2. Evading a Fast-moving Enemy at a Given Distance

An evasive maneuver against a fast-moving enemy at a given distance is executed whenever the enemy is proceeding on a steady course and the distance to him is greater than his detection range.

Let us suppose that maneuvering vessel M is at point Mo. The enemy assault group sought, with three escort vessels, steering course Kk at speed Vk with Vk > Vm, is detected from bearing Bo at distances Do

Кие

M,

Fig. 119. Evading a fast-moving enemy at a given distance.

Plotting the positions of escort vessels on a chart and calculating the width of the search area, we see that the initial position of the maneuvering vessel lies within this area (Fig. 119). The following plots are drawn for the evasive action of a maneuvering vessel against escort ships at distance D1. From the initial position of the escort vessel Ko furthermost from the maneuvering ship we describe a circle having a radius equal to the selected distance of evasion D1. Then from the initial position of the maneuvering vessel we draw a tangent to the circle and use it as the relative course K,. The point of tangency M1 is connected by a straight line with point Ko. In determining the evasive course of the maneuvering vessel, we construct a speed triangle at the point of tangency M1, in which the velocity vector Vm indicates the desired evasive course K, and the value of the relative velocity vector, indicates the relative speed Vp Then the maneuvering time

t =

(173)

3. Taking Evasive Action Against a Fast-moving
Enemy on an Extreme Course

An evasive maneuver against a fast-moving enemy on an extreme course (maximum distance) is executed when the latter is detected at small distances

(for example, at distances at which radar and sonar contact, etc. is established).

Let us suppose that at the initial moment the maneuvering vessel M is at point Mo, and an enemy antisubmarine vessel K (Fig. 120) is at point Ko. The initial distance between the vessels is D, and VmVk. In order to determine the evasive course of the maneuvering vessel, we construct a speed triangle at point Mo with critical relative bearing Q, assuming that the enemy vessel will proceed on course Kk. Actually, the position of the vessels at the end of the maneuver will be determined by constructing a position triangle. For this purpose, from point M1 we plot the course of the antisubmarine vessel Kk to its intersection with course Km and obtain point M1. From this point, at the moment of relative beam, we plot the bearing to its intersection with course Kk and obtain point K1.

The time of arrival at convergence distance D1 is calculated as the ratio:

In the process of maneuvering for an advantageous position vis-à-vis enemy forces, it is sometimes necessary to change bearing on the enemy a given amount (Fig. 121).

Let us assume that at the end of the maneuver the bearing on the enemy should be B1. In order for the maneuvering vessel to assume bearing B1 in the 1 shortest possible time, it must proceed on the shortest route. This route will be perpendicular to the line of bearing B1. Consequently, in order to change bearing in the shortest possible time, it is necessary to steer a course perpendicular to bearing B1, i.e., Km = B1 ± 90°.

Fig. 121. Changing bearing on the enemy in the shortest possible time.

Км

The sign is "-" with a clockwise change in bearing, i.e., if the starboard side of the target is visible. The sign is "+" with a counterclockwise change in bearing, i.e., if the port side of the target is visible.

SECTION 49. DETERMINING THE MANEUVERING
PROPERTIES OF THE TARGET

1. Determining the Course and Speed of the Target
from Bearings, Crossing the Target Course

This method is used when it is impossible to measure the distance to the target, but it is possible to cross its course.

We measure three bearings: the first at moment T1 several minutes prior to crossing the course, the second at the moment of crossing T2, and the third at some time T3 after crossing the target course. We plot them on a chart (Fig. 122) from the calculated positions of one's own ship, corresponding to moments at which bearings are taken.

The target course is equal to the reciprocal bearing on the chart at the moment the second bearing is taken in crossing the target course. The target speed is equal to:

Vk

=

K1K3
T3 - T1

(175)

2. Determining the Course and Speed of the
Target from Distance and Three Bearings

This method is used if there is a possibility of measuring one distance to the target and taking three bearings.

Having measured the distance and bearing to the target and having plotted it from one's own calculated position, we obtain point K1. Then through equal time intervals we take the second and third bearings and plot them (Fig. 123). From point K1 we plot a secondary straight line, perpendicular to the line of the first bearing. We measure distance K1A, intersected by the line of the second bearing, and plot from point A. To the obtained point C we plot a line, parallel to the second bearing, to its intersection with the third bearing and obtain point K3. Connecting point K1 with point K3, we obtain target course K1K3. The target speed:

This method is used when it is impossible to determine the target course and speed using other methods.

Three bearings are taken on the target through equal time intervals on the first course of one's own vessel. Let us assume that the target at the moment the first bearing is taken was at point Fat distance D1 (Fig. 124). From point F we plot secondary line FM perpendicular to the first bearing. From point A we plot a segment equal to FA, and from the obtained point B we draw a straight line parallel to the second bearing, to its intersection with the third bearing.

At the moment the third bearing is taken one's own vessel executes a tum to a new course and changes its speed significantly. The fourth bearing is taken on the new course. We plot the imaginary position of our own vessel M4, where it was located at the moment the fourth bearing was taken, if our own course and speed did not change. From point C, continuing line FC, we plot a segment equal to AC, and connect the obtained point D with point M4 with a straight line. If our own ship has not changed course, then at the moment the fourth bearing is taken the target will be on the line of imaginary bearing MD. Actually, at the moment the fourth bearing is taken the target will be on the line of the fourth bearing B4. Therefore, it must be assumed that at the moment the fourth bearing is taken the target will be at the point of intersection of these two