How Life Jackets Work

If you were to fall off a boat, it's unlikely that your first thoughts would involve the ancient Greek philosopher Archimedes. However, Archimedes' principle helps to explain what is happening to your submerged body. As an object is submerged in water, it moves, or displaces, water according to how much it weighs. Archimedes found that the water will push upward against the object with a force equal to the weight of water that is displaced.

How much water is displaced is determined by the density of the object. Density is the measure of how much mass is in an object, related to its volume. A bowling ball and a beach ball may have the same volume, but the bowling ball weighs much more, and is much denser, than the beach ball. Meanwhile, a block of solid steel and a steel ship may both be heavy, but the steel ship has a greater volume of steel (in addition to weightless air). It displaces enough water to match its own mass, so it floats.

When that solid, heavy bowling ball is dropped in water, the water pushes up on it with a force equal to the weight of water it displaced. The ball weighs more than the amount of water it displaced and will sink. The beach ball, meanwhile, displaces very little water, and the air inside is much lighter than the weight of the water that was displaced. The buoyant force from below keeps the beach ball afloat. If you were to try to push the beach ball down into the water, the push back that you would feel is the buoyant force of the water at work. Objects that displace an amount of liquid equal to their weight will float because they receive that upward push from the water. You can read more about buoyant forces in How Hot Air Balloons Work.

Buoyancy is the upward force we need from the water to stay afloat, and it's measured by weight. Buoyant forces are why we feel so much lighter when we're in a swimming pool or bathtub. Our bodies are mostly water, so a person's density is fairly close to that of water. Because of this, an average person needs only about seven to 12 pounds of additional buoyancy to float [source: Personal Flotation Device Manufacturers Association]. A life jacket provides this extra lift.

The material inside of a life jacket, which we'll investigate in the next section, traps air when the jacket is submerged. The trapped air weighs much less than the weight of the water it displaces, so the water pushes up harder than the life jacket pushes down, allowing the life jacket to remain buoyant and float. This buoyancy is strong enough to hold up additional weight without sinking.

Life jackets are measured according to how much additional weight they can support. Jackets for adults provide, at a minimum, buoyancies ranging from 15.5 pounds to 22 pounds (7 kilograms to 10 kilograms)­ [source: U.S. Coast Guard]. Since the average person in water needs about seven to 12 additional pounds of buoyancy to float, a life jacket does not have to support the entire physical weight of a human body. Instead, it supports those seven to 12 pounds, with a few pounds to spare. You could attach an item that weighs less than the minimum buoyancy provided by that life jacket, and the jacket would hold that item without sinking.

But what exactly in a life jacket is trapping the air? We'll take a look at how life jackets are made on the next page.

The outer shell of a life jacket is usually made of nylon or vinyl with material that keeps the life jacket afloat sewn inside. There are three classifications for the substance inside a life jacket: inherently buoyant, inflatable and hybrid.

Historically, cork, balsa wood and kapok (a naturally buoyant material from a tropical tree that's also used in mattresses and insulation) have been used in inherently buoyant life jackets, but today, the most commonly used materials are plastic foams, such as polyvinyl chloride and polyethylene.

Life jackets with foam are classified as inherently buoyant because a person does not need to do anything to activate the flotation. These foams use closed cells that trap air in pockets when the jacket is submerged.

Life jackets can also be classified as inflatable. These life jackets have cartridges of carbon dioxide gas sewn into them. When activated, the gas will release and fill the chambers of the jacket.

Some models will activate the release of gas automatically when the jacket is submerged, thanks to a small dissolvable stopper, like a bobbin or a pill. These devices are made of a water-soluble material, and when they dissolve, the gas is released and inflates the jacket. Other inflatable models require the wearer to pull a tab to activate inflation. When the user pulls the tab, it pierces the carbon dioxide canisters to release the gas. Both of these inflatable devices include tubes that a person can blow into, providing a backup way to inflate the jacket.

Inflatable life jackets have been in use since World War II, but the Coast Guard did not approve them for recreational use until the 1990s because of the extra steps involved. However, inflatables have become more reliable, and many people prefer inflatable life jackets because they aren't at full size until inflated. They're less bulky and more comfortable for all-day use. Inflatable life jackets are recommended only for adults who are strong swimmers.

Hybrid life jackets are a mix of natural buoyancy and manual inflation. Inherently buoyant, inflatable and hybrid life jackets all work the same way, though. No matter how the trapped air gets in there, it weighs much less than the weight of the displaced water, and holds the person in the life jacket afloat.

There are more choices to make, though, in determining what kind of life jacket to wear. We'll take a look at the different categories of flotation devices in the next section.

The Coast Guard regulates life jackets and other flotation devices in the United States. The Coast Guard has approved five different categories for personal flotation devices (PFDs):

Types I, II and III are the flotation aids most commonly worn by recreational boaters.

Generally, PFDs with lower numbers provide more buoyancy. A Type I, or an off-shore life jacket, will turn an unconscious person into a face-up position most of the time. In order to accomplish this turn, more buoyant material is positioned near the front of the life jacket, so that the jacket rolls users over onto their backs. Because of this, Type I devices are very bulky in the front.

Near-shore buoyancy vests (Type II) are less bulky but may not always turn a person face-up. Flotation aids (Type III) are the least bulky and are more comfortable for all-day use because the buoyant material is more evenly distributed around the vest. However, these devices require some effort on the part of the wearer to keep the head above water. While the aid may not turn the wearer automatically, it does help a user remain vertical and afloat.

When picking out a life jacket, it's important to consider these characteristics, as well as the types of boating that one will be doing. A Type I PFD is best for open, rough waters and can keep a person afloat for several hours if a rescue will be slow in coming. Type II and Type III PFDs are better for inland and calm waters, when a person can be pulled back aboard a boat fairly quickly. The flotation type classification can be found on the label of the device. In the next section, we'll take a closer look at qualifications to consider when selecting a life jacket.

Life jackets are not one size fits all, nor does one life jacket fit every situation. Because the amount of buoyancy a person needs to remain afloat is dependent on his or her size, life jackets won't work if they don't fit.

Life jackets come in infant, child, youth and adult sizes, and they include a weight and chest measurement that is appropriate for that jacket. A life jacket should fit snugly so that you don't slip out of it. If a life jacket is too big and rides up, then it could obstruct your nose and mouth and keep you from breathing. If it's too small, the snaps and buckles might break.

It's important to ensure that each person has a life jacket that fits. Simply strapping a child into an adult's life jacket won't work because a child's body weight is distributed differently than an adult's. Since that additional weight is mainly in the head, infant life jackets have flotation collars to better support the head and keep it out of the water. Infant and child life jackets also usually come with a crotch strap to hold the jacket in place.

As we discussed in the previous section, some life jackets are better suited for sailing in rough waters than in calm waters. Similarly, some flotation devices are made differently for different boating activities. For example, jackets for fishermen and paddlers have larger armholes, while devices for kayakers might be shorter and less bulky.

Flotation devices designed for activities such as water skiing are impact-tested to ensure that they will survive hitting the water at a high speed. Water skiers should look for a flotation device that has been tested for impacts at 50 mph (80 kph). This means that the flotation device will withstand the impact of a fall at that speed and will keep you afloat. A life jacket might help cushion fall, but the testing only ensures that the impact won't break a zipper or seam. It does not guarantee any bodily protection (or that you won't crack a rib if you smack the water at 50 mph).

All of this information can be found on the flotation device's label:

If a person does a variety of boating, from sailing to waterskiing to deep-sea fishing, it may be necessary to buy more than one life jacket.

A life jacket should be tested for proper fit every year. To learn more about how to test a life jacket and how to maintain it properly, see the next page.

You've picked out which life jacket works best for you and the type of water activity you'll be doing, but you shouldn't head out for the boat just yet. Test the life jacket in shallow water to make sure that it fits. When correctly fastened, the life jacket should stay in place and not ride up. You can test the fit of a child's flotation device by picking the child up by the shoulders of the jacket.

It's also worthwhile to test a life jacket in slightly deeper water to measure the freeboard. Freeboard is the distance between the surface of the water and the bottom of the user's mouth. With more buoyancy, the freeboard will be higher. In rough water with big waves, a greater amount of freeboard will make it easier to keep your head above water.

A life jacket should be tested every year to ensure that it still fits properly and for signs of aging, such as fading or tears. As the jacket ages, the foam will disintegrate, or, in the case of a hole or a tear, fall out of the jacket. When a life jacket is past its useful life, it should be cut into tiny pieces before it's thrown away. This will prevent someone from finding and using a degraded life jacket.

Life jackets will not last forever, but good care and maintenance will help them work properly while you have them. Most life jackets may be washed by hand in mild detergent, and then they should be allowed to drip dry. Do not dry clean a life jacket, because the solvents and the heat can disintegrate the foam. Store the life jacket in a well-ventilated place.

In caring for a life jacket, it's important to avoid extreme heat, such as from clothing dryers, because this will quicken the disintegration of the foam. Life jackets shouldn't be left sitting out in direct sunlight. Compression, including using a life jacket as a pillow or a seat cushion, will also compromise the integrity of the foam.

Inflatable devices require slightly different maintenance than naturally buoyant life jackets. Inflatable PFDs must have full cylinders of carbon dioxide, and the cylinders must be replaced each time the jacket is activated. An inflatable device must be regularly checked for leaks. Indicators on inflatable jackets will glow green when the canisters of carbon dioxide are installed and ready for use. If the indicator is not glowing green, then the device is not approved by the Coast Guard for use. With an automatically inflatable jacket, the water-soluble device must be replaced as well. If the life jacket is never activated, the bobbin or pill should still be replaced every one to three years. Additional cylinders and stoppers are available in a re-arming kit.

Your life jacket is tested and ready to go, but do you really have to wear it? We'll take a look at the laws on life jackets in the next section.

The U.S. Coast Guard has several federal requirements regarding flotation device use. Recreational boats must carry one wearable PFD for each person aboard the boat. One throwable PFD, such as a ring or a cushion, must be present on any boat 16 feet and longer, although canoes and kayaks are exempt from this rule. These devices should be readily accessible, meaning they're not in the bottom of a trunk or tied to the side of a boat.

In addition to these federal regulations, many states have laws regulating the use of life jackets, particularly for children and for activities such as water skiing. However, there is no law that a person has to wear a life jacket. In 2005, the Coast Guard found that only about 23 percent of recreational boaters were wearing personal flotation devices [source: U.S. Coast Guard].

Many states have tried to pass laws mandating life jacket use for all adults, but they have not been successful. Those who support mandatory rules for adults point to laws regarding seat belts and helmets as instances where legislation increased use and saved lives. Boating, however, is seen as more of a leisure activity, where those who participate are choosing to do so at their own risk. In a 2002 survey, 55 percent of boaters disagreed that all persons should be required to wear a flotation device while aboard a boat [source: U.S. Coast Guard]. Boaters were more likely to support enforcing laws related to boating under the influence or stricter control of reckless boaters.

Some people may choose not to wear a life jacket because they are strong swimmers, and they count on providing additional buoyancy through actions such as treading water. A life jacket has additional safety purposes to consider:

For more information on life jackets and how other things float, check out the links on the next page.