A snorkel is a tube with a mouthepiece attached to one end. This allows a diver or swimmer to swim with their face in the water while breathing. When snorkels were first invented there were several types. Today, snorkels are a certain length and diameter, made for efficient breathing.
Early snorkels could have too large an airway, too narrow an airway (which restricted breathing), be too long or too short. There were also various attempts made at keeping water out of the tube. This was accomplished through a ball and cage in the beginning. However, the ball could lodge in the cage and prevent a person from breathing. I have never found it hard to lift my head out of the water, but some people were injured by this design. Today, we see models with splash guards that do not close but try to abate the waves, and purge valves to try to ease clearing a snorkel. If the snorkel is too large or long, a diver will have a difficult if not impossible time clearing their snorkel. There is a certain possible pressure from our lungs for easily clearing a snorkel. Most types today will serve any diver. However, smaller and bigger divers may need to consider the tube size still.
Also, the curvature in the tube of the snorkel contributes to air resistance. The curvature is necessary to follow the contour of a person's face. However, it was found that the J, or U formation of the curvature inhibited breathing, and created excessive dead air space. To breath, the snorkel must be short enough that all the dead air space, a.k.a. stagnant carbon dioxide that was exhaled, can be cleared. Otherwise the diver would receive too little oxygen. This is also why you cannot breath off of a long pipe (or reed like in the movies). The hoses that extend to divers are filled with pressurized air. The diver exhales into the water.
My question with the snorkel lies in my friend's safety concerns.
