Diving Lore

Diving Activity

Low Visibility Diving

Overview

Low visibility is the reality for much of the diving that takes place in coastal and inland waters in Britain and northern Europe. There are many reasons why the visibility is restricted. Some of these are seasonal and others depend on the location.
In addition to visibility, there are other factors, which reduce the diver's ability to see and communicate underwater. Darkness is an obvious example, but even highly overcast conditions can make a significant difference to underwater visibility. Artificial situations such as the interior of a wreck can also reduce the divers ability to see. It is particularly important to avoid stirred-up sediment on dives since this will quickly reduce visibility even in areas of quite good visibility.

Reasons for Low Visibility Diving

Many professional and commercial divers have no choice in the matter. The work they are contracted to carry out may occur in waters which have naturally low visibility or the act of working itself may cause a disturbance which reduces visibility. Sport divers, however, have a choice as to where they dive and consequently they may or may not want to dive in low visibility waters.
Many of the wrecks, which attract divers lie on stretches of the coast here one or more of the conditions mentioned above, are prevalent. If you want to dive such wrecks, then you need to train to cope with the conditions.
Marine archaeology is another area of sport diving which is likely to require diving in poor, if not zero visibility conditions. In this case, most of the cause is likely to be the diver himself as he disturbs the bottom or uses an airlift to clear an area.
Other activities on the bottom, such as searches, are likely to cause similar problems. Grasping for old bottles with your arms filled with mud or searching for marine organisms that live in the bottom will tend to reduce visibility.
Since divers wish to take part in these activities in spite of these conditions, it has been necessary for the evolution of low visibility dive techniques.

Techniques for Low-Visibility Diving

The major problems that low visibility diving causes to the diver is that it reduces his ability to communicate with his buddy and, unless steps are taken to compensate for this, both the diver and his buddy are put at increased risk.

In good visibility the diver and his buddy can swim up to several meters apart if they wish and still maintain good contact and communication. As visibility is reduced, maintaining contact between the divers becomes an increasing problem.

With visibility of above 5 meters there should be no problem for the divers.
At 2 - 5 meters, divers should close-in, with good technique and discipline they should maintain contact, a buddy line should be considered for making the task easier.

Below 2 meters a buddy line is essential for maintaining contact and below 1 meter seriously consider a lifeline from the surface tethered to either yourself or buddy.

There are a number of techniques which can reduce disturbance of the bottom, and hence increase visibility. The first is to master the art of buoyancy control so that the diver can move along above the bottom. Secondly, avoid using legs and fins for propulsion as this causes severe bottom disturbance. Pull yourself along using hands and arms. Boulders, kelp, etc, make good anchor points to pull yourself along. Keeping your fins still has the added advantage that your buddy, following close behind and trying to maintain contact , is less likely to have his mask dislodged by them.

Descents and ascents need special care in low visibility water. Many divers descend feet first to avoid banging into the bottom with their head. Surfacing is usually performed cautiously, with a hand above the head, to avoid bumping into surface objects such as jetties or moored boats.
Loss of confidence is also a major problem in low visibility, especially for the inexperienced diver. In this sort of situation, a buddy line is highly recommended. Even for experienced divers the buddy line can be a valuable tool as it gives the freedom to go out of sight of each other yet still remain in contact. Communication between divers using the buddy line should be by the standard signals known to one another.

The use of the buddy line between divers in no way conflicts with the use of a lifeline to the surface and a surface marker buoy. The two lines serve quite distinct roles. The first allows constant communication between the diving pair. The second keeps the surface party in contact with the divers.

There is a temptation to simplify matters and dispense with the buddy line by allowing the second diver to hold on to the surface marker buoy line some distance away from the diver with the reel. This method suffers from the fact that a diver holding onto the line makes the surface marker buoy even more difficult to control, and the natural tugging of the buoy on the line makes it very difficult for either diver to know whether messages are being sent or not.

Navigation while using a buddy line can be much more difficult. It is very easy for the leading diver to be drawn off course by a constant pull on the buddy line from his buddy. In low visibility it is very difficult to realize that this is happening. The effect on navigation is exactly the same as that of a tidal stream onto one side of the divers and needs to be allowed for or guarded against.

Buddy lines have limited application for low-visibility diving in confined spaces or where there are obstructions on the bottom. Divers should carry a knife in case of inextricable entanglement, but in using it contact with the buddy and surface is likely to be lost. In these conditions other techniques may be more appropriate.

Roped-Diver Operation

For diving in very low visibility of 1 meter or less it is wise to consider replacing independent swimming divers with a diver secured by a lifeline to the surface and tended by another experienced diver.  
This technique avoids the likelihood of divers getting in each other's way or displacing equipment, especially if a task of work is to be performed. The disadvantage of the technique is, of course, that any equipment failure, such as regulator malfunction, is less easily dealt with if there is no buddy close at hand. This means that the link to the surface tender is vital. The tender should be an experienced diver who understands the problems faced by the roped diver. As a back-up to the diver, a fully kitted stand-by diver should be ready to enter the water and assist the rope diver should this become necessary.
The line itself needs to be of reasonable thickness. If it is too thin or too thick it is very difficult for the divers to handle. It should also be capable of taking the breaking strain of a fully kitted diver. An 8 - 10mm rope with a breaking strain of around 450 kg is appropriate. The length required might depend somewhat on the circumstances of the dive, but care should be taken not to let the rope become too long, about thirty meters is a manageable length. Any longer, and the likelihood of tangling or snagging increases. It is a good idea to mark the line in a suitable way at, say, 5 meter intervals, so that the tender can keep an easy check on how much line is out.

The line must be securely tied to the roped diver. The rope should be passed round the body under all the equipment. The diver should be able to jettison BCD and weight belt without having to remove the rope. The other end of the line should be in the hands of the tender, but the tender should tie the free end to an appropriate object (e.g tree, boat, etc) so that if it is jerked from his hands it is not lost.
Both diver and tender must know and recognize the standard signals, the communication procedures and what to do if messages are not properly received and understood. It is good practice for the diver and tender to rehearse the signals together before the diver enters the water.

The tender should be prepared to give all his attention to the diver while he is in the water. If he is not paying attention, he may miss some signals. The tender should ensure that the line is kept free of obstructions and that the line does not become tangled. Coiling and uncoiling the line as it comes in and out is good practice. The line should be neither too tight nor too slack. In the first case, the diver's progress will be impeded, while in the second, signals are difficult to pass on.

The tender must be prepared to react to any obvious difficulty or if signals remain unanswered or confused. The best action is to mobilize the stand-by diver, who will go down the lifeline of the diver in trouble, but secured by his own line and tender. Trying to haul a diver to the surface on his lifeline may only compound the situation if the line is caught on an obstruction, it is therefore something of a gamble attempting this method of recovery.

The tender should remember that the safety and comfort of the diver is his responsibility. It is bad practice to change tenders while the diver is down as signals may be lost or the diver be otherwise confused by the change.

From the diver's viewpoint, patience is the keyword. He must be prepared to wait until the line is right and the appropriate signals given before proceeding. Swimming against a taut line will only exhaust the diver. Similarly, if he returns faster than the line is recovered, he may miss a vital signal. If he should become entangled in the line, it is imperative that he remains calm and then quietly sorts it out.
The stand-by diver should be fully kitted-up and ready to enter the water before the first diver actually enters the water himself. The stand-by diver needs to be prepared to act quickly in an emergency situation by going down the line and getting to the distressed diver quickly. A spare cylinder of air clipped to the stand-by diver is a good idea in case it takes time to get a trapped diver clear of an obstruction.
In very low visibility small problems can be magnified. Claustrophobia can develop and the panic threshold can be lowered. It is important that in these situations the diver remains calm and gets himself out of his problem by slow, premeditated actions. A knife is essential, and a torch is a very good idea. If both hands are required for a task, the torch (or torches) can be mounted on a light plastic helmet, which can be worn by the diver.

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