Everything You Need To Know About Life Jacket Buoyancy

December 11, 2022

Sometimes walking into a store, and picking a PFD can seem easy. If you’ve done your research on how to choose a life jacket, then you’ll probably know how to select one based on the circumference of your chest area.

But the confusion comes in when you start comparing the numbers. How does a PFD with a buoyancy rating of 10 pounds hold up a person weighing 80 kilos compared to one weighing 200 pounds? In this guide, we take a look at the numbers behind PFDs and how to determine the best buoyancy rating for your size.

Types of PFDs

1. Type I PFDs

These have a PFD buoyancy rating of 7 pounds for infants, 11 pounds for children, and 22 pounds for adults. They have the highest flotation rate and are designed to handle rough water. Even if someone was unconscious, they will not only hold their heads above water but will also roll the person in a head-up position.

Type I PFDs are also referred to as offshore life jackets

2. Type II PFDs

They are also called near-shore buoyancy vests as they are best suited in calmer waters where people tend to congregate and the chance of rescue is high.

Type II PFDs are rated 7 pounds for kids, 11 pounds for children, and 15.5 pounds for adults.

3. Type III PFDs

The type III life jackets have a similar rating to type II life jackets. Also known as floatation aids, they are lightweight and are meant for areas where the water is calm and there is a high chance of rescue.

These are among the best SUP life vests and are therefore highly popular with stand up paddleboard enthusiasts.

4. Type IV PFDs

These are not necessarily used as primary floatation devices. They, however, are used to help the rescue process. They come with a PFD buoyancy rating of between 16.5 pounds and 18 pounds. Type IV PFDs are also called throwable devices.

5. Type V PFDs

They are also known as special use devices and have a buoyancy rating of between 15 pounds and 22 pounds. Type Vs are more commonly used by professionals and rescuers working with strong currents such as in white waters.

6. Inflatable PFD

There are two types of inflatable PFDs—manual and automatic. With the former, you pull on a chord that activates a CO2 cartridge which releases the air and inflates the vest.

With the automatic inflatable, it will activate when it comes into contact with the water after the plug soaks up and dissolves. These types of PFDs are also common with stand-up paddleboarders.

Understanding the math behind PFDs

Everybody is encouraged to exercise regularly and become physically fit. But in a watery environment, a little fat on your body can do you some good. Think of it this way, our bodies are made of 80% water.

This means that when we get into the water, our weight doesn’t count. Since fat is lighter than water, the fat on your body will help you float.

What this means is that the less fat you have the more buoyancy you will need.

To best understand the concept, let’s crunch up some numbers. We will use two people one weighing 125 pounds and the other 200 pounds.

125x 80% body water =100lbs

125x 15% fat =15lbs fat

125 minus 100 minus 15 =10 lbs.

This means that a person weighing 125 lbs. Will weigh just 10lbs in the water. The best life jacket for him would be a type III which has a minimum life jacket buoyancy rating of 15.5 pounds which is more than needed to keep him afloat.

Next, let’s take a look at a 200lbs guy

200x 80% water =160lbs water

200x 15%fat=30lbs fat

200 minus 160 minus 30 =10lbs

Interestingly, the larger 200lbs guy will also weigh just 10lbs in the water. A type III vest with a minimum buoyancy rating of 15.5 is also more than needed to keep him afloat.

If you are lean with only 10% body fat, you might weigh 20lbs in the water and thus will require a PFD with a much higher buoyancy rating.

The best way to test the life jacket buoyancy is to get in the water and simply relax. Then tilt your head back and see whether your chin remains above the water. If water is getting to your mouth then you need to switch up to a higher buoyancy rating.

Life jackets for kids

Before you can even think about buying a life jacket for your kid, or infant life jackets, you should first and foremost ensure that it is coast guard approved. You can find this out by writings located on the side of the jacket or at the back.

Another important thing to check is the number of straps on the life jacket. Make sure it is between 2 and 3. Also, you can read the US coast guard requirements for inflatable PFDs, and don’t forget to check the life jacket laws by state to ensure that you comply with the regulations.

How life jacket buoyancy works

There are life jackets for boating, fishing life jackets, wakeboard life jackets, life jackets for jet ski, and sailing life jackets to mention but a few. All of them, however, work the same way.

According to the Archimedes principle, when you fall into the water, you will displace just as much water as your weight. However, you will also receive an upwards force equal to the water that you displaced and which is also equal to your weight.

The density of the object that is dropped in water will, therefore, determine the amount and volume of the water that is displaced. Density simply means the mass of the object that is dropped in the water.

Look at it this way. A basketball has the same volume as a bowling ball. But the bowling ball is denser and therefore much heavier.

If you were to drop a bowling ball in the water, the volume displaced would be less than the weight of the bowling ball and therefore the bowling ball would sink.

On the other hand, if you dropped a basketball in water, the volume of the ball plus the fact that the air inside is lighter than water means that the ball would experience an equal upwards pressure causing it to float. This force is buoyancy.

If for example, you tried to push the ball into the water with your hand, you would experience an upward force equal to the force you applied which again is the buoyancy of the water.

But you’ll probably not be thinking about Archimedes and his buoyancy laws when you fall in the water. You’ll be thinking about how to stay afloat long enough to get rescued and that’s where life jacket comes into play.

The material used to construct the life jacket has air spaces that trap the air when under the water. Like with the basketball, the air is lighter than the water causing the water to push up against the life jacket. The greater the force when a person is wearing the life jacket, the greater the buoyancy force by the water.

The average human being will require between 7 and 12 pounds of additional PFD buoyancy to float on water. This is why the life jacket doesn’t have to cover the entire body as long as it provides that extra 12 pounds.

But what is contained in a life jacket that enables it to trap the air?

The material that absorbs and traps the air is held inside a nylon or vinyl case. Before we get to the material itself, it’s important to take note of the three main classifications. These are inherently buoyant, inflatable, and hybrid.

Inherently buoyant PFDs materials such as balsa wood, kapok, and cork were traditionally used for life jackets. Modern designs however mostly include foam, Gaia, and polyvinyl chloride. They are referred to as inherently buoyant since there is no action required on the part of the user to activate the life jacket buoyancy.

Inflatable life jackets contain cartridges of carbon dioxide. When a cord is pulled, the gas is released into chambers in the life jacket giving it buoyancy.

The second type of inflatable life jacket features a water-soluble stopper. Once it is submerged, the stopper dissolves releasing the carbon dioxide into chambers thus inflating it.

Life jackets for water sports come in different sizes and care needs to be taken when choosing one. It should fit snuggly around your chest. Too big a life jacket will obstruct your breathing which is not something that you want when you are in the water. On the other hand, if it’s too small the straps might snap.

If you are going to be on the water with your dog, then you should also get them a life jacket. Just like humans, dogs will also tire after swimming for a long time. A crash could also injure your dog and make it harder for him to swim.

A good fitting dog life jacket will keep your dog afloat until you can reach them and pull them out of the water.

Related Review: Life Jackets For Water Sports

Testing for buoyancy

When buying your life jacket, it’s good practice to test it before deciding on which model you are going to take. While life jackets are durable, they should be tested every year for integrity. If you notice that it is torn and the foam has started to come out of some areas, then the best thing is to cut it up to smaller sizes and dispose of it. This will render it completely unusable and will prevent others from trying to use it which could put their life at risk.

To test how buoyant your life jacket is, wear it, get in the water and lie on your back. If your mouth and your chin are above the water surface then the life jacket buoyancy is just right. You will need a life jacket with a higher buoyancy rating if your mouth keeps dipping into the water.

It is paramount that your chin doesn’t go below the water surface. In an accident situation where you are unconscious, a life jacket that can’t keep your entire head out of the water will cause you to swallow water which could be life-threatening

While different models and types of PFDs will offer different levels of buoyancy, most will offer more PFD buoyancy than needed just to keep you on the safe side. Sometimes it’s all about checking the label to find out the buoyancy rating of that life jacket.

Make sure to avoid activities that could affect the buoyancy of the life jacket such as sitting or kneeling on it. Your weight will compress the material causing it to compress and reducing its buoyancy.

Also, make sure that you dry your PFD before storing it. Leaving it soaked in storage will also cause the material to lose its integrity and compress. Storing without drying can also create the perfect habitat for mold and bacteria to grow. This can result in some serious health concerns.

While being simple to maintain, it’s important to know how to clean life jackets. While using a mild detergent is completely fine, avoid dry cleaning as this can cause the foam to disintegrate.

You might also like: What Are The Advantages Of A Type IV PFD?

Main types of floatation materials

Modern life jackets contain the main types of floatation materials as we saw above. These include PVC, Kapok, and Gaia. Let’s take a closer look at each of these to see what sets them apart.

1. PVC

PVC is the most popular type of floatation material used in personal floatation devices. It is what you will find in your attic, foam balls, or even in the foam pits. Its main advantage is that it is inexpensive yet very durable.

The downside to PVC is that it also contains chlorine and oils which means it’s not exactly what you would consider environmentally friendly.

2. Kapok

Unlike PVC, kapok is 100% environmentally friendly. It gets its name from its source which is the kapok tree. Coupled with being highly durable, kapok is also lightweight and its fibrous nature makes it even more buoyant than regular foam.

Its main advantage is at keeping the water out of the PFD. You don’t have to always worry about drying your life jacket.

3. Gaia

This is by far the best floatation material available. Gaia is much lighter than foam and is both cold and heat resistant. This also means that the PFDs made from Gaia will be lighter in weight yet more buoyant.

Gaia is also an environmentally friendly material.

Globo Surf Overview

Life jackets come with different buoyancy ratings not to mention coming with different qualities and features. The right one for you will depend on your size, and ironically on the amount of fat on your body. Fat people are more buoyant than people who are lean because fat is less dense than water. That is not an excuse not to hit the gym though!