Merchant and passenger ships are generally required to have a life preserver for every person aboard and in many cases, a certain percentage of smaller sizes for children. According to United States requirements, life preservers must design, reversible capable of being quickly adjusted to fit the uninitiated individual, and must be so designed as to support the wearer in the water in an upright or slightly backward position. Sufficient buoyancy( 浮力 ) to support the wearer should be retained by the life preserver after 48 hours in the water, and it should be reliable even after long period of storage. Thus it should be made of materials resistant to sunlight, gasoline, and oils, and it should be not easily set on fire.?The position in which the life preserver will support a person who jumps or falls into the water is most important, as is its tendency to turn the wearer in the water from a face-down position to an upright or slightly backward position, with his face clear of the water, even when the wearer is exhausted or unconscious. The method of adjustment to the body should be simple, and self-evident to uninitiated persons even in the dark under the confused conditions, which follow a disaster. Thus, the life be reversible that it is nearly impossible to get it on wrong. Catches, straps, and ties should be kept to a minimum. In addition, the life preserver must be adjustable to the wide variety of shapes and sizes of wearers, since this greatly affects the position of floating and the self-righting qualities. A suitable life also be comfortable to wear at all times, in and out of the water, not so heavy as to encourage to take it off on shipboard while the ship is in danger, nor so burdensome that it hinders a person in the water while trying to swim.