Question: What is the most common type of snorkel like?
About the article "Snorkeling", section "Flooding and clearing".

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A snorkel is a breathing tube used for swimming underwater, called Snorkeling.
The term snorkel may also refer to:  Submarine snorkel  Vehicle snorkel, for a motor vehicle  Hydraulic platform, a type of fire apparatus  Sergeant Snorkel, a character in "Beetle Bailey" comics  "The Snorkel", a 1958 British suspense film, directed by Guy Green   Snorkel (pen), design for easing filling of fountain pens  Snorkel International, a company manufacturing aerial working platforms in the United States and Australia  
Ex- US Navy "Balao" class submarine Fleet Snorkel type: 
The original snorkel parka (USAF N-3B parka, which is 3/4 length and has a full, attached hood; the similar N-2B parka is waist-length and has an attached split hood) was developed in the USA during the early 1950s for military use, mainly for flight crews stationed in extremely cold areas, designed as it was for temperatures down to −60 °F (~−50 °C).
Originally made with a sage green DuPont flight silk nylon outer and lining it was padded with a wool blanket type material until the mid-1970s when the padding was changed to polyester wadding making the jacket both lighter and warmer.
The outer shell material also was changed to a sage green cotton-nylon blend, with respective percentages 80–20, 65–35, and 50–50 being used at various times.
It gained the common name of "Snorkel Parka" because the hood can be zipped right up leaving only a small tunnel (or snorkel) for the wearer to look out of.
This is particularly effective in very cold, windy weather although it has the added liabilities of seriously limiting the field of vision and hearing.
Earlier (Vietnam-era) hoods had genuine fur ruffs on the hoods; later versions used synthetic furs.
Original manufacturers of this parka for the government included Skyline, Southern Athletic, Lancer, Greenbrier, Workroom For Designers, Alpha, and Avirex.
Older nylon-shell parkas have a tendency to exhibit a change in color from the original sage green to a shade of magenta due to long-term cumulative exposure to ultraviolet light from the sun.
To some in the military, this is personally desirable, as it lends to its wearer an aura of seasoned experience (referred to as salty by those in the US Navy and US Marine Corps).
However, considered in a tactical environment, this is a liability, as it decreases one's ability to be camouflaged on the ground.
The basic N-3B parka design was copied and sold to the civilian market by many manufacturers with varying degrees of quality and faithfulness to the original government specifications.
Surplus military parkas are often available for relatively low prices online and in surplus stores; they compare quite favorably with civilian extreme-cold parkas of all types due to their robust construction, designed for combat conditions, and warmth, at (usually) significantly lower prices.
However, one would have to be satisfied with the single color choice of sage green.
In the late 1980s the snorkel parka became unpopular and was associated with geeks and nerds, helping to create the UK term anorak for such people (because trainspotters would supposedly wear anoraks whilst collecting numbers on cold railway platforms).
As such it became highly unfashionable and for a time wearers became the subject of ridicule.
In Europe the snorkel parka started to regain popularity in the late 1990s and early 2000s, being worn by the likes of Liam Gallagher, Kenny McCormick (South Park), and David Beckham.
Around 2004, the traditional association with "Anoraks" had faded and the Snorkel Parka became a main-stream fashion jacket once more becoming particularly popular in the indie scene and with now middle-aged people recapturing memories of their school snorkel parkas.
It is also once again popular as a school jacket, though at nothing like the same level of popularity as it achieved in the 1970s and 1980s.
Most modern parkas more closely resemble the original 1950s design and have lost the orange quilted lining of the 70's and 80's school parkas, however the old school style are now considered highly desirable selling for high prices in vintage clothing shops.
Snorkel jackets remain very popular within Hip Hop culture being worn by both men and women and also have an association with Indie pop. 
This genus of molluscs possessed shells which were shaped like curved caps, with an exhaust pipe shaped "snorkel" emanating underneath where the apex (point of the shell) curves over.
All experts agree that the "snorkel" was used in breathing, with most experts suspecting that it allowed waste water to flow away from the gills. 
The most common type of snorkel is a simple tube that is allowed to flood when underwater.
The snorkeler expels water from the snorkel either with a sharp exhalation on return to the surface ("blast clearing") or by tilting the head back shortly before reaching the surface and exhaling until reaching or breaking the surface ("displacement method") and facing forward again before inhaling the next breath.
The displacement method expels water by displacing its presence in the snorkel with air; it is a technique that takes practice but clears the snorkel with less effort, but only works when surfacing.
Clearing splash water while at the surface requires blast clearing.
Some snorkels have a sump in the mouthpiece to allow a small volume of water to remain in the snorkel without being inhaled when the snorkeler breathes.
Some also have a one-way output valve in the sump, which automatically drains the sump as it fills with water.
A few snorkels have float-operated valves attached to the top end of the tube to keep water out when a wave passes, but these cause problems when diving as the snorkel must then be equalised during descent, using part of the diver's inhaled air supply.
Some recent designs have a splash deflector on the top end that prevents entry of any water that splashes over the open end of the tube, thereby keeping it relatively free from water.
Finswimmers usually don't use snorkels with a sump valve, as they learn to spit water out through the tube, allowing greater speed and lowering the stress of eventual swallowing of small quantities of water, which would impede their competition performance.
There is actually a method to re-train former snorkelers to use sump valve, splash deflector and noseclip to become a proficient finswimmer.
Common problem with all assistive mechanical clearing mechanisms is their tendency to fail if infrequently used, stored for long periods, environmental fouling, or due to lack of maintenance.
Modern designs use silicone rubber in the mouthpiece and one-way clearing and float valves due to its resistance to degradation and its long service life.
Natural rubber was formerly used, but slowly oxidizes and breaks down due to ultraviolet light exposure from the sun.
It eventually loses its flexibility, becomes brittle and cracks, which can cause clearing valves to stick in the open or closed position, and float valves to leak due to a failure of the valve seat to seal.
In even older designs, some snorkels were made with small "ping pong" balls in a cage mounted to the open end of the tube to prevent water ingress.
These are no longer sold or recommended because they are unreliable and considered hazardous.
Similarly, diving masks with a built-in snorkel are considered unsafe and obsolete.
Application of a grease to valve seats can aid in sealing, but is mechanically washed away over time.
A heavy application can cause valves to stick closed and trap grit (foul), which will cause the valve to leak.
Environmental fouling can be caused by beach sand or loose-floating plant or animal matter becoming lodged in the valve, preventing proper operation.
Although swimming with a snorkel is much easier than without, it is important that a novice swimmer also learn surface breathing and floating without a snorkel, in the event that the clearing mechanisms fail, rendering the snorkel useless as a breathing aid.