Im Thurn Torpedo Tactical Instrument
The Im Thurn Torpedo Tactical Instrument was a Torpedo Control instrument designed by Commander John Knowles im Thurn which could be used to work a variety of offensive and defensive torpedo problems.
It was constructed as follows.
- A transparent bar (labelled "A" in the plate) graduated from 24 to 24 through zero is secured to graduated ring "E", which can be revolved through 360 degrees by knobs "K".
- Speed and Time scales "F" which can be positioned alongside "A".
- Transparent bar "B" can slide along bar "A" and is scaled the same as "A".
- A metal bar "G" can revolve freely, pivoted from the center of the instrument
- a slide along its length serves as the pivot for transparent graduated bar "D" which has a hole drilled at each division on its scale
- an engraved matt circular plate "H" revolves freely around the face underneath bar "G"
- outside "E" and secured to the base of the instrument is graduated ring "J"
- bar "G" can be clamped to either disc "H" or rings "E" or "J", as desired
Let us assume the following defensive scenario, as illustrated in the plate. Enemy destroyers bearing red 40 are seen to fire at a range of 20,000 yards. The enemy torpedoes are assumed to run 15,000 yards at 21 knots and our own squadron is making 18 knots. You would work the instrument as follows:
- Place bar "A" fore and aft
- position slider pivot for bar "B" at position 6
- revolve "B" until the seven division cuts bar "G". 6 is being used to represent our speed (18) and 7 the enemy torpedo speed (21)
- clamp bar "G" to ring "J"
- rotate disc "H" until its lines are parallel to bar "B"
- set bar "D" parallel to bar "B"
- bar "D" now shows the torpedo path
- the running range to cross the line is read off "D" where it crosses bar "A": 13,600 yards
- the red diagonal on disc "H" shows the angle at which the torpedoes will cross the line: Green 105
- slide a speed and time scale "F" up to bar "A" to read how long own ship will take to reach impact point and the distance to run: 18 minutes and 11,300 yards
- place a pencil's point in the hole at 15 on bar "D" (15,000 yards) and swing "D" through a circle to draw the danger area
- examine bar "A" and scale "F" to see when own ship enters the danger area: 8,000 yards, 12.5 minutes
- revolve bar "A" by knob "K" until "A" is tangential to the danger area to show the turn required to skirt it: 9 degrees
- the distance own ship has to steam on the new course to clear the danger area and the time on that course can be seen: 15,000 yards, 24 minutes
- if such a turn is not made, bar "D" shows the expected track of the torpedoes, assisting in judging how to comb the track.
- Annual Report of the Torpedo School, 1917. Plate 60.
- Annual Report of the Torpedo School, 1917. pp. 200-201, also Plate 60.
- Annual Report of the Torpedo School, 1917. p. 201, Plate 60.
- H.M.S. Vernon. Annual Report of the Torpedo School, 1917. Originally C.B. 1474. Copy 7 at The National Archives. ADM 189/37.