Buoyancy+Trim | What are they, and why are they so important? - Dive School Extracurriculars -

  • Posted on: 29 September 2016
  • By: adminscubafreedom

This is the first article in a new series I'll be writing called Dive School Extracurriculars. This is a series mostly for divers who have already completed an entry level scuba diving course and are looking for tips/tutorials on advancing themselves as a diver. 

I'd like to start off with probably one of the most common conversations us Pro divers have when we're talking about our clients 

"Wow it looks like we almost have enough experienced divers signed up this week for a dive at a special, challenging dive site... we'd just need a couple more. Hey, what do you know about John and Jane Doe?"
"Oh, I was diving with them yesterday!"
"Really?! How was their Buoyancy?"

EVERY. Single. Time. Trim and Buoyancy ARE the skills we are going to use to judge a diver's ability and fitness to explore more advanced dive sites.

Buoyancy and Trim are keystone skills for any diver looking to become a better diver. These skills and concepts (hopefully) were already touched on during your first diving course, and expanded on in subsequent courses. Unfortunately, although important, these skills don't often get the attention they deserve during diving training for a few reasons which I will outline in a later article. Just for now, trust there is A TON to learn about these skills.  This article is going to explain basically what exactly we mean by Trim and Buoyancy and why it's so important you start working to master them as soon as possible.  There will be more articles coming soon with techniques useful for training your own Trim and Buoyancy.

What is Buoyancy?
What a diver means by buoyancy and what a physicist means by buoyancy are actually two separate things. We will start with the physics definition and basics, because knowing them will help you to understand how we manipulate and use buoyancy to our advantage. We all know gravity is what causes us to be held to the earth and is actively working on everything at all times on land. Gravity doesn't stop working when you enter the water; in fact the force of gravity is trying to pull you to the bottom just as hard as it was on land, but now there is a new force pushing back up and working against gravity.  The force I speak of is the Buoyant Force and is what causes what we know as Buoyancy. In general buoyancy is the force that causes things to float or seem less heavy in water.​ 

The exact calculation of how much upward buoyant force there will be on an object is [volume of liquid being displaced by an object * density of liquid = buoyant force]. Please keep in mind any increase in weight still increases how much force is pulling you down due to gravity. In our case the liquid we're suspended in is water, and the object is us in all of our SCUBA equipment. If we want to float we need to increase Buoyant Force by displacing more water (increasing our volume) without adding much weight.  An example of this is a Life Jacket or your BCD while inflated, notice how both take up a lot of space, but don't weigh very much which is ideal for staying afloat. If we want to sink we need to add weight with a minimal increase in volume.  An example of this is an anchor or dive weight. These are dense, heavy pieces of metal that don't take much space compared to their weight.

We refer to an object with more buoyant force than weight (floaty) as positively buoyant. An object that has more weight than buoyant force (sinky) is negatively buoyant.  When weight and buoyant force are equal an object is neutrally buoyant and should neither rise nor fall. When diving after a long period of no dives or if you have any equipment changes it's best to always check you have a perfect amount of weight so that you are neutrally buoyant with an empty BCD at the surface. To do this hop in the water with your weights and full kit and hold a normal size breath. Then release all the air from your BCD. If you float at or just below eye level or very slowly sink then you are properly weighted.  This will help you tremendously.

What a diver really means by buoyancy is Buoyancy Control. Buoyancy control is your ability to change your volume on the fly which, in turn, changed the buoyant force acting on your body. We have two primary methods of adjusting our volume in the water. The first is your aptly named BCD (Buoyancy Control Device), and the other is your lungs.

The purpose of you BCD is to change your volume so that with a normal size breath in your lungs you are neutrally buoyant.  At the surface you should already be neutrally buoyant, but as you descend underwater your wetsuit will compress slightly and your volume will shrink making you negatively buoyant. You adjust your BCD slightly to compensate this small change in volume. As you ascend or descend with air in your BCD that air will also expand or compress and you will have to add or remove air to keep your neutral buoyancy.

A diver's lungs are really their main tool to control their movement underwater. A single full inhale or exhale can create a surprisingly huge change in volume. Try placing your hand on the base of your ribs and take a huge, full inhale and then a full exhale and you can physically feel the massive difference in volume. Once you have added or removed small amounts of air with from your BCD to attain neutral buoyancy it's very easy to ascend or descend several feet/meters using only the power of your lungs.

A diver's ability to maintain neutral buoyancy with slight BCD adjustments, control their depth changes with their lungs, and maintain a desired depth while still or in motion are measures of their Buoyancy Control.


What Is Trim

Trim is a word we use to describe your body's positioning while diving. The goals of good trim are to maximize your range of vision, efficiency in your kicking, hydrodynamics, and access to all of your gear. The best position for diving is parallel to the surface with your chest out with your head upright and looking forward, an arched back with your hips forward, and your legs being behind you at the same depth as your head.


And why are they so important?

Trim and Buoyancy are keystone skills for diving because they are going to provide a base for every single other action you take underwater. I consider them to be much like a stance and basic movement in any other sport. It doesn't matter how much you practice the other technical aspects of any sport, if you never learn a proper stance or how to move effectively you'll never advance. Imagine a soccer player with a beautiful shot that just tripped over his feet every time he got the ball, or a boxer who had the hardest punch in the world but never had a chance to throw it because he stood with his hands on his hips during his match. Those athletes would have great potential, but only if they went back to master the basics. It's much the same for a diver. In an out of air emergency it's fantastic to know exactly how to donate air, but you won't be helping anybody if you can't control your depth well enough to stay with your buddy while he has your alternate air source (or worse, if you can't get to them in the first place).

Having well practiced trim and buoyancy can save crucial seconds when you need to react to something underwater. Until you are comfortable with your trim and buoyancy you will always have to take a few seconds to "collect yourself" when reacting under, whether trying to snap a quick photo of a passing fish or grabbing your buddy to stop him from swimming into a patch of fire coral. Once positioning is second nature to you you'll never have to think "Okay descend, release air from BCD, breathe out... wait... now I'm getting close enough! What do I need to do now?!" Instead you'll see that you need to descend, you'll already have the right amount of air in your BCD, and your body will naturally move to where it needs to be leaving your brain free to think through what you need to be doing once you get there vastly decreasing your reaction time. These moments may seem inconsequential most of the time, but imagine diving in a serious current and your buddy loses their mask. You'll be happy (and your buddy will think you're a hero) when you can both catch their mask and move straight to them to maintain physical contact without having to stop to collect yourself underwater, maybe losing the opportunity to do both and either losing your buddy or losing his mask and having to end the dive.

Also as I mentioned earlier these skills will open a world of new and advanced dive sites for you. Often an advanced dive site has some fragile elements that dive pros worry divers without excellent buoyancy skills could smash into and break.  Other times they may involve overhead environments and tight spaces which could trap divers that have erratic movement underwater.  Here in Playa Del Carmen my favorite dives are in the caverns of the Cenotes, and while some are pretty open there are more than a few which we can only take divers to after we have observed top notch buoyancy skills because of the tight spaces filled with stalactites that are billions of years old we don't want to risk being snapped off by a diver with poor buoyancy control skills. On dives with current every member of the dive group should have good control or else the group can be easily separated, which is a nightmare on a drift dive. On a deep dive, if everything goes well, it's unlikely buoyancy will make or break your dive, BUT if you go over your deco limit by five minutes you are supposed to do a 15 minute deco stop at 15 feet. That is a long time not to change depth, and the penalty for failure can be pretty drastic. In the PADI Open Water Dive Course students have to hover for one minute and it can be a struggle even in the best conditions. Would you be comfortable to hover at a particular depth for 15 minutes?  What if you're in a ripping current?  Could you do it with breathing off of your buddy's alternate air source? 

There is an almost endless list of applications for these skills. Underwater Photography, Technical Decompression Diving, Wreck and Cave Diving, Drift Diving, working underwater, identifying rare species, self rescue, buddy rescue, controlling a group, spearfishing, the list goes on and on because ultimately anything you do underwater will be complemented by being in control of your trim and buoyancy. 

Tune in to the next post for some tips to go out and start improving your buoyancy right away!The post after that will be some basic drills and exercises you can start to do during the less exciting parts of your dives to improve even faster!


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