ACCURACY OF SHIPBOARD GUN FIRE

18A3. Accidental errors causing dispersion

The problem of spotting is complicated by dispersion. If a battery of guns is fired at the same instant with the same settings in range and deflection, the projectiles will not all land at the same point.

If the battery of guns were stationary and rigidly fixed in elevation and train, variations in range and deflection would be caused by:

(1) differences in weight and temperature among individual powder charges;

(2) differences in projectile weights;

(3) variations in angles of projection-the axes of projectiles diverging, in varying amounts, from the continuation of the bore axis as they leave the guns;

(4) differences in projectile seating, causing variations in density of loading and initial velocity;

(5) differences in erosion among the several guns, with corrections not precisely made;

(6) differences in droop among similar guns, and unlike variations in droop with temperature changes;


(7) variations in amount the gun mount will yield and irregularity in action of recoil mechanisms.

These are sufficient to justify acceptance of the fact that, even under ideal conditions, dispersion in the points of fall of projectiles from several guns, or in several shots from the same gun, may be expected.

If this battery of guns be mounted aboard ship, and each be individually positioned by a pointer and trainer, the rolling and pitching, and the yawing, of the ship itself will further cause dispersion.

The motion of the ship may cause the pointers and trainers of the several guns to misalign their sights on the target when the guns are fired.

The same effect may result from failure to fire exactly simultaneously, causing different guns to fire at slightly different points in the roll, and thus at different velocities of roll. Director-controlled gunfire, although not subject to the same characteristic errors as pointer fire, is subject to its own set of characteristic errors. These are discussed in some detail in
articles l8Al1 through 18A15.

The errors mentioned above as characteristic of pointer fire, and many of those which are characteristic of director fire, come under the general classification of accidental errors. They are revealed by analyses of firings, and their effects are governed by
laws of probability, as will be explained.

Additionaly the sea state and the direction of the ship with relation to the waves and troughs and the height of the waves and the depth of the troughs affects how the ship hogs, rainbows, bends and twists.

How much the ship hogs, rainbows, bends and twists affects how parallel the roller paths of the turret and gun mounts are with each other from bow to stern.

As the roller paths are bowed and twisted out of parallel with each other both trunnion tilt and gun elevation with each turret and gun mount are changed and the guns are all not pointing at the same spot. This affects both the length and width of the pattern.

If the ship is in the troughs and parallel with the waves then the ship is least affected by the current sea state and is only rolling with respect to the waves.

As the ship turns out of the troughs and begins to cross the waves the ship hogs, rainbows, bends and twists more with the maximum hogging and rainbowing occuring when the ship is crossing the waves at 90 degrees of angle. Maximum bending and twisting occur when the ship is crossing the waves at 45 degrees of angle.

As the ship hogs, rainbows, bends and varying amounts at different heights of waves and depths of the troughs and at different angles of travel across the waves and troughs then the length and width of the pattern changes as well.

As the firing range is increased or decreased under these conditions the pattern size in range and width also increases and decreases.

It is therefore clear that there can be no set pattern size for any ship as on any given day the sea state is different and the ships heading with respect to the waves also changes the pattern size.

The pattern size can only apply to the ship on the day of firing and at the particular ships course in relation to the waves on that day.

For these reasons the USN did not and could not produce a table that refleced pattern size for any ship, nor could any other navy.

17B2. Standard conditions for range tables

In the computations upon which range tables are based, certain arbitrary conditions are assumed. These arbitrary conditions are generally spoken of as range-table standard conditions. In order to use the range table under conditions other than standard, it is necessary to provide in the range table, corrections for variations from these conditions.

The standard conditions assumed for range-table values are that:

1. The projectile leaves the gun with the designed velocity.

2. The projectile is of the designed weight.

3. The atmosphere is of an arbitrarily chosen standard density.

4. There is no wind.

5. The gun is motionless.

6. The target is motionless.

7. The earth is motionless.

8. The gun and target are in the same horizontal plane, the plane tangent to the earth’s surface at the gun.

9. The gun is elevated in the vertical plane; that is, the axis of the gun trunnions is horizontal at the time of firing.

Since the range table is based upon these conditions, variations from any one of them may cause a significant error. In addition to the tabulation of trajectories under standard conditions, the range table provides necessary corrections for variations from the first six of these standard conditions. Corrections for variations from any of the last three require separate consideration or computation.

NOTE: Although motion of the target does not affect the trajectory itself, it enters into the problem of determining the trajectory that will reach the target.

Correction for rotation of the earth can be computed from auxiliary tables published with range tables for major-caliber guns, or by fire control instruments. The amount and direction of the error vary with the azimuth of the LOF, the latitude, and the range; but remain small in all cases. The correction is disregarded for guns of 5-inch caliber or smaller. The effect of firing at a target not in the horizontal plane of fire was discussed in
article 17A6. The correction for tilt of the trunnion axis from the horizontal is practical only when using a mechanical solution and therefore is considered in chapter 19. Use of the columns of the range table to account for nonstandard conditions will be taken up in the next section of this chapter.