Chapter 18 Spotting
A. Laws of probability in their effect on gunfire and on spotting
B. The Spotter
C. Methods of spotting
                                                                 B. The Spotter

18B1. General function of the spotter

The primary function of the spotter is the correction of range and deflection errors of the MPI so as to bring the shots on the target, or, in other words, to establish the hitting gun range and deflection. The basis of these corrections is his own observation combined with those of the fire control radar operator. As a rule, in good visibility the spotter will estimate the necessary deflection correction, and range corrections will be obtained from the radar. Under unfavorable conditions both range and deflection corrections will be obtained from the radar. Prompt and accurate correction of initial errors may be the deciding factor in a naval engagement.

In the event of radar failure, optical spotting must be relied upon for all corrections necessary to establish the hitting gun range and deflection. Therefore in the initial discussion of the spotter’s problems, it will be assumed that radar is not available.

18B2. Other functions and duties

The spotter’s detailed duties and spotting procedures are prescribed in the ship’s battle bill and fleet doctrinal publications. Included are the following important duties:

1. Search for and locate the enemy.

2. Describe the enemy forces (number of ships, bearing, etc.).

3. Estimate all values for the initial rangekeeper set-up (enemy speed, target angle, and range).

4. Notify Plot and Control of any changes of target angle, target speed, and enemy deployment.

5. Keep Conn and Control informed of the tactical situation.

18B3. Visual estimation of target course and speed

The spotter who can estimate target angle and target speed with fair accuracy will greatly assist the rangekeeper operator in arriving at an early solution of the problem, and also aid him in checking the rangekeeper set-up. On the other hand, if the spotter’s estimates are inaccurate the range-keeper operator will have to make many changes.

To estimate target angle the spotter must know the structural details of all likely targets. Silhouettes and scale models of all probable targets are furnished each ship. The spotter should study the details of these visual aids, not only for the purpose of recognizing the enemy, but also for estimating target angle. In estimating target angle, the spotter should make use of prominent objects such as bridges, breaks in the deck, stacks, masts, and other features. By observing the opening and closing of the apparent distance between such details, the spotter can estimate the angle the enemy ship makes with the line of sight.

Target speed can at best be only roughly estimated. Here again, knowledge of enemy ships is valuable, particularly in the case of maximum speeds. If in line of battle, target speed may be estimated as about 1 or 2 knots less than the maximum speed of the slowest ship in the formation, considering any damage reports which may have been received.

The best speed estimates can be made only by men who have had extensive training and experience. The aids used by a spotter in a direct estimation of target speed are smoke from the stacks, bow wave, and stern wake.

18B4. Visually spotting error of MPI in deflection

Correcting the deflection is extremely important not only for hitting but also because, in long-range surface fire, it is difficult to spot in range if the deflection error is excessive. A salvo which is widely off a target should first be spotted in deflection, and the succeeding salvo spotted in range. In estimating deflection spots, the estimated target width in mils should be used as a guide. Also, telescopes, binoculars, or other optical devices fitted with mil scales furnish a suitable means of estimating deflection errors.

With a high-speed target, the spotter should bear in mind that the apparent MPI in deflection should be held abaft the point of aim in order to allow for the travel of the target while the splashes are forming. In other words, it must not be assumed that full splashes form instantaneously at the impact of a salvo. The time lag is only a few seconds at most, varying with the caliber of projectiles, but is sufficient to allow considerable movement of a high-speed target.

18B5. Visually spotting error of MPI in range

One of the most common mistakes made by the untrained spotter is to underestimate the amount of the range error at long ranges. This is due to the fact that a given range error will subtend a much smaller angle at long ranges than it does at short ranges. However, with good visibility and a spotting height of 120 feet or more, the error of MPI can usually be estimated with reasonable accuracy at ranges up to 15,000 yards. This is accomplished by observing the position of the bases or slicks of the splashes relative to that of the waterline of the target.

Experience in determining range error can be gained during target practice and by individual practice on a spotting board, which stimulates the appearance of splashes relative to the target as viewed from a spotter’s station. Spotting diagrams are used in conjunction with such training to indicate the relative displacement of a salvo from the target and to estimate ranges.
Figure 18B1 shows a typical example for spotting height of 100 feet and target heights of 10 feet, 20 feet, and 30 feet. Such a diagram is constructed by means of dip curves for the particular height of a spotting station. Range lines are drawn at intervals of 500 or 1,000 yards, from minimum range to that of the horizon. These range lines represent angular distance below the horizon at which any object would appear from a height of 100 feet, if observed on the range lines so marked. The distances between the range lines represent the apparent range differences as viewed by an observer at the spotting height for which the diagram was constructed. The method of construction and the dip curves are available in Fleet training publications and are not described here.

A study of
figure l8Bl makes it evident that salvo A is short of the imaginary extension of the waterline of a target at 12,000 yards by about 500 yards. However, the error of a salvo fired at a target at 19,000 yards is not so apparent. The 500-yard error of such a salvo (indicated on the diagram as salvo B), appears as little or no error in apparent distance from the extended waterline of the target. Thus it can be seen that for shipboard spotting at such a range, the splashes must be in line with some portion of the target before the spotter can reasonably tell whether the salvo is over or short, to say nothing of estimating the amount of the error. In addition to its use for estimating range and range errors, the spotting diagram shows the number of mils a given target length will subtend at any given range. For example in figure 18B1 a 600-foot target will subtend 20 mils at 10,000 yards range.