Italian Circling Torpedo, WWII Tactical and Technical Trends, No. 11, November 5, 1942 (Lone Sentry)

The British Navy has recently made known the recovery of an Italian circling parachute torpedo, which has a number of characteristics that distinguish it from any other torpedo of its kind.

After the torpedo had been rendered inoperative and examined, it was found to have no depth-setting device and would therefore travel on the surface of the water with a probable wake. It is 19 inches in diameter, approximately 8 feet long, and weighs about 700 pounds, the weight of the explosive charge being nearly 200 pounds. The torpedo has a maximum speed of 6 knots, and a running time of about 30 minutes. It is equipped with a three-blade propeller and a 250-volt electric motor.

Features of the torpedo that differ externally from other Italian circling torpedoes are listed below:

(a) The position of the impact fuzes.

(b) The use of a ring bolt for the carrying fitting.

(d) Propeller streamlined flush with the body of the torpedo.

(e) 19-inch instead of 18-inch diameter.

Internal differences which characterize the torpedo include the following features:

(a) 250-volt instead of 220-volt motor.

(b) Motor speed of 3,700 rpm instead of 2,880 rpm (geared down to 750 rpm).

(d) Spring-loaded tail switch operated by a spring-loaded rod inside the after-part of the propeller shaft.

(e) Starboard helm setting. (Others are set for port helm only.)

The rudder of the circling torpedo is actuated by the arm bearing on the eccentric projection of the cog-wheel, which is driven by the worm on the propeller shaft. The torpedo moves to starboard in a series of increasing circles.

Of the three fitted switches, one is an external hand switch on the port quarter and one a mercury switch on the battery, cutting out when the torpedo head lies approximately 45 degrees depression to horizontal. The third switch, which is spring-loaded, is placed inside of the after-part and is held open by a roller bearing and a disc fitted around the propeller shaft. The disc is secured by a spring-loaded rod inside the propeller shaft and projecting inside the propeller boss, where it appears to be held by a parachute lug and a plug, which is soluble. When the plug dissolves, the spring-loaded rod ejects the parachute lug, and simultaneously brings the disc further aft, permitting the spring-loaded switch to close and the motor to start.

It is believed that the torpedo had been dropped about two months prior to its recovery, as it was heavily corroded. Since the corrosion prevented the unscrewing of the impact fuze, it was decided to remove the war head complete and recover the propulsion machinery, etc. This was successfully accomplished and the war head rendered inoperative. Following the examination of the torpedo, the propulsion machinery and other parts of the mechanism were dispatched to London for further study.

It is believed that circling torpedoes have been used only experimentally up to the present time. When employed against convoys, the pilot would probably not have to maneuver his aircraft within close range of antiaircraft fire in order to score a hit, but could drop the torpedo at a reasonably safe distance and immediately resort to evasive tactics. The average running time of the torpedo which is from 30 to 40 minutes, would give an additional advantage. A weapon of this kind would, therefore, present a serious problem to a convoy.

This type of torpedo might also be used against large vessels lying at anchor. They could best be protected against such an attack either by being surrounded with lighters made fast to the ships or by being anchored in an area enclosed by a barrage net extending to a depth of 5 feet.

[WWII Italian Circling Torpedo]

250-volt electric, 3,700 rpm, geared down to 750 rpm.
One external hand switch; one mercury switch on battery, and one in tail which is operated when spring-loaded rod inside propeller shaft ejects parachute lug from propeller boss after soluble plug dissolves.
Vertical rudders driven by motor through worm gearing and eccentric cam working on rudder lever, giving gradual reduction of starboard helm. Torpedo would move in series of increasing circles to starboard.