Wide Area Hand Searches

This blog builds on the concepts presented earlier. Executing a search for a drowning victim requires a team effort. This involves divers, tenders, accurate dive profiles, and experienced planning. This becomes even more critical when the search covers a large area.

A lousy point-last-seen

As we’ve pointed out earlier, the most important part of any underwater search is the accuracy of the point-last-seen.  Sometimes, this is pretty easy: if someone drowns in a river, witnesses are nearby, and the victim will be fairly close, downstream of where they were seen to go down. If a car enters the water, there are usually marks where it left the road. Unfortunately, a good point-last-seen isn’t always available. Sometimes, the best information we have is fairly vague. This is true, even when we have eyewitnesses, because estimating distances and locations over larger spans of water is notoriously difficult. So we might end up with witnesses that see a drowning on a lake that place the event in very different places. Or worse, we might just start with a capsized boat, or some debris.

So what can we do?

Once we’ve decided where to start, the first step is to create a stationary platform on the water. We need a reliable fixed location to put a team on in a lake, bay, or estuary, so we can create a reliable record of what areas have been searched by divers. The search team will operate from a boat at anchor, with tenders, backup divers and profilers working from the boat, while the diver searches underwater. The search area is recorded, so we know what has been searched and what areas remain.

An anchored boat?

If you’ve ever been in a boat on anchor, you’ll probably be thinking that this can’t possibly work, because boats on anchor can move about quite a bit. In fact, the way an anchor works is by having a long offset downwind or downstream that lets the anchor bite into the underwater terrain. Using a technique called “hurricane anchoring”, however, we can create a fixed position and leverage the tools we’ve already described for reproducible searches

First, a little primer on anchors: Anchors come in various shapes and sizes. We generally use what’s called a “Danforth” anchor that gives us the best combination of weight and bite in the waters around here. The key to the anchor biting into the underwater terrain is the chain. A longer and heavier chain will  make the anchor lay down, and dig into the soil. Finally, a long anchor line will allow the chain to lie down, making the whole assembly secure the boat. There are rules-of-thumb about anchor line lengths, anchor weight, and chain weight and length that I won’t go into here. But they give us tried-and-true guidance about secure anchoring.

Hurricane Anchoring

Next, hurricane anchoring requires at least 3 anchor lines to establish a fixed platform. This technique gets its name from its use to secure boats in hurricanes. The wind and/or current direction will determine where we place our leading anchors. A third anchor will add additional security for moments when the wind might go slack, or (in our case) when a diver is pulling hard on the tender line in another direction. The overall layout looks something like this.

Sometimes we use a second boat to set anchors; while it’s possible with a single boat, a long anchor line (optimally 7x the depth) helps with stability, and using a second boat is helpful for managing the long lines.

Once our platform is secured, we can deploy a team in nearly the same manner as we would from a dock or shoreline. We’re usually limited to arc-pattern deployments because of the limited boat size, but this isn’t really a problem in doing extended searches, as you’ll see below.

When we’ve cleared an area without finding our target, what do we do? Because we have a stable platform and we’ve been keeping records, we can shift the boat to a new location and search there. With foresight and long enough anchor lines, we can do this once or twice by simply adjusting the length of the anchor line. Eventually, though, we’ll need to move the boat.

Expanding the Search

GPS is a fabulous tool for keeping track of where we need to move, and what we’ve covered. There are some shortcuts that make this process fairly quick. For example, most modern boat GPS systems use degrees and decimal minutes for latitude and longitude: .001 minute is 6 feet of latitude, and a bit less longitude (here). So if we know where we’ve been, we can estimate where we need to move next. Also, if we have an understanding of wind and current at our location, we can plan our search track for maximal efficiency.

Over time, this kind of search evolves into a large area search that looks like this. While there are still areas that haven’t been fully explored, we know where they are, and we can build an adjacent search path that will thoroughly cover everything. Even without a highly precise point-last-seen, we can find and recover our victim.

Large area hand searches are time consuming, but can definitely be done with efficiency and precision. Wandering about with a SCUBA tank without a full team for documentation and guidance just isn’t a viable option.