Article: Snorkeling
Section: Flooding and clearing

Question: How can you breathe using a snorkel while submerged under water?

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.