written by: Raunekk
• edited by: KennethSleight
• updated: 4/18/2009
Life jackets, also known as personal flotation device, is one of the most important safety device on the ship. Marine Legislature states that each and every person on board a ship should have a life jacket. It is also a mandatory rule to know the working and procedure of wearing a life jacket.
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Today, Life jackets are mandatory at all places where life is at stake due to presence of water. Onboard all types of ships, it is supposed to be considered the most important life saving appliance. Every crew member has his own separate life jacket that is retained in his room. Apart from that, all the working places on the ship including the engine room have a life jacket locker where spare life jackets are kept for emergency situations. According to SOLAS, it is mandatory that every person travelling on board a ship has his or her dedicated life jacket. Not only that, every person on board a ship should be familiar with the working of life jackets and must know how to use it. But do you really understand how do life jackets keep you afloat? If not, then just read on.
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Types of life jackets and materials used
Type of life jacket used depends on the purpose and utility. There are basically two main types of lifejackets. They are as follows;
Foam core type life jacket
This is the most widely used type of life jacket. The foam cores provide high buoyancy which facilitates the person wearing it to keep his face clear of the water. The life jacket consist of an adjustable strap that is worn around the vest. It also consist of a harness like strap that can be worn between the legs to restrict the the life jacket from moving from its position when in water. They are also sometimes provided with headrest flap to support the head when in water.
Life jackets with air chambers
These type of life jackets are used in activities that involved more risk. It generally consists of a pair of airtight chambers made of nylon or other tougher material. Both the chambers are separated from each other, thus in an event of 1 air chamber leaking the other remains unaffected. The air chambers can be inflated manually or with the help of gas canister release mechanism. The life jackets are made in such a way that they automatically brings the wearer in the "Face up" position and their bodies in an inclined backward position.
Both the types of lifejackets are provided with distress signalling instruments and pockets which can be utilized for keeping walkie-talkies, knife, smoke or light flares, energy bars, seasickness tablets, whistles etc.
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So why do the life jacket float?
How can something that is 10 times lighter than you help in keeping you above water?The answer is buoyancy.So how does the life jacket produces buoyancy?
The buoyancy of a substance depends on the amount of weight it displaces.The higher the amount of weight a body can displace, greater the buoyancy it will generate. When a body is in the water, it displays an amount of water equal to its weight. But life jacket consist of a very light material such as form that can display water 10 times its weight. Also, a body when in water creates a buoyant force that pushes it upwards. Thus a life jacket doesn't have to support all the weight of the body as the body itself generates a buoyant force.
Let's take an example, suppose that a solid wood of weight around 60 pounds is placed in water. It will definitely float in water but will be submerged more than half its height. Now take a wooden box of the same size that is hollow inside and weighs around 10 pounds and place it in water. The wooden box has very less amount of weight to displace and thus it will float at a much higher level. Thus the hollow wooden box has buoyancy higher than that of solid wood
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How do they work?
If it's a foam type life jacket nothing has to be done other than wearing the life jacket before getting into the water. But special care should be taken to fasten the straps and safety harness properly.
If it's a life jacket with air chambers then there will be a requirement to inflate the air chambers either manually or with the help of gas canister release mechanism. The canisters contain carbon dioxide cartridges that inflate when pulled a cord. When the cord is pulled, a pin punchers the canister and the carbon dioxide escapes into the air chambers.
There are some life jackets that inflate automatically when they come in contact with the salt water. Nowadays there are "hydrostatic release" life jackets which inflate according to the water pressure acting on the life jacket.