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Use Less Air
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Don't Bully Mother Nature
You
think of whales as gentle creatures, and they
are in an unexpected way. They've figured out in
their millions of years of shouldering aside the
ocean that it pays to be patient. They don't try
to rush through the ocean. Though a baleen whale
can swim faster than you can run, most of the
time it cruises slower than you would walk.
Water is not only much heavier than air, it is
not compressible. You can afford to bully the
air, because when you force your body through
it, the individual molecules in your path will
squeeze closer together without much complaint.
But water molecules are already as close
together as they can get. Whenever a whale or a
human swims through the water, it has to move
its own volume in water out of the way. That's
hard work.
If you are neutrally buoyant, you displace your
weight in water--including tank and gear, say
200 pounds. A gallon of water weighs about eight
pounds, so that's 25 gallons of water you have
to push aside.
Put another way, that's five of those jerry cans
you see mounted on the backs of four-wheel
drives. And even when you swim at a slow 1 mph,
you have to push aside all of those 40-pound
jerry cans every four seconds. Walking the same
speed on land, you're pushing aside only a few
ounces of air.
Doing that kind of work requires a lot of
energy, and that uses up a lot of air, because
your body makes energy by burning glucose.
Glucose plus oxygen yields energy for muscles
plus carbon dioxide. So working hard hurts your
air supply two ways, by using oxygen faster and
by producing more carbon dioxide, which promotes
wasteful breathing.
Whales produce energy the same way and have
learned to shove aside the water slowly, by
moving and swimming slowly, because going slow
saves energy and air. Actually, the problem is
not speed, it's acceleration. Moving your body
forward means pushing aside the water that's in
your path. As you swim forward, you accelerate
every molecule of water in your path from zero
to whatever speed it needs to get out of your
way. And as every motorhead knows, it's those
zero-to-60 times that suck up the horsepower.
For example, Consumer Reports says a 3,200-pound
Chevy Malibu gets to 60 in 8.1 seconds with 144
horsepower. The same weight with a Ferrari
nameplate gets there twice as fast, but needs
almost four times as much horsepower to do it.
The Ferrari is also sucking down fuel and air at
a phenomenal rate, and you will too if you make
those water molecules jump like Ferraris to get
out of your way.
If going faster gets you to your goal sooner,
you might think you'd come out with about the
same overall air consumption per mile. Afraid
not. That's the "drive faster when you're almost
out of gas" theory, and it doesn't work under
water either. Swimming or driving faster
consumes not just more air and energy but a lot
more. In theory, the energy and air cost is
proportional to the square of the speed. That
means going twice as fast costs four times as
much energy and air; going three times as fast
takes nine times as much.
At least that's what the physics books say. To
test the theory, I tried another experiment. I
gave four test divers underwater speedometers,
digital tank pressure gauges and stopwatches and
had them measure their air consumption at speeds
from 0.5 to 2 mph. Admittedly, this is another
shade-tree experiment, but the numbers did show
that when they doubled their speed, all four
divers more than doubled their air consumption.
They were wasting air by going faster.
What's your most efficient speed in water?
Everybody moving through water has a unique
speed limit beyond which the energy cost of
going faster skyrockets. It's a function that is
generally proportional to the square root of the
length. So a 100-foot whale (square root = 10)
can go about four times as fast as a six-foot
diver (square root = 2.45) with the same general
energy efficiency. Whales have apparently found
their most efficient speed by experience. Though
baleen whales can sprint faster than 20 mph,
they cruise long distances at only 2 to 4 mph,
according to Monterey Bay Aquarium Senior Marine
Biologist Dr. Steve Webster. Your most efficient
speed would be about one-fourth as fast, no more
than 1 mph. That's only 1.5 feet per second,
like a very slow walk.
Whales also gain by showing Mother Nature a neat
appearance. Next to their streamlined shape, we
divers are as sloppy as, well, that improvident
teenager again. Whales, porpoises and fish have
smooth skins and no instrument consoles dragging
in the water. A snorkel dangling from a mask
strap, a hose that sticks out, a couple of
unnecessary gadgets snapped to your BC, all stir
the water as you swim through it. That wastes
energy and air. Likewise, keep your arms at your
sides or behind your back--inside your
slipstream.
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