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Chapter 6 — 2
The "Half Tank Rule".
The best technique is to plan to swim into the current for the first half
of the dive and use it to drift back to the boat during the second half. The dive is divided into
halves on the basis of the air supply. After subtracting the air pressure needed to drive the
regulator and as a reserve, allow half the remaining gas for the swim into the current and return
using the second half of the supply. In tidal areas, it is necessary to anticipate any change in
direction as the tide turns, or both halves of the dive may be into current. The best time to dive
in tidal areas, for both ease and visibility, is usually during slack water - between tides.
As an example, if there is 200 ATA (or Bar) in the scuba cylinder, 40 ATA would be needed as
a driving pressure for the regulator. That leaves 160 ATA for the dive. Eighty ATA could be
used to swim into the current, and that leaves 80 ATA to drift back to the boat, allow for
navigational errors and perform a safety stop. If one had allocated gas for emergencies, this
would reduce the dive times accordingly.
The
anchor line
can be used to advantage. It is much easier to make headway against a current
by pulling along a rope or chain, than by swimming. If a rope is attached loosely to the anchor
line at the surface and run around the side of the boat, divers can enter the water holding onto
this and use it to pull themselves to the anchor line. By using the anchor line, divers can pull
themselves down to within 2 metres of the bottom, where the current is often less strong. Avoid
swimming onto or dislodging the anchor, which can cast the boat adrift or lift and injure the
diver. Swimming around the anchor allows the diver to check the anchor’s security before
continuing upstream.
Another rope (a
floating
or
Jesus line
) should drift with the current from the back of the boat,
for 50 metres or more. This should be supported at regular intervals by buoys or floating plastic
containers. This line has earned its name by "saving the sinners" who have missed the boat or
surfaced down-current.
Another technique used in locations with strong currents is
drift diving
. Because of the fast
currents, all equipment should be firmly attached and snagging on environmental hazards and
other divers must be avoided. A
float
is towed to mark the diver's position and allow for signals
to be sent to the surface craft. A rescue or pickup boat must drift with the divers and the current
to another location, where the divers are hopefully recovered. Any such boat should have a
propeller guard
if it is to be used to rescue the divers.
Divers being left and lost at sea is unfortunately not a rare event, and it is often difficult for
boatmen to find their divers if the sea surface is choppy (bubbles not detectable) and waves or
swells block the diver from the low gunwaled boat. A divers float or a
"safety sausage"
(a long
fluorescent inflatable plastic float) is a useful backup for a lost diver after an ocean or drift dive,
and can be seen for a kilometre or so. A whistle can be used to attract attention, but is difficult to
hear over engine and ocean noises. Other means of attracting attention of boatmen are low
pressure horns (with >100dB sounds, for > 1km.), signal mirrors (if the sun is shining, for many
km.), and waterproof
smoke flares
(lasting a minute or so, and visible for up to 10 km.). EPIRB
and other electronic signalling devices may send emergency signals to commercial transports,
including aircraft, over many km. See Chapter 5 for information on equipment.
Surge
In shallow water affected by waves, a
to-and-fro surge
which is too strong to swim against,
may be encountered. This is best dealt with by gripping the bottom (with gloves) during the
adverse surge, and moving with the favourable one. A diver contending with a powerful surge
can become disoriented from the violent movement, injured from impact with rocks and can
succumb to panic.