Buoyancy and Trim

  • Buoyancy and Trim

Some diving tips for new Aqua Tutus Dive Club members by Neil Benjamin & Dennis Hocker

  1. GET THE WEIGHT JUST RIGHT. Goal is perfect weighting @ end of dive (nearly empty tank <500psi, empty BC, breathe normally, relax without swimming) just able to float neutrally buoyant ~10ft/3m down. If you checked your weight so you can just sink from the surface at the start of the dive (with a full tank) and add ~5lb (2Kg) more you should be just about right when you re-check your buoyancy again before you surface at the end of the dive, as the air in a full tank weighs about 5lb. Ideally you want to dive with exactly the minimum amount of weight you need, but better to be a pound or so heavy which you can compensate with a little air in the BC rather than too light which will have you searching for a heavy rock to carry around. To do the weight check get fully geared up and empty your BCD/dry suit, then get in water a few feet deep and use the method you were taught (armchair heel pivot/knee bumps/vertical eye level breathing at surface/fin pivots). Remember you may want to take 5lb off to do the check with a full tank. When your weight is just right you should be able to rise in the water column as you breathe in, but descend away from the surface when you exhale fully.
  2. RECORD YOUR LEAD WEIGHT IN YOUR LOGBOOK, along with details of exposure suit, salt vs fresh water etc. so when you go back to the same spot with the same gear you’ll know how much lead to start with. Remember if you change from a steel tank to an aluminum add 5lb and vice versa as different tanks have different buoyancy characteristics (typical steel tanks => -5lB full, 0lb empty vs. typical aluminum tanks => 0lb full, +5lb buoyant empty). If your body weight changes, you have a new wet/dry suit, you go to different salinity or tropical vs. temperate conditions then re-weight.
  3. BE A FISH AND GET THE RIGHT ATTITUDE! Fish (mostly) swim horizontally, neutrally trimmed and streamlined, and so should you. Does it matter where you carry the lead on your body? It sure does, if you put all the lead around your waist on a weight belt it is hard to handle, and similarly if you put it all in the BC dump-able waist pouches. If you put too much weight there it will tend to drag your hips down which you compensate by adding air to your BCD, pulling your torso up, and you become a “fin dragger”. Swimming vertically like that with your fins facing down you will kick all the bottom life and stir up all the sand and silt, making you unpopular with everyone following, and not least pity the poor marine life you are destroying. Distributing the right amount of weight and adjusting it along your body using a combination of tank/shoulder weights/back-plate/back trim pockets/dump-able BCD waist pockets/weight belt/ankle weights helps you trim out horizontal without excessive effort. A rule of thumb is no more than 20lb in any one location, and trim so you can hover in any orientation, and it also makes it safer if you ever have to dump some weight. Remember to tuck your hoses, SPG/gauge console, octopus (must still be able to directly deploy it), lights and other gee-gaws away and you will be streamlined and ready to go be a fish!
  4. USING THE BCD, this is your “coarse buoyancy control”. As you descend the air in your exposure suit and BCD compresses, so to keep your buoyancy constantly neutral you have to progressively add air using your BC power inflator or by oral inflation.  In either case, be cautious, add a little at a time, and give it some time (a breath or two) to work before adding more. Conversely, as you ascend the air in your suit and BC expands, so you must let some out, but again just a little at a time does the trick. Most BCDs have an air dump line to the left shoulder at the corrugated hose elbow, or you can dump through the corrugated hose itself by holding it up (not forward, but up, and roll slightly right so your left shoulder is higher, ‘cos bubbles want to float up, very important if you have a U shaped wing rather than an oval), or when your head is lower than your feet I suggest you try using the rear “butt dump” valve (if one is fitted) on either side towards the bottom of the BC. Always remember the biggest rate of change is near the surface so buoyancy is hardest to control as you ascend in the top 15ft or so, and slower (<30ft/min) ascents are safe ascents, so watch your depth gauge and follow your pea sized bubbles.
  5. BREATHING AND THE BRAIN, “the fine control mechanism”. Besides the BC you have one other way to increase and decrease your air volume, which is by inhaling and exhaling, so you bob up and down a little with every breath. Actually, you can easily change your average buoyancy by as much as 5lb by changing your breathing pattern. The first step to optimal buoyancy control is to stop moving and THINK: am I going up or down, do I want to, do I need to add or dump air? Then stay relaxed and calm and breathe continually taking full proper deep breaths from your diaphragm, sort of underwater yoga. Remember do not hold your breath (especially with full lungs and/or while ascending and even more so near the surface). If you breathe with your lungs more full than empty more of the time (like ∩∩∩∩∩) you will be more buoyant than if you breathe with your lungs more empty than full more of the time (like ᴜ ᴜ ᴜ ᴜ ᴜ). Only when you run out of breathing range because you have gone up or down by more than a few feet will you need to adjust your BC.
  6. PUTTING IT ALL TOGETHER. Think ahead, be aware of your surroundings, and anticipate “what is going to happen next”. Make small corrections, allow a little time (~2 breaths) between each adjustment to see what effect each adjustment has had. While diving, you should keep an eye on just about everything at once. Air supply: SPG, when should we turn the dive, do I have enough gas to get back safely; Navigation: where am I going, depth control and BUOYANCY, compass direction, natural navigation, where’s my buddy/guide; Swimming: am I and my buddy comfortable with this pace, is my kick disturbing the bottom (try frog kicking and sculling rather than flutter kicking all the time) is my TRIM horizontal, periodically stop and check BUOYANCY, turn horizontal (helicopter turn) and looking around for your buddy while still horizontal, by looking behind with your head between your legs (if you lose sight of buddy try looking above you). Neutral buoyancy skills are the key to being a great diver, and you can spend a lifetime perfecting them. The best way to do that is to keep on diving both by yourselves and with the club, and keep on practicing.