the horizontal trim buoyancy

WHAT IS A PROPER TRIM BUOYANCY

Proper trim is not really the same as neutral buoyancy, but rather a subset of it. Proper trim, as defined by modern scuba divers, is the perfect horizontal body position which makes any scuba diver look like he or she is laying down on his stomach on an invisible platform, similar to a prone down position. To attain this position, the scuba diver’s legs are bent at the knee, so the fins are higher than every part of his leg. Fins are generally parallel to the seabed. The arms are extended to the front of his body, and his arms are at the same level with his stomach or just below it. No other parts of the scuba diver’s body appear below this horizontal line.

WHAT ARE THE BENEFITS OF THIS POSITION

Many experienced divers feel that this is the ultimate position and anyone not in this position is not a true diver. I am really uncertain about this statement but well, since so many people are wasting heaps of money on courses and equipment trying to attain this trim position, I thought I should share my two cents worth of opinions. 

But in reality, there are some benefits of this trim buoyancy position. This position reduces the effort required to move through the water due to the streamline position, thus reducing drag. As a result, scuba divers will actually improve air consumption and allow a longer diver. Another distinct advantage is, a trimmed scuba diver moves gracefully through the water, avoiding sand and silt stir up, prevent damaging the delicate aquatic life. And the most important reason why scuba divers should care about their trim is the absolute control and confidence they will have.

According to some instructors (whom I do not have much respect for), you either dive in a trim position or you are not a scuba diver. The importance of getting into a trimmed position has become sort of an ego and pride among divers, which I totally detest.

Enjoy the process of scuba diving and getting into a trimmed buoyancy position. DO NOT OVERLY STRESSED YOURSELF UP. One step at a time and you will eventually reach there, I promise.

GIVE ME THE HIDDEN SECRETS TO A TRIMMED PERFECT BUOYANCY

HIDDEN SECRET 1

 The weights. During your scuba diving jackets, on a rental jacket BCD, your instructor probably have “advised” you to use more weights that it is actually required. Most inexperienced instructors have opted this solution is to weigh scuba divers down to prevent them from “bobbing up and down” like a yo-yo. I have often heard scuba diving instructors passing irresponsible remarks “Oh, you can’t maintain your buoyancy, add another weight for the next time”. This sweeping statement will go on and on for each dive, till the point that the scuba diver has so much weight that the BCD is struggling to provide the right lift to the scuba diver.

What I am gonna say next might come as a surprise to you, and would probably be disagreed by most instructors. Perfect Trimmed Buoyancy is both science and an intuition. You got to understand how weights works, Archimedes’ principle on the physical law of buoyancy, your displacement (with and without equipment). Once you got that sorted out, the rest will be rather easy.

I have loosely classified weights into two categories. Weights for trimming and weights to make you slightly negative when you are down to 50 bar of air.

Weights for Trimming are the weights needed to get you into a trim position. Trimming is different for everyone and there is no such thing as one perfect formula for all. For an instant, the human anatomy of a man is that the legs are heavier, and the human anatomy of a woman is that their legs are more buoyant. A scuba diver getting into trim must first find out the CG (Centre of Gravity) of his body and finding out where the trim weights are best for him or her.

Once we have understood the CG of the scuba diver, the next thing we want to do is find out how much weights is really needed (assuming that breathing rate and method is consistent and a constant). To achieve this, we bleed a scuba tank down to 50 bar/750psi (please note that we are using Aluminium cylinders in this example) and make the diver lay down. We will slowly hand over 1kg/2lbs weight, piece by piece, to see how much weight is really needed.

For those who do not understand, a quick overview of what we are doing here. Everything affects buoyancy. The type of weight suit, fins, BCD type, your body anatomy and most importantly, a diver MUST KNOW THIS, an empty or low on air aluminum cylinder is more buoyant (at the base of the cylinder) compared to a filled up tank.

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HIDDEN SECRET 3

Control your equipment and environment. Do not allow the environment control you. I find the below phrase of Bruce Lee very relevant to perfecting your buoyancy. Many times, I have seen a diver try to hard and have failed. They become so stiff and rigid, making themselves like a log. A log can NEVER move fluidly in water, and when you cannot move fluidly, you cannot master the trim buoyancy.

“You must be shapeless, formless, like water. When you pour water in a cup, it becomes the cup. When you pour water in a bottle, it becomes the bottle. When you pour water in a teapot, it becomes the teapot. Water can drip and it can crash. Become like water my friend.”

Bruce Lee

A lot of new divers struggle with their buoyancy for a few reasons.

  1. Not knowing and understanding how their BCD works
  2. Ill-fitting buoyancy
  3. Unable to control their BCD or BC Systems

This is an important step to learning how to be comfortable. I find that the easiest way to learn to be comfortable is to teach scuba divers how to be REALLY UNCOMFORTABLE. What we usually do at this stage is to teach students how to control their BCD systems by asking them to over-inflate their BCD and learn how to roll on the surface.

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Lester Kwok
PADI Master Instructor at Orpheus Dive
Lester Kwok enjoys teaching scuba diving and seeking for new ways to improve the diving industry. He does this when he is not busy attending to his animals as a canine physiotherapist.
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