Excerpt from the book With Naval Wings, by John Wellham.
With Naval Wings, by John Wellham, © John Wellham 1995, Spellmount, Staplehurst 1995, distributed in the US by Stackpole Books,
Mechanicsburg, PA. Foreword by RAdm. I.D.G. Garnett, RN, Flag Officer Naval Aviation.
p. 84
Our aircraft were to carry a new type of torpedo which had not been used before. It carried a Duplex
contact/magnetic pistol designed to pass under the battleship and explode below its soft underbelly:
it would also explode on contact with the target. We had agreed that these would be set to run at
27kts and at a depth of 33ft. A device consisting of a cable unwinding from a spool had been designed
to avoid the weapon porpoising, or diving too deeply and sticking on the bottom of the harbour. They had
been set to a low safety range of only 300 yards. We hoped that they would work better than the Italian
ones; our torpedo men lavished care on them and we had never suffered a failure in the past.
There it is again!
Looking at the appendix drawing on p. 191 describing the action at Bomba, and at maritime charts' water depths, it seems clear that Lt. Cheesman launched from the shore and that, either the Mark XII was a good shallow-water weapon on its own, or the planes were already fitted with the depth-controlling wire.
As we said elsewhere, no one would be foolish enough to go about an operation like Taranto without previously well-tested equipment, especially saddling torpedoes with restraining wires.