Usborne Fall of Shot Indicator
Usborne's Fall of Shot Indicator was a specialised timer to help in assisting spotting by sounding an audible alarm when one's own shots were about to fall about the target. It was illustrated and described in the Annual Report of the Torpedo School, 1916, printed in May 1917.
The device was run by clockwork running at a constant speed and was wound up by a handle. A disc which revolved once every 8 seconds ("3" in Plate 88) was visible through a window to help judge the proper operation. The user pressed a plunger to release one of 36 steel balls to fall into one of 40 hollows in a spinning drum ("4") to travel along a spiral groove. At the proper time, it would strike a contact lever ("7") to close a circuit and sound a buzzer before falling out of the hollow after 40 seconds, which was the maximum time of flight the device was designed for. The balls would be automatically returned to the feed tube for successive use.
The gun range in use was entered through a range dial atop the instrument, which converted the range to the corresponding time-of-flight and adjusted the position of the contact lever within the device.
It should be noted that adjusting the range would alter the timing of all balls within the spiral — a limitation that made it unsuitable for tracking two salvoes with different times of flight, and one that would prove a nuisance when the Spotting Rules were devised which permitted use of double salvoes in ranging ladders.
An "automatic gun contact box" accessory was added to electrically open the gate to release balls into the device when any individual gun was recoiling, and was already in use in Revenge and Renown classes some time around 1916 or early 1917. The device also worked for sub-calibre firing, which presumably required a different range dial atop the fall of shot indicator.
In 1917, an alternative device, a time-of-flight instrument designed by Lt. J. A. Whitelock of Canada, was not approved for manufacture, as the Usborne gear was performing to satisfaction, except that the "hooter" was judged to be not loud enough, mandating their replacement with night alarm rattlers sans amplifier.
After the war, the Grand Fleet Dreyer Table Committee complained that the design in service was unreliable and did not meet the demands of double salvo firing. It was suggested that device be made to work with the "timing shaft" and gun range on the Dreyer Table.
- Annual Report of the Torpedo School, 1916. Plate 88.
- Annual Report of the Torpedo School, 1916. p. 152.
- Annual Report of the Torpedo School, 1916. pp. 152-153. There is a woodcut of the device on p. 153.
- Annual Report of the Torpedo School, 1917. p. 230.
- Reports of the Grand Fleet Dreyer Table Committee, 1918-1919, p. 11. The editor surmises that the "timing shaft" was the drive shaft for the Dreyer's plots.
- H.M.S. Vernon. Annual Report of the Torpedo School, 1916. Originally C.B. 1329. Copy 4 at The National Archives. ADM 189/36.