Buoyancy basics for Scuba Divers

Your comprehensive guide to buoyancy.

2 March 2021

By Danel Wentzel

Imagine yourself out on a date at a the fanciest restaurant in town. The mood is set. There are candles on every table, elegant flowers placed throughout the restaurant and an orcestra playing in the background. The ambience couldn't be more serene and peaceful.

But then.

Another couple walks in through the door. You hear a load thud as the guy slams the door behind him. He continues to move erratically through the restaurant, stepping on someone's foot and knocking over the beautiful vase of flowers. He then tries to take a photo of his date, but in the process pulls the table cloth from underneath you, sending your food flying everywhere!

Do you know any divers like this? Nobody wants to be that guy, or even that guy's buddy. Learning to control your bouyancy is a vital skill that every scuba diver should aim towards mastering.

Why is buoyancy control so important?

Buoyancy control is what prevents divers from falling deep into the abyss when diving in open water and from shooting up to the surface like a cork when ascending. It is what allows underwater photographers to get up close and personal with macro creatures that hide beneath coral outcrops and it’s what allows cave and wreck divers to safely penetrate into overhead environments.

Good buoyancy control is what separates seasoned pros from newbie divers, and it is the single most important skills any diver can master. Once a diver becomes a proficient practitioner of buoyancy, they are introduced to a world of benefits which include:

Increased Safety: According to Diver’s Alert Network (DAN), overweighting and poor buoyancy are often contributing factors to scuba diving accidents. Good buoyancy reduces the risk of decompression illness by helping divers control their ascent and descent rates.

Protected Marine Environment: Divers with good buoyancy do damage fragile marine environments by crashing into rocks and corals. Instead, they glide effortlessly through them, taking in the sights as they go, zooming in close when they want to, and pulling out without leaving a trace.

Improved Air Consumption and Reduced Fatigue: Divers with bad buoyancy tend to flail around and move their limbs a lot. This causes them to exert a lot of energy and consume air quickly. By contrast, neutrally buoyant divers do not have to work very hard to maintain their position in the water, allowing them to conserve air and energy, and prolong their bottom times.

Better Diving Experience: Being able to control your movements makes each dive more enjoyable. Diving becomes a more relaxed activity, you can get a closer look at interesting marine animals, and it improves your confidence in your abilities.

What is buoyancy?

Buoyancy is an object's (or diver's) tendency to float. You can think of buoyancy as an object's "floatiness". In scuba diving, we use the term buoyancy to describe not only an object's ability to float in the water but its tendency to sink or to do neither. Scuba divers use the following buoyancy-related terms:

(1) Positive Buoyancy/ Positively Buoyant: The object or person floats upwards in the water or remains floating on the surface.

(2) Negative Buoyancy/ Negatively Buoyant: The object or person sinks downwards in the water or remains on the bottom.

(3) Neutral Buoyancy/ Neutrally Buoyant: The object or person neither sinks downwards nor floats upwards, but remains suspended in the water at a single depth.

Archimedes' principle

A simple way to determine whether and an object will float, sink, or do neither, is to use Archimedes'. Archimedes's Principle explains that there are two forces at work to determine if an object will float or sink.

  1. Gravity and the Weight of the Object - This pushes the object down​

  2. Buoyancy or Buoyant Force - This pushes the object up

Easy! If the force from the weight of the object is greater than the force from buoyancy, the object sinks. If the buoyant force is greater than the force from the weight of the object, the object floats (Hint: iPhones sink).

Now all that is left is to figure out how much the buoyant force for a given object is. The simplest way to do this is to weigh the water that the object displaces. The buoyant force on a given object is the same as the weight of the water it displaces. It follows then that:

1. An object floats up if the weight of the water it displaces is more than its own weight.

2. An object sinks down if the weight of the water is displaces is less than its own weight.

3. An object remains suspended at one level if the weight of the water it displaces is exactly the same as its own weight.

In diving, we want to sink at the beginning of the dive to get down our desired depth, and then remain neutrally buoyant until we ascend. We can't change from negative to neutral buoyancy on a whim because we can't change the amount of water our bodies displace. Therefore, divers control their buoyancy using an inflatable jacket, or buoyancy control device (BCD) to displace more water (by inflating it and increasing their buoyancy) or less water (by deflating it and decreasing their buoyancy).

Let's take a look at the different ways to improve buoyancy on dives.

  1. Balance and breathing

Basically if you want to achieve the perfect neutral buoyancy there are two major factors you need to be aware of.

  • First: wear the right amount of weight for the dive. This will differ depending on your exposure protection and conditions.

  • Second: breathe slowly and evenly.

Get those two things sorted and you are the master of neutral buoyancy, right? Well, not exactly. When I started diving, I always seemed to need a lot of weight to get down. But, at depth I felt over-weighted. I was constantly adjusting the air in my BCD and couldn’t maintain that effortless hover others seemed to manage so easily. Now, as an experienced diver, I see other new divers with the same problems. So what’s the secret?

When diving you always want to make sure you are correctly weighted. You can do this by preforming a weight check at the surface before your dive. To do this release all your air from BCD and observe:

  • If you have the correct weight, with full lungs you will be at chin level (1). you will be floating at the eye level while breathing normally (2). You will begin to go down (3) with empty lungs.

  • If you have too much weight, you will go down even with full lungs. A little extra weight can be helpful if you are still new to diving and have trouble with buoyancy at the end of the dive. You become negatively buoyant as the air in your tank is used up.

  • If you do not have enough weight, you will remain on the surface even after you empty your lungs.

2. Another key is to just get comfortable and relax!

Underwater, if you’re nervous and/or wearing unfamiliar gear, you’re not setting yourself up for success. Long, relaxed breaths allow you to maintain control over your buoyancy. Diving can be exciting, but don’t let adrenaline take over.

I also realized: as a new diver I would unconsciously hold my breath because I was nervous. This teeny bit of breath-holding made it hard for me to descend, so I’d wear more weight to compensate. But wearing too much weight screwed up my buoyancy. It was a vicious cycle!

Once I got my own dive gear and started getting more comfortable, I was able to wear less weight and as an added bonus - my air started lasting significantly longer. It was only until my Dive masters where I knew exactly how much weight to use and where to distribute it, no more guessing. As a result, I finally nailed my crossed legged hover I dreamt of for so long – and became the really good diver with skills others looked up to.

3. Try work on your trim

Just like any other sport, when diving we want to make sure we have the right technique. This means knowing what your body position is when diving. Divers refer to this as your trim. Your center of gravity should balance around your navel. The correct neutral position is when you are horizontal (2) to the seabed and your knees bent 90 degrees and fins pointing backwards. If you have too much weight your knees will sink (3) and you will be positively buoyant. Conversely, if you have too little weight your head will point downwards (1) and you will be negatively buoyant.

So I tried these tips, but I am still struggeling with my buoyancy, what do I do next?

You might want to consider seeking the help of a PADI professional. A PADI Pro will help you figure out the exact weight you need and where it should go on your body. Few divers wear all their weight around their waist. You’ve probably seen tank weights, ankle weights, and other configurations. The “right” distribution for you depends on body composition and desired body position (vertical for wall diving vs horizontal for a silty lake bottom).

So in short,

Buoyancy is an essential diving skill and life-long practice. No matter what your area of interest (shooting video, doing research, public safety diving, wreck diving) perfect buoyancy is the key to success. Even if you don’t want to do anything but look at fish, the process of achieving good buoyancy should allow you to drop weight and extend your dive time.