Gun Deflection (or Scale in American parlance) is the angular difference between a gun's orientation in bearing and the line of sight to the target. It was sometimes referred to simply as Deflection, although that can be an ambiguous term used to refer to Gun Deflection or Dumaresq Deflection. Gun deflection was intended to place the shell on target after factors that cause lateral deviation of shell or target during time-of-flight, such as wind or drift in the former case, and relative motion in the latter case.
Naval gun sights were designed to permit the sighting telescopes to be angled in pitch and yaw so that proper elevation and deflection could be established to hit the target while it was centred in the telescopic sights. These angles were usually established by sightsetting equipment.
In all navies, deflection is an angular measure. The names of the units and their measure varied widely.
The Royal Navy denoted gun deflection in "knots", but these were actually angles and not a speed, per se. Moreover, a knot of deflection on one sight might differ from that on another sight. The awkward use was an artifact of an earlier time when fighting ranges were short and fairly fixed. The angle chosen for a knot then corresponded to the Speed-Across of the enemy at a reference fighting range.[Inference] For instance, if the enemy was at the chosen range, and the control officer felt the ship were doing 4 knots to the left, the deflection would be "four left".
When ordering a spotting correction in deflection, the British prefaced the change with the number of knots. When stating a given deflection, this was reversed. For instance, if the deflection were to be altered 4 knots to the left relative to its present value, one would command, "left four".
It was not until after the war that proposals were firmly voiced by the Grand Fleet Dreyer Table Committee that the service should standardise deflection angles as a given number of minutes per knot for all heavy guns.
The U.S.N. used "Scale" for deflection. Scale always range from 0[Fact Check] to 100, and 50 meant "no deflection". I am not sure if the angle of a unit of scale was fixed or variable.[Citation needed]
German sights were calibrated with deflection units of a fixed angle of 1/16 of a degree.
- The reference range was 1500 yards for heavy guns (1000 yards for light guns) until 1901 when these were doubled. By 1907, heavy guns were increased to 5000 yards with lights remaining at 2000 yards. Manual of Gunnery for HM Fleet, Volume I part 1, 1907, p. 42.
- Reports of the Grand Fleet Dreyer Table Committee, 1918-1919, p. 15.
- German Navy: War Vessels, Naval Ordnance, Torpedoes, Mines, etc. 1917, ADM 186/383, p.27.