TETHERED DIVER SYNDROME
“We couldn’t find it. We missed it by only five feet.”
All too often teams ask the same questions after a dive: “Why couldn’t we find the victim?” or “Why did we have to re-search the area? Obviously, the answer is. You missed it—probably because you didn’t plan your first search correctly.
Tethered divers—for safety and efficiency reasons—should be used whenever there is lowor no-visibility water.
Let’s look at a hypothetical scenario. A witness tells the team that the victim last was seen approximately 40 feet from shore. The tender sends the diver out to the approximate spot—or perhaps a little farther out—and the diver descends to search the bottom. The tender most often will use the safe technique of “bringing the diver home” toward shore while he works his pattern. This technique allows the diver to be more psychologically and physically comfortable because he knows—as his fatigue grows —that he always is getting closer to the shore. Another benefit of this technique is that when looking for large objects such as cars, the diver’s line might catch on the object and reduce the search time. Snags of this type, however, usually involve luck; finding a body depends primarily on a great deal of organization, knowledge, and preplanning.
In the case of the victim last seen approximately 40 feet offshore, there is more than a good chance that if the diver descends at the location, he will not find the victim.
A major reason for this is that judging distance over water is difficult. Witnesses typically are short by about 20 to 25 percent when judging how far an object is on the water. This is particularly true in instances where the body of water is large and the opposite shore is far away or not visible. Many people, in fact, misjudge distance over land by about 10 to 15 percent. In my training programs I have found that about 75 to 80 percent of them underestimate distances involving water.
TEST YOUR TEAM
The following exercises will help test your team’s distance-judging abilities. Take your team out to an open site and premeasure the distance within a range of from 60 or 65 feet to 200 feet to objects such as cans, trees, tables, and garbage cans. Without telling the team members that most people underestimate distances, have them stand in one place and estimate the distance of a particular object.
To simulate an experience closer to what a witness to a drowning sees, toss tennis balls on the ground while the team watches. Once the balls stop rolling, wait two seconds and then ask the members to turn their backs to the balls and write down how far away they think the balls are. Finally, have them measure the actual distances to determine how close their estimates came.
Another exercise involves setting marker buoys out on the water at various distances and asking team members to guess how far away each is. In such cases, witnesses’ estimates usually are 20 to 25 percent short.
In the event of an actual incident, therefore, if the victim last was seen approximately 40 feet away from the shore, you must add 20 to 25 percent of 40 feet —or 8 to 10 feet (the usual degree of distance underestimated). Round the figure to 50 feet. The diver should descend with at least 50 to 55 feet of line.
Measuring the line is imperative. Following is the system we use for measuring the line; it has been used successfully by major teams in at least eight countries.
Begin with a ⅜- to ‘/2-inch piece of polydacron rope; it will not absorb water, stiffen, or lose line signals over distances less than 150 feet as a nylon line will. Put a ½to 1-inch-wide piece of duct tape at five-foot intervals. Make sure the rope is dry and that the duct tape is wrapped around two or three times. Then make easily spotted reference marks at even’ 25-foot interval. At these intervals, different tape stripe patterns can be used to mark 50 feet, 75 feet, 100 feet, and so on. Using a different color duct tape for these intervals will make the tender’s job easier.
If a high-quality line is used, linesignals will travel successfully a distance of up to 1 50 feet, which should be the maximum line length. If you are using ½-inch nylon line, the signals may not travel effectively beyond 100 feet, due in part to the increased drag of that size and type of line. The maximum of 150 feet is set for the diver’s safety—you never want your diver out more than 150 feet from the shore, boat, or platform, where the backup divers, extra air cylinders, EMS personnel, and tools are.
Even after the line has been measured and the diver sent out to about 50 to 55 feet, you still cannot be certain that when the diver descends at that point he will end up 50 feet away from shore. The depth of the water also must be taken into account-failure to do so is another reason searches for victims fail. If the depth is not known, estimate that it is at least 25 to 30 feet.
The following exercise shows how water depth can affect the rescue procedure.
Take a 5-inch piece of string and lay it over the top of the square shown above. Hold the left end of the string in place on dot X and then swing the string down to each 10-foot depth interval. Notice that at each interval, the diver is brought farther away from the desired location of 50 feet from shore. You can see how far thediver would be from the desired 50-feetfrom-shore location in the situation described above if the water’s depth was 30 or 40 feet.
The formula for determining how far out a diver should be sent to compensate for depth involves the Pythagorean theorem as shown in the Line Distance Chart (below). You are probably thinking—and rightly so— that the last thing you need is to have to make mathematical calculations at an incident site. The following approaches expedite the process.
Using the Line Distance Chart, find the intersection between the water’s depth and the desired distance from shore. Let’s look at the case in which a witness reports that the drowning victim was approximately 40 feet from shore.
The formula would be witness 4misjudgment correction factor = true distance out.
40 + (20 percent x 40) = 48 feet; round to 50 feet.
This distance applies to a body of water such as a pond or small lake.
The team captain estimates that the depth at 50 feet out is probably 30 feet. Locate on the Line Distance Chart the intersection of 50 feet horizontal surface distance and 30 feet vertical depth, which is 58 feet. The diver should be sent out 58 to 60 feet so that he properly can begin his search at the bottom 50 feet from shore.
A quick rule of thumb if you don’t have access to a distance chart is to add 50 percent of the depth to the line for distances up to 100 feet from shore and 25 percent of the depth for distances more than 100 feet from shore. For example, if the actual distance out is 60 feet and the depth is 40 feet, then add 20 feet for a total line distance out of 80 feet. If you look that problem up on the chart you get 72 feet, which means that the rule-ofthumh gives a more conservative answer—or a larger area—than the chart.
If the actual distance out is 140 feet and the depth is 52 feet:
25 percent X 52 feet = 13 feet
140 + 13= 153 feet distance out
Standing on a platform or boat puts a tender more than two or three feet from the water line, and that distance must be added to the depth before making your calculations. For example, if a tender is standing on a 12-foot pier, 12 feet must be added to the depth
The importance of this adjustment will become evident as you practice using the chart for different combinations of depths and distances out. When you work with short distances out (less than 45 feet), the depth becomes an even more significant factor. When you use the rule-ofthumh technique for distances greater than 45 feet out, your answer will be more conservative than the actual numbers found on the chart. But if you use the rule of thumb for distances less than 45 feet, you see that your answer comes up short and that you could miss your victim. That is because when the distance out is less than 45 feet, the rule of thumb works for sloping-bottom profiles (the rope follows the slope) but not for dock/ pier/boat work where there is a straight drop to the bottom from the tender. When working in the latter situation, add the depth to the distance out for the correct line distance.
Let’s take the example of a tender standing on a good-size fire boat with a distance out of 30 feet and a water depth of 50 feet. In this case just add 50 + 30 = B0 feet, and put the diver out 80 feet—which is more conservative than the distance given in the chart (58). But, you might say, a large fire boat probably puts the tender approximately 10 feet above the water line, which means that the depth is 60 feet and the diver must be sent out 90 feet. This approach gives us a good chance of finding our object.
The tender’s physical location is also important. Often the tender works his diver right at the water’s edge. When the diver runs out of time, a new diver-tender team comes in. The new tender, however, might prefer to stand five feet up the shore to avoid wetting his feet instead of right at the edge of the shore. Instead of the new team picking up where the last team left off—hopefully with a few feet of overlap —the difference of five feet between the positions of the two tenders easily could cause the team to miss some bottom area. To prevent the possibility that the second diver will miss the object, consecutive tenders must stand in the exact spot; this practice will keep the search pattern accurate and efficient. The way to do this is either to draw the tender’s location on the profile sheet or to permanently mark the tender’s location.