If you’re a fairly observant swimmer, you may have noticed that your body tends to resist staying underwater while you’re trying to glide through the depths. This can be quite annoying, especially if you intend to hone in on your underwater swimming skills. So why does it even happen in the first place?
The reason you always float when trying to swim underwater is because you’re positively buoyant. Your body weighs less than the water that it displaces, causing your body to rise as you swim below the water’s surface. This positive buoyancy phenomenon can be attributed to your body composition.
Positive buoyancy can be a bit of a tricky concept to understand if you go about it the wrong way. Below, we will break down the concept of positive buoyancy step-by-step so that you can have a thorough understanding of why your body cannot seem to stay underwater. Read until the end to discover some neat tricks on how to counteract your positive buoyancy and stay beneath the water’s surface.
Why Your Natural Buoyancy is Making You Float Underwater
Every object in the world has a natural buoyancy level, from rocks to sailboats to humans. As a quick reminder, buoyancy refers to an object’s natural propensity to float—or sink for that matter—when it’s completely or partly immersed in a fluid (source).
When early scientists tried to pin down exactly why certain objects float and other objects sink, they were at a complete loss. It wasn’t until a Greek physicist by the name Archimedes came upon a groundbreaking revelation that humans were finally able to understand why some objects float and why some objects sink.
This discovery later became known as the Archimedes’ Principle. In order to understand why you float while swimming underwater, we first need to take a bit of a detour and learn the fundamentals behind this principle.
Brief Explanation of Archimedes’ Principle
Have you ever eased yourself into a hot bath and noticed the water level rise as you slinked further into the water? If you have, then you’ve experienced firsthand what went through Archimedes’ mind when he uncovered his groundbreaking scientific discovery.
The increase in water level proved that submerged objects displace a certain amount of water. Ultimately, the amount of water that has been displaced by an object is key to whether that object will sink or float underwater.
Archimedes’ findings can be summarized with the following statement:
Put simply, the more water that’s been displaced by an object, the stronger the upward buoyant force. Though, it’s important to note that an object’s volume isn’t the only determining factor of its ability to float.
Mass is just as important to consider when determining buoyancy. Fortunately, an object’s mass and volume can be simplified into one quantifiable numerical measure: density.
How Density Plays into Archimedes’ Principle
Recall that density is just the amount of mass something has compared to its volume. A bowling ball is considered to have a high density, since it’s packed completely full of material. In contrast, a basketball is considered to have a low density, since it’s hollow and mostly occupied by air.
The reason that density is such a crucial component to Archimedes’ Principle is that an object’s density dictates whether the downward gravitational force or the upward buoyant force will prevail. Whichever force wins is what decides whether an object will sink or float.
Going back to the bowling ball versus basketball example, you can see the logic behind why one ball floats and the other ball sinks by looking at the table below:
|Ball Type||Sink or Float?||High Density or Low Density?||Weighs More or Less than Displaced Fluid?|
|Bowling Ball||Sinks||High Density||Weighs More than Displaced Fluid|
|Basketball||Floats||Low Density||Weighs Less than Displaced Fluid|
Many people are under the misconception that the capacity to float in water only has to do with weight. This is completely false.
If flotation only dealt with weight, it would be hard to explain why a cruise ship weighing in at 200,000 gross tons can float on the water, while a lightweight computer laptop cannot. In short, density is king when it comes to determining an object’s ability to float or sink, not weight.
To better reinforce concepts you’ve just learned, check out the following clip on Archimedes’ Principle:
How These Concepts Relate to Your Flotation Issues
Now that you understand the concepts behind natural buoyancy, Archimedes’ Principle, and object density, it’s time to jump back to the main topic at hand.
Ultimately, the reason why your body is floating underwater is because your body is less dense than the displaced fluid. With the amount of space your body takes up underwater, your body simply doesn’t have enough mass to weigh you down. This is why the upward buoyant force is able to win against the force of gravity.
Other people that do not typically float when swimming underwater have a different buoyancy level from you. Rather than being positively buoyant, they generally tend to have a neutral buoyancy slightly underneath. the water’s surface.
However, there are rare exceptions where a person may be negatively buoyant. Under these circumstances, the person cannot float at the water’s surface at all.
You can learn more about the topic of negative buoyancy as it relates to people by reading through Why Can’t Some People Float? (Everything You Need to Know!).
Factors that Enable People to Be Positively Buoyant
So now you’re probably thinking to yourself, “Why does my body not have enough mass to weigh me down underwater?” The answer to this question lies with your body composition.
No two people have the same exact body makeup. We all may be made up of the same general parts, but these parts differ in a lot of ways from person to person, particularly when it comes to three specific components of body composition:
- body fat percentage
- muscle mass percentage
- bone density
High Body Fat Percentage
A person’s body fat percentage is a significant factor when it comes to natural buoyancy level. The reason being that body fat is less dense than water. To give you some numbers, the average density of body fat is 0.9 g/mL, whereas the density of water is 1.0 g/mL (source).
Thus, the more body fat that a person has, the more prone to flotation they will be. If body fat percentage is kept to a normal range, however, being positively buoyant is far less likely.
Low Muscle Mass Percentage
The next component of body composition that affects buoyancy level is the amount of muscle an individual has on their body. Unlike body fat, muscle is denser than water, with an average density value of 1.1 g/mL compared to 1.0 g/mL (source).
At first glance, a difference of just 0.1 g/mL may appear to be rather trivial. Since there’s such a large degree of muscle found all over the body, this seemingly insignificant deviation can actually be the root cause of whether a person floats or sinks.
For this reason, individuals that do not carry a high amount of muscle on their body tend to float underwater.
Low Bone Density
The third and final body composition variable we will be analyzing is bone density. Bone density can fluctuate markedly individuals, as bone density can be affected by a multitude of factors, such as (source):
- calcium consumption
- physical activity
- vitamin intake
Bone density can be tough for researchers to quantify, so they typically just measure the bone mineral density (BMD) to help estimate bone density. The literature has shown that bone mineral density values vary significantly even between genders.
According to the literature, men have a bone mineral density of 3.88 g/cm² while women have a bone mineral density of 2.90 g/cm² (source). If you think about it, that’s quite the distinction!
With such a high variability in bone density, it’s no wonder that this such a prominent factor in natural buoyancy level. Those with low bone densities likely face the same trouble that you’re facing now with staying underwater, since there’s not a sufficient of bone mass to counteract the upward buoyant force.
Simple Tricks to Stay Underwater While Swimming
Reconfiguring your body composition can take weeks, months, or even years to get it to a point where you can remain underwater effortlessly. Although taking the time to reconfigure your body composition is certainly an option, there are other short-term fixes that you should explore at the very least.
Empty Out More Air from Your Lungs
One easy trick to stay underwater is to breathe more air out of your lungs as you’re swimming.
Any excess air in your lungs will cause your chest cavity to expand and displace more water. As aforementioned, the more water that’s been displaced, the greater the upward buoyant force. If you want to remain below the water’s surface, this is obviously a practice you want to avoid.
Plus, filling yourself up with air will slightly lower your body’s overall density. This seemingly minor decrease in body density may be enough to cause your body to rise in the water.
For these reasons, you should try to push beyond your normal exhalation threshold and see if it resolves this problem.
As a side note, you should know that this technique comes with a trade-off. Since you’re keeping less air in your lungs, you won’t be able to stay underwater for as long as you normally would. The lack of oxygen will take its toll and you will have to resurface more frequently.
If you have no interest in dealing with this trade-off, you may want to consider a more convenient fix, such as the one we will discuss next.
Put On a Free Diving Weight Belt
Achieving neutral buoyancy is extremely critical for free divers that want to glide underwater freely without being hindered by their natural buoyancy. Positively buoyant free divers that want to do away with the personal buoyancy restrictions often wear a weight belt to compensate for their low body density.
Fortunately, you can implement this same tactic to achieve neutral buoyancy for yourself. Strapping on a free diving weight belt equipped with weights will allow you to overcome any obstacles presented by your body composition.
When the appropriate amount of weight is attached, you will be able to glide underwater without your body wanting to float or sink. Your body will simply respond to your swimming strokes and not much else.
Of course, this strategy comes at a financial cost, as opposed to the previous tactic. If you’re curious as to what the prices are for a free diving belt, check out the Riffe Rubber Weight Belt on Amazon. This belt has all the features you need for a smooth underwater swim, such as quick detachability, an adjustable clasp, and a rubber texture that locks weights in place.
Swim Farther Down Where the Pressure is Higher
The last trick for staying underwater is to swim deeper into the water. At these lower depths, the water pressure is much higher. This extra pressure compresses the air in your lungs, so your chest cavity doesn’t displace as much water as it normally would.
By reducing the amount of water displaced, you also reduce the amount of water that’s contributing to the upward buoyant force. This makes you slightly less prone to floating as you swim underwater.
Unfortunately, there are drawbacks to this method as well, namely excess sinus pressure. Just like the water pressure bears down your lungs, it also bears down on your sinuses, which can lead to extreme feelings of discomfort. This discomfort may even reach a point where it’s simply not worth swimming underwater any longer.
So if you can deal with the sinus pressure, by all means try out this strategy! Just know that this trick isn’t made for everyone.