■Search and Recovery

■ Underwater Navigation

■ Maintenance and Repair Instrument Implantation ■Salvage

■ Photography.

7.1.1 Search and Recovery

Search techniques all rely on one common element: the adoption and rigid execution of a defined search pattern. The pattern should commence at a known point, cover a known area, and terminate at a known end point.

Search patterns must be selected to fully utilize the capabilities of the search equipment, whether it be sonar, visual search, or diver hand-held sonar. Search patterns are conducted by carrying out search sweeps that overlap. To be efficient, the overlap should be minimal. The initial step in a search is to define the general area and the limits to be searched. If the search is being conducted to locate a specific object, the last known position of the object is the starting point for defining the search

area. The drift in the open sea resulting from sea and wind currents, the local wind condition at the time the object was lost, and the leeway, or movement through the water from the force of the wind, should be studied. The sea currents can be estimated for a particular area using NOAA Tidal Current Tables and Tidal Current Charts and the U.S. Navy's Atlas of Surface Currents (1944, 1947, 1953). Wind currents can be estimated using the following table:

Wind speed (knots)

Wind current (miles/day)

1-3

2

4-6

4

7-10

7

11-16

11

The leeway is generally calculated at 0 to 10 percent of the wind speed depending upon the area of the object exposed to the wind and the relative resistance of the object to sinking. The direction of leeway is downwind, except for boats that have a tendency to drift up to 40 percent off the wind vector. The calculation of a value and direction of leeway is highly subjective for objects that will float or resist sinking; however, if the average wind velocity is relatively low (under 5 knots), or the object is of a nature to sink rapidly, the leeway has little or no effect on the calculation of a probable location.

Once the vectors of water current, wind current, and leeway are added vectorially and applied to the last known position of the object, a datum point is defined. The datum point is the most probable position of the object.

Once the datum point is defined, the search radius around the datum point is selected. The search

key to success (U.S. Navy, Deep Water Search Procedures Report 1972). A good search technique must assure complete coverage of the probable area, clearly define areas already searched, and identify areas to be searched.

Some standard search techniques, are summarized in Table 7-1 and discussed in subsequent paragraphs.

The visibility, bottom topography, number of available divers, and size of the object(s) to be located are prime factors in selecting the best method for a particular search.

In favorable conditions (bottom is free of projections, the visibility is good, and the object(s) to be found is (are) reasonably large), and where the area to be searched is small, the Circular Search is probably the best. In areas larger than the tethered range of the diver, however, either the 'Z' or Jackstay should be used to eliminate segments of incomplete search.

For large areas in which visibility is good and time is of the essence, the Jackstay is usually the most desirable method.

The 'Z' Search is most efficient, however, and is more likely to locate very small objects, especially in low visibility or where conditions exist that prevent visual coverage. Because much of the search is conducted by touch and feel, and because it covers most of the area twice, the 'Z' search is the slowest of the three.

7.1.1.1 Circular Search

A search line is anchored to the bottom or tied with a bowline around the bottom of the descent line and used to sweep the area. In order to determine when a 360-degree circle has been made, a marker line should also be laid out from the same anchor as the search line. This marker line should be highly visible and numbered with the radial distance from the anchor.

Where current is noticeable, this marker should be placed in the down current position such that the diver always commences his search from the position where the possibility of entanglement is decreased. When more than one circle is to be made with tethered diver(s), the direction of travel should be changed at the end of each rotation to prevent the possibility of fouling lines.

The circular search has many modifications depending on the number of divers and the thoroughness of the search required.

The standard technique is to station one or more divers along the search line close to the center of the desired search area. The marker line can be used to assign precise distances. The divers swim a circle while maintaining taut holds with one hand on the search line until they return to the marker line and are certain that 360 degrees have been covered. The divers increase the radius for the next search, moving out a distance that will permit good visual coverage. This procedure is continued until the outermost perimeter is reached.

When two divers are used . . .

Effectiveness can be increased as shown in Figures 7-1 (a) and (b), by having one diver hold the circling line taut and swim the outside perimeter of the area to be searched while another diver sweeps back and forth along the taut circling line. As shown in Figure 7-1(a), the first search will cover a full circle bounded by the outside diver's path. The search starts and finishes at the marker line. The search may be extended by the pattern shown in Figure 7-1(b). In this case, the circling line is marked where the outside diver was previously stationed. The outside diver then moves to a new position, farther out on the circling line, and the inside diver sweeps back and forth between the marker and the outside diver's new position. The outside diver covers a considerably greater distance than those on the inside and therefore must swim faster. Positions may be changed at regular intervals if fatigue becomes a factor. This can be done at the end of each sweep by having the outside diver hold his position after moving out one visibility length. The other diver(s) move outside taking up positions for the next sweep.

7.1.1.2 Jackstay Search

Both the Jackstay Search and the 'Z' Search require a rectangular boundary. The layout procedure described below is recommended for use with both searches.

Step 1. A weight is lowered near the center of the search area.

2

NOTE: Steps 3, 4, and 5 are more easily achieved using two divers. With one diver, the procedure is the same except that the diver must be able to locate the free ends of the two lines.

The search commences by laying two 87' Jackstay lines parallel to the boundary line of points 1 and 2 (Figure 7-2). The first line should be laid only as far from the boundary line as visibility permits. The second Jackstay line should then be placed the same distance as the first (to the limit of visibility). The divers then search between the boundary line and the first Jackstay line. As soon as they complete that area, the first Jackstay line can be leap frogged over the second line to the limit of visibility (Figure 7-3).

When the search is completed in the 87' x 100' area it is only necessary to shift anchor points Nos. 2 and 3 to establish points Nos. 5 and 6, using

the same technique as shown in step 3. A new search area cornered by points Nos. 1, 4, 5, and 6 is now formed.

If the area to be searched extends beyond the combined area of the two halves (100' × 174' total), the weight must be moved to the positions illustrated (Figure 7-4), and the search continued from step 2.

7.1.1.3 'Z' Search

The layout of the 'Z' Search boundary lines is done exactly as the Jackstay Search. The search is conducted by both divers traveling on the same anchored search line. Each diver maintains one hand on the line with the free hand searching to the extent of the arm reach, while slowly moving along the search line. At the end of the traverse, the anchor is shifted with the opposite end remaining unchanged. Upon searching in the opposite direction and reaching the opposite distance line, the same procedure is followed. The search pattern resolves itself into a 'Z' pattern on each sweep (Figures 7-5 and 7-6). This results in almost all of the search area having double coverage. The remainder of the search continues until the area has been covered.

7.1.1.4 Searching Without Lines

When conditions are such that it would be unfeasible to use search lines, a search can be conducted using an underwater compass. Many patterns may be developed which will ensure