Buoyancy skills

Buoyancy skills (Part 1)

My observations and thoughts lead me to the conclusion (possibly erroneous) that personal buoyancy skills are less than perfect in many of our less experienced divers, and I have to include myself in this category. Basic buoyancy skills are taught as part of the novice pool training, and examined in one or more of the tests. But few novices will claim that their buoyancy is good, and although they manage to pass each test, it is frequently with considerable concentration, and can hardly be described as 'unconscious control'. Once into open water, with the complication of drysuits, yards of compressible neoprene, the loss of buoyancy at depth, and the changing cylinder weights, bouyancy becomes somewhat more approximate. Only once divers are expected to perform 'free' decompression stops does the need for precise bouyancy assert itself again.

Fewer to these problems occur in warmer waters, but the American diving community is probably much more environmentally conscious than us Brits, and are much more concerned about such things as coral damage by uncontrolled divers. This has lead to a much greater awareness of buoyancy skills, with many Carribean dive centres offering courses in 'post-novice' buoyancy control.

Interestingly they classify buoyancy control as being typical of particular grades of diver, as follows.

BASIC - heavy reliance on BC/(suit) making gross adjustments by inflation and deflation. Frequently too much lead is used. Skill level may last from first open water dive to 25 dives.

NOVICE - when you realise you can actually control your approximate position at any depth. There is still a heavy reliance on the BC/suit and overweighting is still often a problem.

INTERMEDIATE - Beginning to use your lungs and breathing pattern to fine tune your buoyancy, although you still rely pretty heavily on your BC/suit. You're happy with your buoyancy when you're moving, but when you stop it gets harder. When you're vertical in the water, you find it much harder to stay in the same place, and have to make constant fine adjustments to your BC/suit when doing free decompression stops.

ADVANCED - You rely more on breathing for control. You feel your skills are excellent, and unconsciously forget to fine tune them, but dive in a 'comfort zone'. Getting really close to a reef, using a camera or hanging stationary in blue water (without moving your fins/arms/etc) can still challenge you. You still do quite a bit of finning during free decompression stops.

ULTIMATE - There is continuous feedback from breathing patterns and water/depth conditions. You can 'feel' fine changes in depth as easily as looking at your depth guage. You can hang motionless during free decompression stops without hanging onto the shot line.

How do you rate yourself? Do you feel as if your personal buoyancy skills could be improved with practice? I'll continue next month with an analysis of the components that make up buoyancy control, and how you can put all those together to make your buoyancy better.

There is no doubt that the thickness of suits that we have to wear in the UK to keep warm considerably complicates matters by providing gross changes in buoyancy which we have to compensate for, but it easy to blame our equipment for our own lack of skills. Often it is poorly adjusted or tuned for optimum performance. The result is increased air consumption, a sure giveaway of the diver who does not have good control of his or her buoyancy and must compensate by overinflating and overdumping, or who uses more air by virtue of the sheer physical effort required finning to stay where he wants to be.

Our pool does not necessarily provide the best environment to practice in, as when hanging vertically there is very little 'space' to move up and down before you reach the surface or your fins hit the bottom. For this reason, I am proposing hiring either a deep pool (Crawley or Leatherhead diving pool) early one Sunday for any interested people to come along and have a good long session fine tuning their buoyancy in a decent depth of water, and trying out what works and what doesn't. Those taking part will have to fund it themselves, but an hour's hire should be no more than about 30, which, if split among enough people should be quite reasonable.

I hope also to be able to provide a number of experienced instructors to give advice, and help you try out new techniques. Those of you planning warm water holidays, or who are interested in underwater (especially macro) photography may find such a session extremely valuable. So let me know if you are interested (I'll probably put up a sheet on the noticeboard), and watch this column next month for hints on how to make it all better.

Buoyancy skills (Part 2)

Last month I talked about buoyancy skills, and outlined a number of 'grades' of buoyancy, ranging from basic to ultimate. We continue this month by looking at the sort of things that affect your buoyancy, and how to properly hone your skills.

So what really affects your control of buoyancy? Let's start at the beginning.

Natural Body Composition - In a swimsuit some people sink, and others float, no matter what they do or try. Others, perhaps the lucky ones, float with a full lung of air, and sink when they breath out. They have the capability to fully control their buoyancy from positive through neutral to negative by control of their breathing. How do you rate? This is really the first thing that you want to find out.

Go into the swimming pool in the deep end, take a full breath in and relax with you head tilted back. You can wear a mask and snorkel if you like, it makes little difference. Find out what you are naturally, and if you fall on one side or the other, take a full breath in (to float) or a full breath out (to sink) to see if you can achieve neutral buoyancy. The majority of people should be able to float at about eye level with a full lungful of air. Practice this, and once you are comfortable, try breathing out at the surface, bit by bit, until you start to sink. With practice you should be able to breath out just enough to leave you floating genuinely neutral in mid-water.

You will be surprised how much force you can exert just by moving your hands and feet about, and I promise you that you will never achieve proper neutral buoyancy in the water unless you leave your arms and legs motionless, whatever their desire to move on their own. Try crossing your legs at the ankle, or going into a cross-legged sitting position, and fold your arms over your chest.

Lung volume - despite the BS-AC assertion that the average lung volume is 6 litres, people vary considerably. The average woman has a lung capacity of 4.5 litres, with some more and some less. My own lungs have been scientifically measured (using a radioactive argon dilution technique) to be just over 8.5 litres. The total amount of air that goes in and out on every breath is also very important. Larger volumes tend to make fine tuning buoyancy harder, unless you very carefully keep your breathing perfectly regular, and adjust your tidal volume around your point of neutral buoyancy. This is easier, and more comfortable to do with a more emptied lung than a fuller one.

Breathing pattern - Novice divers breathe more rapidly, more deeply, and less regularly than experienced ones, making it impossible to get to advanced stages of buoyancy control. A normal breathing pattern is essential for the last two stages of mastering buoyancy.

State of mind - Regardless of how you feel on the surface before a dive, being relaxed once you get into the water is very important in achieving advanced and ultimate buoyancy control.

Note that considering all the above points, you can get into a swimming pool, and have a good try at achieving neutral buoyancy. If you really know that you are positively buoyant, and some men and ladies are, then take a weight belt with some one and two kilo weights to fine tune your natural buoyancy until you get it in a range where you can float neutrally underwater by controlling your breathing. Nothing is going to benefit your buoyancy control more than practice, and more practice, in the swimming pool without any other diving kit to confuse the issue.

You will in time get a good feeling about your own buoyancy. You will be able to float at the surface, and be able to quickly breath out exactly the right amount of air to make you neutrally buoyant. You will be able to float about in the pool, and 'feel', even when your eyes are closed, whether you are going up or down in the water. Once you have reached this point, and it may take a few sessions, you can progress to fine tuning your buoyancy with your diving kit on.

As a sort of postscript, there is co-incidentally an article on buoyancy in this month's DDRC newsletter. They say that they get students to do a one finger handstand on the bottom of the pool. Too much buoyancy and you will float away, too little, and your finger is going to hurt!

Sorting out the kit - Once you are happy in a swimsuit, check whereabouts in the water you float with a full lung of air. Many people float with the water line at about eye level (you will need to be wearing your mask and a snorkel to properly check this). Try it for yourself, and remember where you float.

The trick is to put on all your basic diving kit, including suits, knives, fins etc, but leaving off the stab jacket and aqualung itself, and adjust your weights until, when you hang motionless with a full lungful of air, you float at the same level as you did when you had no equipment on. You should now be very close to neutral in your diving kit, and should, by exhaling the right amount of air, be able to float about in mid-water like you were doing before. Remember the amount of weight, and write it down as "Kit only weighting". Practice now in the pool with this equipment on, as you did before in a swimsuit.

Once you are comfortable like this, then put on your stab and aqualung, first seeing whether it sinks or floats when there is no air in the stab. If it sinks, then you will have to start taking some weights off to compensate, and if it floats, then you will have to put some on. Practice again in the pool until you can again float motionless at the right level with a full lungful of air, and can breath out a little to become properly neutral.

Cylinders contain a lot of air, and this may weigh between 2 and 4 kilos. Unfortunately as your air goes away, so does this weight, so it is best to practice with about 50bar of air in you tank to simulate the lightest position at the end of the dive. Remember also to always keep an upright and vertical position in the water when floating motionless. 'Flaring' your body, or floating horizontally in the water is going to exert considerable resistance to upward and downward motion - you may find it easier to maintain a depth in the water this way, but you are not properly developing your buoyancy skills. Remember the amount of weight your finish with, and write it down as "Full kit weighting".

If you are a small bodied person, or have an exceptionally large cylinder, the you might find that the weight of the cylinder rotates you backward, as it tries to find a balance for your centre of gravity. I find that wearing my weights as far around the sides and front as I comfortably can, and this usually sorts the problem out. You might like to try this if you too are being pulled backwards by your cylinder.

Adjust for the sea - Messing around in waves, adjusting your buoyancy is not to be recommended. Luckily once you have established your buoyancy properly in the swimming pool, it will be correct for fresh water diving, such as Stoney Cove and we can adjust it scientifically for the sea. As we all know, the sea is saltier, and is slightly denser, and exerts a greater upforce on us (Archimedes principle) than does fresh water. All we need to do is calculate what that greater force is, and compensate accordingly.

The clue to this is that we know (again by Archimedes principle) that when we have achieved neutral buoyancy in the swimming pool, then the amount of water that we have displaced exactly matches our weight. We have adjusted the whole of our diving 'unit' including body, suit, cylinder etc. to match the density of water. Easy eh.

To find out how much water we have displace, all we have to do is weigh ourselves, in full kit. The number of kilograms will equal the number of litres of water that we displace. Let's say for example that it is 110 litres.

Now seawater varies in its salinity, from 35% in mid-ocean, to 40% in high evaporation areas like the Red Sea, to lower levels in places where it is mixed with fresh-water river run-offs. We will work on the average figure of 35%. At this salinity, the weight of a litre of seawater is 1.026 kilograms compared to 1.000 kilograms for fresh water. Each litre weighs 26 grams more, and for each litre that we displaced in the pool, we will have to add 26 grams of weight when we go into the sea.

So calculating the extra weight is easy now. If we displaced 110 litres of water, then we need to add (110 x 26) grams of weight when we go into the sea. That works out to 2860 grams, or nearly 3 kilos of extra lead. Easy isn't it? You will find it much more comfortable to dive correctly weighted, you will use less air, and have to work less on your dives. Get it right first time, and do it scientifically, rather than in a hit or miss manner. And don't forget to re-trim your buoyancy properly as you get deeper. On a final note on sea weighting, remember the loss of buoyancy with depth.

I dive in a membrane drysuit, and admit that I dive with more weight than I need, as it allows me to put a larger amount of air into my suit which makes it more comfortable. But I do this knowingly, and know how to achieve proper neutral buoyancy in the sea. The DDRC article recommends that divers trim their buoyancy so that they are neutral at 3 metres depth rather than at the surface, and consequently positively buoyant at the surface. Start practicing those surface dive techniques!

Pool practice - Our pool does not necessarily provide the best environment to practice more advanced skills in, as when hanging vertically there is very little 'space' to move up and down before you reach the surface or your fins hit the bottom. It is however a good place to start to develop your buoyancy skills, and master the basics to fine weighting and breathing control. For this reason, I am proposing hiring either a deep pool (Crawley or Leatherhead diving pool) early one Sunday for any interested people to come along and have a good long session fine tuning their buoyancy in a decent depth of water, and trying out what works and what doesn't. Those taking part will have to fund it themselves, but an hour's hire should be no more than about 30, which, if split among enough people should be quite reasonable.

I hope also to be able to provide a number of experienced instructors to give advice, and help you try out new techniques. Those of you planning warm water holidays, or who are interested in underwater (especially macro) photography may find such a session extremely valuable. So let me know if you are interested (I'll probably put up a sheet on the noticeboard). I am particularly pleased at the number of people who have already shown an interest in this, since I first mentioned it last month.

Acknowledgements due to articles in Skin Diver magazine in March/April 1992.

This page was last updated on : 06 Sep 2011