One of the most common problems for scuba divers is overweighting, and many divers believe that excess weight keeps them safer by preventing them from accidentally floating to the surface. Mark ‘Crowley’ Russell tells us why the exact opposite is true, and why excess weight is dangerous for divers.
One of the most chronic problems I faced during my years as a dive professional was overweighted divers, with issues ranging from moderately annoying to seriously dangerous.
These problems arise from a fairly simple misunderstanding of why divers need to wear weights, and they arise from both divers and – sometimes – the dive professionals who train them.
I have seen seriously overweighted divers from all the major training organisations. It is not limited to a particular set of training standards, it’s a global phenomenon.
As we know from our basic training, divers need to add extra weights to their equipment (most commonly in the form of small lead blocks) to compensate for the increased buoyancy of the exposure suit and buoyancy control device that we are wearing.
How much lead we need to add to the system depends on the type and thickness of the suit, the construction of the BCD, the tank we are using, and our own human physiology.
The word ‘system’ is a good term to start with when considering weights. Underwater, we need to think of ourselves as one, holistic, integrated system, rather than individual components. Each has an effect on the other, and even minor variations can have a dramatic impact on the diver.
We also need to understand that this system is not rigid. It varies over time as equipment ages and bodies change. It can be noticeable between the start and end of a dive vacation: two weeks and 25 dives is enough to permanently compress a new exposure suit and shed a bit of body fat, both of which will impact the amount of lead you need to wear.
As we descend underwater, the pressure causes the equipment we are wearing to lose its positive buoyancy. The tiny bubbles contained within the material of our exposure suits will compress, for example, and the weights we are wearing now override the buoyant support of the equipment.
To compensate, we add small amounts of air to our BCD (jacket, wing, hybrid or sidemount backpack, they are all buoyancy compensators) to balance out the system – to make it ‘neutrally buoyant’. If we dive deeper, the air we put in our BCD also compresses and provides less support, so we need to add more air to the jacket. If we ascend, we need to let some of the expanding air out again, or we become too positively buoyant and find it harder to stay underwater.
This is where overweighting becomes dangerous. If a diver is wearing too much weight, they will have to add an excessive amount of air to their jacket. If they should ascend more than a couple of metres, the expanding air very rapidly overcomes the negative effect of the lead.
The problem escalates very quickly. Divers often attempt to swim themselves back down while depressing the deflator button on the low-pressure hose. This has no effect because they are pointing down, and air only travels upwards. The expanding air accumulates towards the base of the jacket – now pointing to the surface – and hence we have the classic feet-first, uncontrolled buoyant ascent.
If this occurs after a lengthy deep dive, the risk of decompression sickness (DCS, the bends) or lung overexpansion injury increases dramatically. If a boat should happen to pass overhead – the results could be even worse.
It may seem contradictory, but the simple fact of the matter is that the number one cause of floating is too much weight.
The problems are often compounded by the fact that excess weight tends to drag the lower half of the body down, which means that for every fin kick, the diver propels themselves upwards a little, and become too positively buoyant as a result. Overweighted divers also tend to kick more often and more powerfully – often continuously – to maintain their depth.Many divers forget – or have been improperly trained to recognise – that the deflator mechanism does not work unless you’re in a reasonably upright position. Many divers do not know the location of, nor how to use, the rear dump valve.
I’ve heard too many divers say that wearing excess weight is somehow safer. It never is. At best, it causes the diver to work harder during the dive, constantly bobbing up and down, inflating and deflating the jacket as a result, swimming harder to get anywhere, all of which leads to a lot of discomfort, rapid breathing and faster air consumption. It just makes diving a lot less fun. At worst, as mentioned above, it’s outright dangerous.
The most common excuse used by both divers and dive professionals alike is that the extra weight is a safety measure to stop people pinging to the surface. The usual scenario is that a diver is having problems descending, and hence more weights are put into their BCD pockets to help them sink.
The most common reasons that divers don’t sink, however, is because they are constantly moving their fins, waving their arms, and – very importantly – breathing too rapidly and too deeply. The human lung can, on average, hold up to 6 litres of air, more than enough to keep a diver at the surface who – when correctly weighted and with a full tank – should be around 2kg negatively buoyant. Every waggle of the fin prevents them from descending; every attempt to push themselves down using their arms often brings them straight back to the surface again as they lower their arms for the next attempt.
Instead of taking a few moments to get the diver to relax, control their breathing, and explain what they’re doing wrong – or even hang on to the diver and let the laws of physics work as two divers descend together – the immediate reaction is to add more lead. The diver starts to sink rapidly, and consequently adds excess air to the BCD, in which case we go back to the beginning of this article and start again.
Proper buoyancy checks are essential, of course, but divers should also learn to be more aware of problems associated with overweighting and not be shy of discussing them with their instructor or buddies. Always having to inflate and deflate for even small changes in depth, finding yourself unable to maintain a horizontal position in the water and having a very full BCD at any given depth are all classic signs.
It is often an imperfect science, and sometimes it’s worth having a more experienced diver (who can deal with the extra weight) carry a couple of extra kilos just in case – but for any diver who is having issues with buoyancy control, it’s far better to get these fixed as early as possible before entering water where excess weight may lead to serious complications.