Features needed
US Navy Explosive Ordnance Disposal dive equipment
A frogman\’s breathing set should:[opinion][citation needed]
Be as silent as possible in use.
Have a full face diving mask:
To let frogmen communicate underwater.
Be less easily knocked off underwater.
Be much less easily lost if the frogman becomes unconscious underwater.
Be securely fastened, see here about full-face diving masks. It should have as little as possible (e.g., an excessively bulky or projecting set/air valve) that can catch on things or that an attacker could easily grasp.
Be a dull color to avoid being seen from out of the water. Many are black, but the Russian IDA71\’s backpack box is mostly dark green. No large bright-colored badges or manufacturer\’s logos.
Contain as little iron or steel as possible, to avoid detection by magnetic sensors. This is also useful when the frogmen have to remove or defuse mines underwater.
Be as light and agile as possible, as far as is compatible with an adequate dive duration:
Be well streamlined, and as small and light as possible for the dive duration. With a combat diver this may mean removing safety features such as an open-circuit bailout that would add bulk. Long trailing hoses (e.g., regulator hoses) are easily fouled and or pulled at and add to drag. If an underwater fight, or a quick need to escape, develops, agility and lack of cumbersomeness could be vital. This applies to:
Streamlining in straight swimming, as he may have to swim fast and far.
Streamlining when he rolls over and twists about.
The diver\’s inertia when he must roll over quickly.
The risk of snagging on things in dark water, or being taken hold of by. The Russian IDA71 military and naval rebreather is a good example here.
Have a long dive duration.
The front of the frogman\’s abdomen should be clear so he can easily climb in and out of small boats or over obstacles, particularly out of the water.
Have its breathing bag toughened against stabbing and scratches, or safely inside a hard backpack box.
All controls should be where the frogman can easily reach them, and not projecting. Turning the usual type of sport diving scuba\’s air off or on is easy for an attacker from above but difficult or impossible for the diver himself (and has been known to happen by itself when a diver pushes through thick kelp), unless the cylinder or cylinders are mounted inverted. However, that needs more pipework, and it is easy to bump the valvework on things, including when taking the set off.
Have its working parts and breathing tube or tubes should be safe from snagging on things in dark water, and from attack in an underwater fight, including in the risk of being jumped from above.
Long trailing breathing tubes or regulator hoses may snag on things in dark water and can easily be grasped and pulled.
Older Siebe Gorman-type rebreathers (see Siebe Gorman CDBA) had one breathing tube, which was in front of the chest and easier for the
frogman to keep track of.
Not open-circuit scuba
As a result, the frogman\’s breathing set should be[opinion][citation needed] fully closed circuit rebreather, preferably not[opinion][citation needed] semi-closed circuit and certainly not[opinion][citation needed] open-circuit scuba, because:
Open-circuit scuba makes large amounts of bubbles, showing where the diver is.
Open-circuit scuba makes noise (on exhalation, and regulator valve intake hiss as the diver breathes in) showing underwater listening devices where the diver is.
There have been experiments with making released air or gas come out through a diffuser, to break the bubbles up; this may sometimes work with the small amounts of gas that are sometimes released by rebreathers, but open-circuit scuba releases so much gas at every breath that a diffuser large enough to handle it without making breathing difficult would be too bulky and would interfere with streamlining.[2] Holding the breath to avoid making noise at critical moments is not recommended and very risky: see diving hazards and precautions
The bulk of an open-circuit set makes the diver heavy and cumbersome in rolling over and changing course or speed.
The dive duration of open circuit sets is much shorter than the dive duration of naval rebreathers, in proportion to bulk. However, some technical diving rebreathers are very burdened with safety devices such as inflatable flotation and open-circuit bailout. (Some modern rebreathers, such as the Draeger, are lighter.) The rebreathers which are the most compact in proportion to dive duration are oxygen rebreathers, but these are depth limited to about 8 metres (26 ft) because of the oxygen toxicity risk.
The common sport open-circuit scuba set is not recommended for a fight against a trained naval or combat diver, because in any sort of underwater combat, a man with a large aqualung has a high rotation-inertia and is very unstreamlined in the twisting and turning involved in fighting and straight swimming, and his maneuvering is slowed critically compared to a man with a light streamlined rebreather with all parts close to his body.
Combat frogmen sometimes use open-circuit scuba sets during training and for operations where being detected or long distance swimming are not significant concerns.
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