Snorkel
From Wikipedia, the free encyclopedia
A snorkel (also spelled "schnorkel" or "schnorchel") is a tube that allows a person, vehicle, or vessel to draw air while submerged under water. A snorkel can also refer to a special type of fire apparatus.
The term "snorkel" can also be used to refer to an apparatus that acts as a sort of fume hood in a chemistry lab. It is essentially an extentable tube that folds down from the ceiling in order to provide collection of fumes from a lab bench.
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[edit] Swimmer's snorkel
A swimmer's snorkel is a tube about thirty centimetres (twelve inches) long, usually J-shaped, fitted with a reasonably comfortable mouthpiece, and constructed of rubber or plastic. It is used for breathing air from above the water surface when the mouth and nose are submerged, either when during snorkeling or during a surface swim before or after scuba diving. The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask, as sticking the snorkel in between the strap and the mask could cause the mask to leak, or risk losing the snorkel should the diver choose to switch to scuba.
The most common type of snorkel is a simple tube that is allowed to flood when underwater. The snorkeller expels water from the snorkel either with a sharp exhalation on return to the surface or by tilting the head backwards once the head is above water.
Some modern snorkels have a sump in the mouthpiece to allow a small volume of water to remain in the snorkel without being inhaled when the diver breathes. Some have a one-way output valve in the sump, which automatically drains the sump as it fills with water. Some snorkels have float-operated valves attached to the surface end of the tube to keep water out when the snorkeller submerges. Snorkels used to be sold that had ping pong balls,(in a cage), at the end of the tube. They are no longer sold or used, as they are considered hazardous to the snorkeler, as is the obsolete snorkel built into the diving mask.
The maximum usable length of the snorkel tube is around fifty centimetres (twenty inches). A longer tube would place the lungs in deeper water where the surrounding water pressure is higher and the lungs would be unable to inflate when the diver inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure. Snorkels also create what is called "dead air space." When the user takes in a fresh breath, some of the previously exhaled air remains in the snorkel and is recycled into the lungs, reducing breathing efficiency. The greater the volume in the device, the more this problem is magnified.
[edit] Submarine snorkel
A submarine snorkel is also a device that allows a submarine to operate submerged while still taking in air from above the surface. It was invented by the Dutch just before World War II and copied by the Germans during the war for use by U-boats. Its common military name is snort.
Until the advent of nuclear power, submarines were designed to operate on the surface most of the time and submerge only for evasion or for rare daylight attacks. In 1940, at night, a U-boat was safer on the surface than submerged because ASDIC could detect boats underwater but was almost useless against a surface vessel. However, with the continued improvement in methods of detection and attack, as the war progressed, the U-boat was forced to spend more and more time underwater running on electric motors that gave speeds of only a few knots and with very limited endurance.
The 1940 defeat of the Netherlands by the Wehrmacht was a stroke of luck for the Kriegsmarine. The Dutch had been working on a device that they had named the snuiver (lit: "sniffer"). The Dutch navy had been experimenting as early as 1938 with a simple pipe system on the submarines O-19 and O-20 that enabled them to travel at periscope depth operating on its diesels with almost unlimited underwater range while charging the propulsion batteries.
The Kriegsmarine, at first, gave some consideration to the snorkel as a means to take fresh air into the boats but saw no need to run the diesel engines underwater. In 1943, however, as more U-boats were lost, it was retrofitted to the VIIC and IXC classes and designed into the new XXI and XXIII types.
The first boat to be fitted with a snorkel was U-58 which experimented with the equipment in the Baltic during the summer of 1943. Boats began to use it operationally in early 1944 and by June 1944 about half of the boats stationed in the French bases had snorkels fitted.
On Type VII boats the snorkel folded forward and was stored in a recess on the port side of the hull while on the IX Types the recess was on the starboard side. The XXI and XXIII types both had telescopic masts that rose vertically through the conning tower close to the periscope.
Snorkels created several problems for their users. A U-boat with a snorkel raised was limited to six knots to avoid breaking the tube and its sound detection gear was deafened by the roaring of the air being sucked down the tube. A submarine that stayed underwater for more than a few hours encountered various disposal problems and had to store garbage internally, further fouling boats already infamous for their odors.
Most dramatically, snorkels were equipped with automatic valves to prevent seawater from being sucked into the diesels, but when these valves slammed shut the engines would draw air from the boat itself before shutting down, which was extremely painful to the ears of the crew and sometimes even ruptured eardrums. This last problem still exists in modern submarines, though their larger internal air volume mitigates the pain somewhat.
Modern snorkel induction masts use a fail-safe design consisting of compressed air, controlled by a simple electrical circuit, to hold the "head valve" open against the pull of a powerful spring. Seawater washing over the mast shorts out exposed electrodes on top, breaking the control circuit. This vents the compressed air and allows the head valve to slam shut. When the electrodes are again clear of the water, the circuit is reenergized and the valve reopens.
[edit] Vehicle snorkel
Military wheeled vehicles, like Jeeps, are capable of mounting snorkels for the air intake and engine exhaust, to allow them to wade through relatively deep water, limited by the height of the driver's head. In the case of a Jeep, all of the engine openings and wiring are sealed, and the driver must first operate a damper that over-pressures the engine vacuum, to prevent water from entering. After fording, the vehicle's wheel bearings must be repacked by a mechanic.
Such snorkelling equipment is available as an aftermarket accessory for some four wheel drive vehicles. The snorkel is typically routed out through one of the front wings and up beside the "A" pillar to the level of the roofline where it is terminated with either a mushroom intake or a forward-facing intake.
[edit] Deep-wading tanks
Deep-wading equipment for armoured fighting vehicles was developed in the Second World War, to allow them to come ashore and support infantry during an amphibious landing.
During the planning of the proposed invasion of Britain in 1940 (Operation Sealion), the Germans developed the Tauchpanzers, modified Panzer III and IV tanks, to be dropped from a landing craft around 1,500 metres (1 mi) offshore. A rubber hose supplied the engine and crew with air and allowed the waterproofed tanks to drive on the seabed up to fifteen metres (50 ft) deep, making it an extreme example of a wading tank. Some were used by the 18th Panzer Regiment during River Bug crossing in Operation Barbarossa.
The German Tiger I tank, too heavy to be supported by many bridges, was designed to ford four-metre deep water. This required unusual mechanisms for ventilation and cooling. Submersion required about thirty minutes of preparation. The turret and gun had to be locked in the forward position so they could be sealed.
The Allies fixed extended trunking to their tanks to allow them to come ashore in a few metres of water—no more than the height of the tank. British Churchill tanks were so equipped for the Dieppe Raid, and U.S. Forces used similar equipment for tanks, tank destroyers, and wheeled vehicles. The swimming Duplex Drive tanks were also used in the D-Day landings.
Many modern tanks and other heavy armoured fighting vehicles have snorkel equipment for wading across rivers up to about five metres deep (many lighter vehicles can swim). Deep-wading operations are very dangerous, and not normally attempted unless there is no alternative.
Western tanks' wading snorkels usually include a wide tube attached to the commander's hatch on the turret. In case the tank stalls under water, the crew can escape through a ladder inside the snorkel. The snorkelling equipment is bulky, and only issued to tank crews when needed. Soviet/CIS forces use such equipment only as a training snorkel. In the field, each tank is normally equipped with two narrow snorkels, and the tank crew is issued with rebreathing respirators, in case of emergency.
Wading operations are only attempted where a shallow river bank approach and bottom have been reconnoitred and prepared in advance by engineers. Tanks under water are somewhat buoyant, and tend to turn wildly. Each tank's snorkel has a flag affixed, and the tank company commander remains on the river bank to give navigation orders to each crew as they traverse the river. In the case of the Soviet T-72, a stalled engine means loss of over-pressure, and a quickly flooding engine compartment (and subsequently the crew compartment). Underwater escape is extremely hazardous, and crews have been known to panic and drown during training.
[edit] Sources
Snorkelling for All, BSAC, ISBN 0-09-188304-0
