The main function of a PFD, of course, is to keep a person afloat, or buoyant - even with heavy clothes and gear. Its buoyancy also helps a stranded person save energy, because without a PFD a swimmer has to continually work to stay afloat.
Some types also help keep a victim's face out of the water.
A PFD's buoyancy is the amount of weight it can keep afloat. High buoyancy is over 20 pounds. This may sound low, but water provides lift, so the average adult needs only 7 to 12 pounds of buoyancy. Five Approved Types But higher buoyancy means a bulkier PFD, which may interfere with activity or be unnecessary in certain situations. Therefore, the US Coast Guard has approved five types.