What is Buoyancy?

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

What is Buoyancy? 

Let's start with an example. Think about a bowling ball and a beach ball that are the same size, they have the same volume or displacement. The heavier bowling ball will sink because it weighs more than the amount of water it has displaced. The beach ball has the same volume as the bowling ball (it would displace the same amount of water if forced under the water) but because it weighs less than the water is displaces, it floats. If you push down on the beach ball, you will feel the buoyant, upward push from the water.

Simply put, the buoyancy we care about is an object's ability to float (the beach ball is more buoyant than the bowling ball.) 

In more technical terms, buoyancy is a force that acts on any object when it is completely or partially submerged in a fluid (e.g., the bowling ball and beach ball in water.) The object is acted upon by an upward, or buoyant, force—the amount of force is equal to the weight of the fluid displaced by the object.

The upward force equals the weight of the displaced water, minus the weight of the object itself. Say the bowling ball displaces water that would weigh 5.6 kg (12.3 lb.) and the bowling ball weighs 7 kg (15.4 lb.), so it weighs more than the upward buoyant force and it sinks. The beach ball also displaces water that would weigh 5.6 kg (12.3 lb.) and it weighs only a fraction of a kilogram, 0.1kg (0.22 lb.), so it will float and would provide 5.5 kg (12.1 lb.) of buoyant, upward force.

Buoyancy is usually expressed numerically in kilograms, pounds, or Newtons of force.  

 The Goal of Buoyancy in a PFD 

The role of buoyancy in PFDs is to get your nose and mouth out of the water; we call this airway protection. The higher the buoyancy of your PFD, the more upward force is applied to your body, helping get 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 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. 

How Buoyancy is Achieved in PFDs

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. 

Choosing the Right Buoyancy  

Many factors can contribute to our buoyancy needs: intended activity, body composition, height, shape, weight distribution, or other unique physical body aspects. Because of the complexity of these factors, choosing the right device means understanding how those factors and your activities impact your choice.

Most adult swimmers 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 your clothing, gear, and environment. For example, will you be boating in exposed offshore waters? Or will you be wearing foul weather gear or carrying heavy equipment?  

The most important thing to remember when considering buoyancy is that a lot of the complexity has been simplified for the consumer. An approved PFD for your size, activity and age is designed to float you. 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.’  

 EXPLORE FLOTATION

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, the environment and activity must also be considered when selecting the correct PFD. For more information on selecting a PFD with these parameters, please check out our ‘How to Choose a PFD’ resource.