Dip is the difference in elevation angle the line of sight to an object from one vantage point (say from a director, or simply a sighting telescope mounted a few feet above the barrel) and the bore of a gun.
dipAngle = arctan(heightOfSightAboveGun / range)
The dip angle would be negative when the gunsight or observer were below the bore of the gun (requiring him to look higher than normal).
Gun sights were almost always positioned quite near the bore of the gun they controlled, but sometimes they were a few feet above or below. While this seems like a small matter, at short ranges, the Royal Navy considered it worthy of consideration and so this sight dip angle (which depended only on the vertical offset) would be incorporated into the design of a gun sight. As an example given for Africa, where the telescopes were 3.77 feet above the bore, the dip angles at various ranges were:
The great height difference between a director and the guns it controlled also required separate dip corrections at each gun, given their different heights above the water.
Since a ship had more than one director, the Royal Navy defined a shipwide "datum point" (often, the armoured director position, or the trunnions of the turret highest above the water) for each ship which would share precise gunnery pointing angles, as in director firing, and provide mechanisms that would convert angles from that position to their own. This permitted the director's elevation transmitters to send out these reference elevations, confident that each receiver would massage the angle a bit as their own position on board required.
For instance, the Royal Navy's elevation receivers often featured a "dip strip" that could be shipped on their face. These corrected a number of small issues, but chief amongst them was that which earned them their name.
- Convergence, the analogous correction in yaw (training)
- Manual of Gunnery (Volume I. Part I.) for His Majesty's Fleet, 1907. p. 38.
- Admiralty, Gunnery Branch (1907). Manual of Gunnery (Volume I. Part I.) for His Majesty's Fleet, 1907. (G. 4117/1907) Copy No. 783 is Ja. 254 at Admiralty Library, Portsmouth.