Fix the Small Leaks
Even a tiny stream of bubbles from an O-ring or an inflator swivel adds up over 40 minutes, and may be a sign of more serious trouble ahead anyway. Ask your buddy to look behind you to be sure. A mask that doesn't seal is another kind of leak in that you have to constantly blow air into it to clear out the water. It's also a source of stress, which needlessly elevates your breathing rate and thereby reduces your breathing efficiency.
Inexperienced divers are famous for burning through their air supply at a furious rate. The reason is anxiety. A new diver is understandably nervous, and his body's automatic response to danger is to raise his metabolism, his heart rate and his breathing rate. It's hard-wired, the body revving its engine to be ready for fight or flight, though the result is a lot of air cycled through his lungs but never used, just dumped into the ocean.
Take a Class
Any class, almost, will reduce your gas consumption just by making you feel more accomplished and therefore more comfortable. But the best bet is probably a class to improve your weighting and buoyancy control. When you get that dialed in, you can control your altitude mostly with your lungs, so you're not squirting that valuable gas into your BC and then venting it to the ocean. Most important, you can now forget (nearly) about the mechanics of diving, drift like a fish, and relax.
The energy cost of speed is even more than you might think because it's an exponential function proportional to the square of the speed. So swimming twice as fast requires four times as much energy and air. But the reverse is true, too: Swim half as fast as you do now, and you'll use only one-fourth as much air.
Minimize the Lead
If you're over weighted, you have to put more air into your BC to float it and be neutral. The inflated BC is larger and requires more energy and oxygen to push it through the water.
An extra eight pounds of lead means your BC is one gallon bigger when inflated enough to make you neutral. Imagine the extra effort of having to push a gallon-sized water jug through the water.
Adjust Your Trim
If your body is horizontal in the water, when you swim forward, your legs and fins will pass through the "hole" in the water made by your head and shoulders. You'll disturb less water and expend less energy and air. Adjust your trim by moving some lead from your hips to your back--to trim pockets on your BC or to your tank.
Seek Neutral Buoyancy
Always being exactly neutral is the key. If you're not, if you're slightly heavy or light, you're constantly using fin power (and air) to maintain a constant depth. If you're not neutral, you can't glide between fin strokes and you can't hang effortlessly.
Streamline Your Gear
All fast-swimming fish have smooth skins with few or no protuberances. That minimizes drag so they can swim with the least energy and oxygen consumption. Divers, by contrast, have rough, convoluted surfaces with all sorts of attachments from scuba tanks to whistles. Anything disturbing the flow of water past your body creates drag and wastes air.
Streamline Your Movements
Keep your arms close to your body. Straighten your legs and keep them as close together as your fins will allow. Kick with short strokes so your fins stay within the slipstream of your body. Some fins do require a wider stroke so you have to compromise between efficient propulsion and streamlining. But usually you're better off finning faster instead of wider.
Get in Shape
Diving can be surprisingly strenuous because water is so much denser than air. Swimming into a current, it's not difficult to elevate your breathing to the very wasteful rate of huffing and puffing. But even much lower levels of exertion will cause your breathing rate to rise. How much it rises and how soon depends mostly on your aerobic conditioning. A diver in better condition will have less increase when the workload goes up, so he will use less air. The other part of getting in shape is to lose fat and achieve a more streamlined shape.
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