Torpedo Track Indicator
The Torpedo Track Indicator was a device described in Grand Fleet Gunnery and Torpedo Orders No 182 of 21 October, 1916, intended to be used by torpedo lookout officers to help them judge whether a given torpedo sighting endangered his fleet and therefore grant him the means of making an intelligent decision as to whether to report it. Seemingly, this was meant to provide a negative check against "Periscope Fever" and similar contagion.
Design and Function
A revolving disc (B in diagram, rotated by use of handle H) with parallel lines drawn 200 yards apart (at a scale of 1 inch for 400 yards) was mounted on base (A) with radius proportional to the speed of the enemy torpedo (at perhaps 0.25 inches per knot). A "Speed of Fleet bar" (C) is scaled in knots the same as that used on (A) is fixed to the centre pivot (P). A small, non-revolving bearing plate (D) can be clamped to any position along (C) and is marked off in 5 degree increments of relative bearings to own fleet. The centre of (D) is to represent own ship.
A sight bar (K) fitted with open sights and marked in range pivots at the center of (D) and slides through a pivot (G) on the circumference of the revolving disc (B). A slider (E) on this bar carries a long, light pointer (F). (E) would be set, on (K), to the range at which the torpedo or track is observed and pointer (F) rotated parallel to the observed track.
To use the device, a man would
- align the device so that (C) is aligned with his own ship's keel
- position and clamp (D) to own fleet's speed
- revolve disc (B) until the sights come onto the track
- set the slider (E) to the estimated range
- rotate pointer (F) to the observed inclination of the track
Whichever parallel line on (B) which is under slider (E) is the dangerous inclination. Where the pointer (F) crosses the bar (C) indicates where the torpedo will cross the line, and its distance from own ship can be read by counting the parallel lines at 200 yards each.
In the illustration, the black pointer (F) shows the torpedo is coming right for us (it is parallel to the dangerous inclination line on disc B). The depiction of the torpedo arrow in red shows a torpedo that would pass 250 yards ahead.
History and Use
I am not sure I have seen mention of this elsewhere.
- Grand Fleet Gunnery and Torpedo Orders. No. 182 of 21/10/16, p. 86.
- H.M.S. Vernon. Grand Fleet Gunnery and Torpedo Orders, Issued 1st October 1915 with reissues and additional and revised pages issued up to the 8th February 1919. The National Archives. ADM 137/293.