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DAN Europe: Mastering Proper Trim

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Audrey Cudel focuses her attention on mastering proper trim and how being able to maintain this position offers many benefits while diving.

While mastery of breathing and buoyancy control is the ability of a diver to achieve and hold a specific position in the water column, trim defines the angle of the body in the water, in either static or propulsion mode.

Have you ever spotted a seahorse? Its vertical trim in the water is typically the opposite of what a diver's trim should be. In fact, in theory, a diver's trim could be identified as neutral, positive (slanting upward), or negative (slanting downward). However, in practice, and apart from constraints imposed by overhead environments, keeping the trim as neutral as possible throughout the dive and avoiding slanting upward or downward is the true skill to master.

Mastering Proper Trim

Suppose you think of a diver as a helicopter taking off, flying at various altitudes before landing. A diver's body line should remain horizontal at all times, knees and ankles bent 90 degrees to keep the fins above the body level parallel to the bottom, like a helicopter's blades rotating parallel to the ground. Laying face down as if on a virtual platform, the diver's hands, arms, chest, hips and upper legs are all at the same level, and no part of their equipment should dangle below the body line. Beyond being environmentally friendly, the less resistance a diver creates passing through the water and staying aligned with the direction of motion, the better the hydrodynamics, the less the swimming effort and subsequent gas usage, and the safer the dive.

DAN Europe: Mastering Proper Trim
DAN Europe: Mastering Proper Trim

Many factors can offset a diver's horizontal axis. However, apart from the body tension required in the shoulders, core and gluteal muscles, holding a horizontal posture should not be too much of an effort, provided all weight components and gas distribution don't alter the diver's centre of gravity.

As Greek mathematician and physicist Archimede once observed, ‘equal weights at equal distances are in equilibrium, and equal weights at unequal distances are not in equilibrium but incline towards the weight at the greater distance'. Achieving proper trim is essentially a matter of weight positioning. In the case of a diver, the weight components are cylinders (and all related equipment: valves, regulators, backplates), ballast weights and potentially fins. Whether you're diving a single or twinset, there is a limit to the adjustments that a diver can make to the cylinders' position relative to the body, be they back- or side-mounted, independent of the type of cylinder. Also, for safety reasons, divers need to be able to reach their valves in case a valve shutdown is required.

However, the distribution of a diver's ballast weights is a major contributing factor to their trim and something that they can act on. Once a diver has determined the amount of ballast weights required, wrapping a metaphorical anvil around the waist on a heavy, ill-fitting weight belt is arguably not the smartest nor the safest strategy. The effect is the one of an unbalanced seesaw that can force a diver into a vertical position, many divers with poor skills experience before surfacing, usually with back pain. Instead, they need the right amount of weight, positioned and secured in the right place. Securing one's ballast weights in the proper location not only guarantees that a diver will drop none of the weights accidentally, but it avoids having them shift in a dissymmetric way that would make the diver roll sideways.

Fins can also significantly impact a diver's trim – travelling considerations should not be the primary concern when choosing their dry weight. Beyond apparent requirements such as an appropriate foot pocket size and a blade surface matching the diver's leg power, the dry weight and salt water buoyancy weight can vary tremendously from one model to another and from one size to another. Selecting the appropriate size and weight of fins makes ankle weights unnecessary and prevents the knees from dropping under the horizontal axis.

Provided the weight is distributed correctly, enabling the diver to position themselves face down, gas distribution is the second major factor to consider when tuning a diver's trim. The action of inflating or deflating a wing (or buoyancy compensator device), a drysuit, or ensuring the right amount of gas flows through a rebreathers diver's counter lungs during the dive are done to maintain buoyancy and comfort. However, where the gas flows, the diver goes.

Provided the design and sizing of such equipment are appropriate, finding the balance between the centre of gravity and the centre of buoyancy is the skill to master. Wings and buoyancy compensator devices come in different designs with different gas distribution characteristics. For example, gas spreads more efficiently in a donut-shaped wing bladder compared to a horseshoe design. To be in equilibrium underwater, the centre of buoyancy must be directly above the centre of gravity. Any variation requires exertion on the part of the diver to maintain a hydrodynamic position. This can increase gas consumption when static in the water compared to the propulsion phase, where the speed makes up for a positive or negative trim.

Mastering Proper Trim in a Drysuit

Drysuits tend to be disregarded by many divers who find them hard to manage and only see them as providing thermal comfort. However, the amount of gas required to provide thermal protection while avoiding squeezes or vasoconstriction plays an active role in a diver's trim and should allow for slight trim adjustments. This is accomplished through the efficient gas distribution within the suit, which the diver can only achieve in a horizontal or neutral trim position.

Once a diver has performed a buoyancy check, a trim check will enhance their underwater experience. It only takes a few minutes to maintain proper body tension, remain still in shallow waters, deflate his drysuit, look forward, find neutral buoyancy by inflating the wing and adopt a normal breathing pattern to find out whether they shift forward, backwards or sideways. This check is not about a diver's ability to perform but about verifying proper weight distribution together with the alignment of centres of buoyancy and gravity.

Trim mastery and breathing and buoyancy control are two fundamentals of safe and advanced diving. Any deviation can create numerous hazards and jeopardizes the diver and the team's safety and the environment – loss of buoyancy and breathing control, along with the seesaw depth profile created by being out of trim, can negatively impact team awareness and ability to communicate effectively, impact the environment, create depth and gas management issues, and even result in less than optimal decompression. Once entropy has turned into equilibrium, the resulting balance and order enable the diver to focus on their surroundings and the team rather than themselves, perform tasks, and move on to the next level of their ‘House of Cards'.

You can see more content from Dan Europe from their regular column, or check out the DAN website for more information about medical advice and diver insurance.

About the author

Audrey Cudel is a cave explorer and technical diving instructor specializing in sidemount and cave diving training in Europe and Mexico.

She is also renowned in the industry for her underwater photography portraying deep technical and cave divers. Her work has appeared in magazines such as Wetnotes, Octopus, Plongeur International, Perfect Diver, Times of Malta, and SDI/TDI and DAN (Divers Alert Network) publications.

Scuba Diver Magazine
Scuba Diver Magazine
Scuba Diver Magazine is a global publication serving all the major English speaking markets in print and digital format.
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