How far should my diver go?
Maximum tether line length
Part II of the Lifeguard Systems Public Safety Diving Standard Operating Guidelines
The information presented here is from the Lifeguard Systems Public Safety Diver Standard Operating Guidelines by Walt Hendrick.
What do your Standard Operating Procedures/Guidelines (SOP/G) state is the maximum line length you can let a diver out for a standard dive operation with no extreme conditions? As we said in SORTIE issue no. 4, in the "Maximum Depth Standard" article, having a clear, thorough SOP is critical for the safety and liability protection of both individual team members, the team as a whole, and the department. If the victim is believed to be 300 feet from shore and there is no boat on the scene yet, should you let your diver out on 300 feet of tether line? If your SOP states a lesser maximum line out, and you end up in court because you waited for a boat, and the family is upset about that, then your SOP is there to protect you. And better yet, your insured trainers who taught you those standards and the trainers certification agency are moved to the front line in court.
Okay, let us begin to address the question at hand, and again we will do it by presenting you with ideas so you can come up with a logical answer. To find the logic, we first need to review the main functions of a tether line.
Back-up Diver - Help is on the way!
As stated in several articles in past issues of SORTIE, one of the most important functions of a tether line is to provide a direct-line access between an entangled or entrapped primary diver and a tender-directed, tethered back-up diver (warm, rested, with a full-tank, and no other job than saving the primary diver). Without this direct line access, it could take far longer to find and reach a trapped primary diver than the divers air supply will allow. That amount of time is not an acceptable gamble, as has been very, very sadly proven during too many operations.
So, how long can you survive without air? What would be the maximum time you would want your back-up diver to take to reach you? Thirty seconds? Sixty seconds? Two minutes? Do not forget that the amount of time it takes for a back-up diver to reach you depends on the length of your tether line!
In low or no-visibility water, how can free-swimming divers know where they have been, areas they have missed, and where they need to go? They cant, but thats what a tether line is for. Free-swimming divers in such poor visibility find things more by luck than by proper procedure.
As past articles have stated, a tether allows dive operations in low or no-visibility water to have accurate records of what areas have been successfully searched, what areas were searched but were not searched well enough to be secured, and what areas still need to be searched. If a one foot by one foot area was missed, by Murphys Law, where is the weapon or the baby? If you do not have that information written on a profile map, then how do you know what areas can be secured and what areas need to be searched? You dont. So, a tether line is very important for maintaining accurate and thorough search patterns, and knowing what areas still need to be searched and which can be secured.
To maintain a good search pattern, tether lines (preferably of minimal-drag 3/8-inch diameter line) must be kept taut by divers. However, the more line they must haul through the water, the more drag there will be, and the more difficult it is for the diver to maintain a taut line. If there is too much line, when the diver moves, the line will bend.
Another important function of tethering is communication between divers and tenders. Even if you use an underwater communication system, line signals are important for when the system goes down. If you are in a rescue mode, can you afford to abort dives just because the communication system failed? If a primary diver requires assistance and communications go down, then line signals become even more imperative. So the next logical question is, what is the maximum distance line signals will be felt down a 3/8" line? (A larger diameter line will have a lesser maximum distance.) Under good conditions, the answer is 150 feet for 3/8" line, and that number is decreased when current, other water movement, or wind are added.
Entanglement and Snags
The longer the tether line, the more chance the tether line may snag or entangle.
Sweep Length and Diver Exertion
The longer the tether line, the longer each sweep of a pattern must be if the pattern is based on a stationary tender. Going back to Making a Molehill out of a Mountain in SORTIE no. 4, look at how many running feet a diver should expect to be able to get out of a 20 to 25 minute dive. For a handgun-sized object, a diver will typically search an average of 300 running feet. A tethered diver put out 150 feet, for example, doing a arc search pattern will have a 236 foot arc. That means one arc for the entire dive, which is not very efficient.
Also if a diver is sent out 200 or 300 feet just to reach the search area, the result is lost time, a poor use of air, and a fatigued diver.
A well-trained back-up diver can reach an entangled/entrapped primary diver in less than a minute with the primary diver on a tether line less than 150 feet - if no problems occur. The greater distance away, the more likely the back-up diver will experience problems.
Our recommendation is to mandate a maximum diver distance from shore or diving platform of 125 feet. We recommend using 150 foot deployment tether line bags to give you an extra 25 feet of line for securing the end of the bag to a non-moving object. Longer distances risk the chance that signals will not be received, back-up divers will not get there in time, the diver will become more fatigued, the pattern will be lost because of bends in the line, and psychologically divers will know that they are that much further from help and home. So, a 125 foot maximum is the best choice for tether lines. If your team has a shorter maximum distance, you will have an even higher level of safety. As a final note, we recommend that new search divers be trained with a maximum of 80 feet of line, and that their maximum be gradually increased as they become more experienced and competent.
If you have any questions or comments, we would love to hear from you. Please contact us at Lifeguard Systems.
For a related article, click here.
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