What is Buoyancy?

Ever wonder what the buoyancy ratings on PFDs (personal flotation devices) and life vests mean? It’s often confusing, especially when understanding how it applies to you and your marine activities. Below we explain what it does and how it should impact your PFD selection. 

What is Buoyancy? 

Let's start with an example. Think about a bowling ball and beach ball that are the same size, the denser and heavier bowling ball will sink because it weighs more than the amount of water it has displaced. The beach ball displaces only a small amount of water, and because the air inside it is also much lighter than the water displaced, it floats. If you push down on the beach ball, you will feel the buoyant, upward push from the water. Therefore, the density of the object plays a significant role in buoyancy.  

Simply put, buoyancy is an object's ability to stay afloat (the beach ball is more buoyant than the bowling ball.) 

To get a little more technical: buoyancy is the upward force of a body of water applied to any object entering it and how it opposes the weight of that object. The buoyancy applied to an object depends on how much water it moves (or displaces) when it enters the water. If the object moves less water than its weight, it'll sink (the bowling ball). It will float if it moves more or an equal amount to its weight (the beach ball.) 

The Goal of Buoyancy in a PFD 

The role of buoyancy in PFDs is to get your head and mouth as high out of the water as possible. The higher the buoyancy, the more lift/upthrust to take you further up and out of the water.  

PFDs are designed for specific weight distribution depending on body composition; for example, a child needs to use a PFD made specifically for them because an adult vest isn’t designed for their smaller bodies. Therefore, the fit is an important consideration when selecting a PFD so that the design and buoyancy are effective. Mustang Survival offers PFDs in infant, child, youth, and adult sizes, as well as PFDs compatible with belt extenders, to ensure everyone can find a snug fit that allows the PFD to work to the best of its ability. 

Choosing the Right Buoyancy  

Many factors can contribute to our buoyancy needs; body composition, height, shape, weight distribution, and unique physical body aspects. Because of the complexity of these factors, this is no ‘one equation’ to find the in-water weight for an individual.  

Most adults only need 7 to 12 pounds of buoyancy (31 to 53 Newton) to keep their heads above water. For context, the MIT 70 Automatic Inflatable PFD—the smallest of its kind—offers 15.7 pounds of buoyancy (70 Newtons). 

However, buoyancy needs may increase based on the clothing and gear worn, and environment you’re in. For example, will you be in exposed offshore waters, or will they be wearing heavy equipment?  

The most important thing to remember when choosing correct buoyancy is that the complexity has already been simplified for the consumer and that an approved PFD for your age and size is designed to float in a ‘worst case scenario.’ Also paying attention to the company’s description of intended use is helpful. For example, our MIT 70 Automatic Inflatable PFD description states, ‘Whether cruising the shoreline or confidently navigating shallower waters, the MIT 70 PFD guarantees smooth sailing every step of the way.’ As opposed to our MIT 150 Convertible A/M Inflatable PFD, which states, ‘cruise coastal waters, supported by an impressive 38 lb of buoyancy in this low-profile life vest.’  

How Buoyancy is Achieved  

In general, PFDs create buoyancy by trapping air in their material which weighs less than the water it displaces. So, the water pushes the life vest back to the surface (remember the beach ball example) with force, creating buoyancy strong enough to support your weight.  

Foam PFDs use closed cells to trap air pockets and create buoyancy. When wearing one, there is nothing you need to do to activate the flotation.  

Inflatable PFDs use cartridges of carbon dioxide gas to inflate air bladders. Some models will deploy automatically when they become wet or when submerged and under pressure, and some require manual activation with a pull tab.  

Hybrid PFDs, like the Mustang Survival Khimera, use a combination of foam and inflatable air bladders to create the necessary buoyancy. This allows for some inherent buoyancy and automatically or manually activated additional buoyancy. 

Understanding PFD Approvals and Performance 

We have created a graphic to help you understand the difference between life jackets, flotation devices, and flotation aids. It charts approvals—including Transport Canada and the United States Coast Guard—and how they relate to performance in the water and the intended environment for use.  

Conclusion  

When it comes down to it, the engineers designing the PFDs have done the leg work. Approved PFDs go through a wide range of user testing that considers different body shapes, sizes, and compositions, so you can trust in the buoyancy of one that is deemed correct for your age and size (infant, child, adult). That said, there are other considerations when selecting the correct PFD – the environment and activity.

For more information on selecting a PFD with these parameters, please check out our ‘How to Choose a PFD’ resource.