In Part 2 of this series, we discussed how to find the amount of area a diver has searched when doing linear pier walk or box search patterns. However, these are not the only types of patterns divers conduct. Another very common pattern is the half-moon or arc search, so we begin Part 3 by learning how to find the amount of area searched in these types of patterns.
Half-moon searches, perhaps better called arc searches, use arcing patterns that resemble a half-moon, or smaller, pie-slice shapes. They are best used where the shoreline or diving platform does not allow the tender to have freedom of movement. Most large-area searches require platform-diving operations, so the half-moon technique is very popular. To figure out the area searched, let us assume you had your diver begin at a distance of 100 feet from shore or platform. For this dive, the diver would be searching his or her way toward the tender, or "home." Suppose the diver performs 8 sweeps, each 3 feet wide. He ends his dive 79 feet from shore. The question is, from 100 feet out to 79 feet out and searching a 90-degree pattern, what is the running feet per sweep? What is the total area covered?
There are three methods to address this question. The first uses linear feet, as was done with the example from the pier walk search. However, for a half-moon, finding linear feet is a bit more complicated, because the distance covered was over an arc, not an easily-measured, straight line. The first step in finding that length is to find the circumference of the total circle the diver has made if he had continued the arc. Circumference of a circle is found using the formula (2 × radius × pi = circumference).
Suppose that in this case, the diver only searched an area that was one-quarter of a circle. Since the diver only swam one-fourth of a circle, we only use one-fourth of the circumference to find the length of the divers sweep. To simplify the math, we can combine that information into the following formula to find the length of each sweep in one shot (using 3.14 to calculate pi):
Distance out × 2 × pi × fraction of the circle = Length of each sweep
So, if the divers first sweep was made 100 feet out, and he covered a 90-degree arc:
100 × 2 × 3.14 × ¼ = 157 feet
The same calculation is then made for each sweep the diver covered. Hence, if the diver made complete sweeps from 100 feet out to 79 feet out, being pulled in 3 feet after every complete sweep, he would have covered eight sweeps, of 157 feet, 152 feet, 148 feet, 143 feet, 138 feet, 133 feet, 129 feet, and 124 feet. The sum of those sweeps, 1124, is the total running feet the diver searched, based on calculations from the equation above.
Once you know the total number of linear feet, it can be multiplied by the width of the divers search, as in the pier-walk example.
1124 running feet ×3 feet of width = 3372 square feet searched
Although these calculations can be found by using the areas of portions of circles defined by the outer perimeters of the search area, the math is fairly involved. Not only must the areas of the portions of circles be found, but the areas covered by the divers arm span outside and inside of the distance out as defined by the tether line must also be found.
The second method to figure out the linear feet searched, and therefore the area searched, is to use the following chart. This chart was created for a search for an adult-sized object, and is based on pulling a diver in three feet after every sweep. The chart shows the area in square feet covered by a diver as calculated using the above method above, for sweeps done at increments of every 3 feet from 150 feet out to 3 feet out from shore. Just start your diver out at one of the increments on the chart, and add up the areas for each of the divers sweeps to calculate the total area the diver searched. You can use the mathematical methods described above to create your own charts for searches in which you pull the diver in more or less than 3 feet after each sweep.
Table 1 shows the area (square feet) searched on each sweep from 150 feet out to 3 feet out, in three-foot increments, based on the divers distance from the tender. On this table, one sweep = ¼ of a circle.
Distance Area Distance Area from Tender Searched (ft2) from Tender Searched (ft2) 150 708 75 354 147 693 72 339 144 678 69 324 141 663 66 312 138 651 63 297 135 636 60 282 132 621 57 270 129 609 54 255 126 694 51 240 123 579 48 225 120 564 45 213 117 552 42 198 114 537 39 183 111 522 36 171 108 510 33 156 105 495 30 141 102 480 27 126 99 468 24 114 96 453 21 99 93 438 18 84 90 423 15 72 87 411 12 57 84 396 9 42 81 381 6 27 78 369 3 15
Note that the half-moon search pattern will often reward you with less actual area searched, less linear feet than the pier walk or box search. This difference is due to the workload on the diver while maintaining a taut line through the top and bottom of the arc pattern, especially when current is involved. Circular search patterns will reward you with even less area searched per diver.
The third method of calculating the area searched uses a quick and dirty estimation process. All of the highly exact math is wonderful, but there are few people who can do it in their heads without a calculator on a rescue/recovery site. On the other hand, finding the amount of area searched is extremely important to the overall planning and success of the total operation. So, if you do not have the appropriate calculations or charts on a site, you can use the following system of approximation. Start the diver 100 feet from shore as a constant, and assume the first sweep is approximately 160 running feet. For every 2 ½ to 3 feet you bring the diver in after that, subtract 5 running feet from the previous sweep. The total of the number found for each sweep will give an approximation of linear feet searched. All you have to do to calculate the area is to multiply the total linear feet by the distance you pulled the diver in after each sweep. With this process, you get a good approximation, you do not have to have a Masters degree in math.
Note that none of the above search patterns should be used in areas with heavy grass. Searches in heavy grass require different patterns and methods. Additional articles will address this issue.
How did we miss it?
Now that you have the ability to figure out how large an area is and approximately how much area is being covered by each diver for different types of searches, you can take several steps to enhance the overall search, and the overall effectiveness of the total operation.
# 1: Are divers covering equal areas?
Decide if each diver is covering an equal amount of space or if some divers are covering smaller areas. If so, you may need to recalculate your time and manpower needs. Delays may be caused by portions of the bottom that require a slower search technique, while other portions of the bottom are clean and allow for very quick and thorough searches. You may wish to place your best searchers in the most difficult areas.
#2: Is the diver searching too quickly?
Since we know and should expect each diver to cover a given area, we should be able to interactively use this information to help determine whether an area was or is being searched well. A diver can only cover so much space in a given amount of time. Hence, when the diver has covered too large of an area in a given amount of time, we may not only be searching too fast, but we may have totally missed a space within the search area. This problem is actually a very common reason for missing items that really were in the area being searched.
#3: Is the diver where he should be?
If a diver is beginning his search at a distance of 100 feet from the tender, and the plan is to bring him in 3 feet on each sweep, then he should obviously be at a distance of 97 feet from the tender on the second sweep, 94 feet on the third, 91 on the fourth, and so on. At this point, you may be asking yourself, "What is the big deal?" Well, the deal is that more often then not we do not monitor these sweeps, and do not realize when a diver has varied from where he should be.
Often, due to a lack of trained tenders, a diver skips or misses an area within the search zone, yet it is not noted. In those cases, we may know that an area was missed, but then are not sure where it was. In fact, the pattern can become so fouled that we cannot be sure that sweeps are consistent or know if the diver has missed large areas within the search pattern. Both large and small portions of the search area may have been missed, but no one notices. Remember that the diver in blackwater will not know where he is or which areas he has and has not covered; he is counting on the tender and or support personnel for that information.
The profiler should always make note of the divers starting location and the beginning point of each new sweep. Hence, the Dive Coordinator should be able to ask at any point the divers location, distance out, and number of sweeps or patterns completed. It does not require a math major to decipher when the diver is too close for the number of swings completed. If he or she is too close for the amount of time in the water or the number of sweeps completed, something is wrong.
If the diver is not holding a good pattern you should see it early and be able to rectify it or remove the diver before the entire dive time is wasted. When the mistake is caught early, it can be fixed or the area can be re-covered without much difficulty. When it is missed, or caught too late, then the whole area must be re-searched. If the mistake is not caught at all, it is a waste of the total operation. Training and practice sessions are the time to allow divers to have extra time and perhaps re-dive a given area.
Most importantly, do not miss what you are looking for because you missed a bad pattern. The total success of a diving operation can be based on one sweep, one diver doing it all right, at the right time. All the rest was securing the area to get you there.
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