which was a tendency to explode
prematurely when running in a swell. Fortunately in the case of the attack on Taranto, such conditions
were unlikely to be encountered, but two other problems presented themselves. To minimise the risk
of a torpedo being dropped at too steep an angle and hitting the bottom before being able to take up
its preset depth, the aircraft must be in level flight or very slightly nose down at an altitude of
150 feet (45 m) when the weapon is released. The depth of water on dropping must not be less than six
fathoms (11 m) and the distance from the target must be greater than 300 yards (274 m) as a safety
device prevents the pistol from being actuated until the torpedo has run that distance. The first
problem could only be overcome by the skill of the Pilot, but the second could be mitigated to some
extent by running off some of the safety range before loading the torpedo in the aircraft. Although
this entailed a certain amount of risk, it was readily accepted by the aircraft crews.
V-Adm. B.B. Schofield's book was published twelve years before Lt. Cdr. Wellham's. Its major difference consists in not mentioning the wire control of the torpedo's plunge. It says something much more damning, though, for those in charge of harbor protection in 1940-41: The depth of water on dropping must not be less than six fathoms (11 m). Either Schofield was not yet allowed to mention the wires or, as said elsewhere, he did not even think of mentioning them, so standard they were. Conversely, if Wellham plain invented the wires, and one would truly wonder why, the Mark XII plunged to six fathoms all by itself when properly launched.
This speaks for itself, however you play the numbers and Washington's 1941 letters to Adm. Kimmel.