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Teaching Neutrally Buoyant and Trimmed
A guide for the instructor who wishes to make the leap, Part 1
By Kosta Koeman
The practice of teaching open water diver students neutrally buoyant and trimmed began over a decade ago and has been steadily growing as progressively more instructors worldwide discover the benefit of students obtaining buoyancy control in less time. There are continuous discussions over its merits and even arguments about whether it is even possible! I assure you it is. Otherwise, I wouldn’t be writing this series.
The purpose of this series of blog posts is to provide my approach to teaching neutrally buoyant and trimmed, which I hope will be used as a guide for instructors who themselves wish to make the leap from teaching on the knees to neutrally buoyant and trimmed from the beginning of the first confined water session and throughout the entire course. Instructors must overweight their students in order to have them firmly on their knees, which results in problems for the new diver. Believe it or not, there can be currents inside pools from water circulation that is sufficient to push over an unstable diver on their knees. I would like to emphasize that this is also a safety issue. DAN’s 2016 Annual Report listed the “Ten Most Wanted Improvements in Scuba”. At the top of the list were correct weighting and greater buoyancy control. This blog series is intended to address those two items and indirectly better ascent rate control. I will not address underweighting, as people figure out that they don’t have enough weight from not being able to maintain a safety stop when nearly empty. Overweighted divers are often unaware that they are carrying too much lead, and as a result, they are at greater risk of injury or even a fatality.
In this first article, I will discuss relevant concepts to teaching open water courses neutrally buoyantand trimmed and share the topics that will be covered later in this series. Before going further, I would like to define proper weighting. Proper weighting is the minimum amount of weight needed to keep a diver at their safety stop (5 meters or 15 feet) with an empty BCD/wing and a nearly empty cylinder (50 bar or 500 psi). If the diver is wearing a drysuit, then the drysuit has the bare minimum amount of gas to keep the diver sufficiently warm. And not a gram or ounce more.
Keys to teaching neutrally buoyant and trimmed
The key to teaching neutrally buoyant and trimmed is proper weightingand weight distribution so students float comfortably in a horizontal position, rising and falling slightly with normal breathing. The scientific rationale is obvious when considering Archimedes’ principle and Boyle’s Law. Let’s start with Archimedes’ Principle as it is covered in the SDI Open Water Scuba Diver course.
You most likely remember reading about the Greek mathematician Archimedes in school. Archimedes discovered that an object, wholly or partially immersed in water, is buoyed up by a force equal to the weight of the water it displaces. We call this Archimedes’ Principle. As divers, we’re chiefly concerned with the three states of buoyancy: positive, negative, and neutral.
Second, let’s remember Boyle’s Law:
During the 17th century, an Irish scientist, Sir Robert Boyle, conducted a series of experiments that would define the physical properties of gases under pressure. Using a U-shaped tube and liquid mercury, Boyle proved that the volume of a gas is inversely proportional to the ambient pressure. In other words, if pressure increases, volume decreases and vice versa. This is known as Boyle’s Law.
Negative vs positive
Open water students must learn to master controlling their buoyancy and depth. Part of mastering this requires situational awareness, which simply takes time to acquire. New divers may not be aware that their depth changes due to task overloading. When you ascend, even slightly, the gas in your BCD expands, as does the gas in your wetsuit or drysuit if worn. You become positively buoyant as you displace more water, and if you do not compensate by dumping gas from your BCD (and/or drysuit if worn), exhaling sufficiently or descending by finning, you will maintain positive buoyancy and be pushed towards the surface.
Likewise, when you descend, even slightly, the gas in your BCD contracts, as does the gas in your wetsuit or drysuit if worn, and you become negatively buoyant as you displace less water. If you do not compensate by adding air to your BCD (and/or your drysuit if worn), inhaling sufficiently or ascending by finning, you will maintain negative buoyancy and sink towards the bottom.
At shallower depths, the change in pressure is greater relative to the change in pressure at deeper depths. Using the diagram above from the SDI Open Water Scuba Diver course, when descending from the surface to 10 meters, the pressure doubles. Descending another 10 meters, the pressure increases by 50%. Descending deeper results in a smaller and smaller percentage increase in pressure. Since open water courses are taught in shallow water initially, new divers are dealing with faster changes in pressure compared to a student taking a deep diver course.
If you are overweighted, you must add additional air to your BCD to maintain neutral buoyancy. If you ascend, even slightly, there is an additional increase in force that buoys you up from what you would experience if you were properly weighted, caused by the additional displacement of water corresponding to that excess weight. Conversely, if you descend, even slightly, there is an additional decrease in force that buoys you up, caused by the decrease of displacement corresponding to that excess weight. Combining this with a lack of situational awareness that one’s depth has changed overwhelms new divers and they often fail to react in time. If they ascend slightly, they continue to ascend to the surface (corking), because by the time they realize they are drifting up, they are not able to dump enough air fast enough from their BCD to reestablish neutral buoyancy. If they descend slightly, they continue to descend to the bottom (cratering), because they realize too late and are not able to add gas fast enough to their BCD. This is a source of frustration that decreases students’ confidence, sometimes eroding their desire to continue to learn to dive.
Compensating for increased changes in buoyancy is often overwhelming for new divers. The only solution for this is being properly weighted: take only the weight that you actually need in order to minimize the change in buoyancy when depth changes. The second key is weight distribution. Weight must be distributed so that a diver is able to float effortlessly in the horizontal position. This increases the diver’s feeling of stability in the water, making it easier to perform skills neutrally buoyant and trimmed. Like maintaining depth, the feeling of stability in the water builds confidence and enthusiasm as students perform skills more comfortably.
For most people, the center of mass is typically around the navel, usually a little higher for men and lower for women. The center of the volume of water displaced by the diver and all equipment is higher in the chest. We also have a large air space in our chests. Our legs are basically muscle and bone and have no air spaces. The difference in the center of mass and the center of displacement result in torque that, without compensation with weight or sculling of the fins, will rotate divers until they are vertical in the water with the feet below the body as if they are standing on land. This is what is meant by being “leg heavy.” Diving in a shorty wetsuit exacerbates this problem due to the lack of buoyancy that the missing neoprene below the knee would provide. I personally cannot trim out properly weighted when not wearing exposure protection, and this includes using slightly positive fins. But more on this topic later.
Without deliberate steps in weight distribution, divers’ centers of mass and centers of displacement typically never match.
It is important to recognize that when using the antiquated method of fin pivots, neutral buoyancy is not taught. Students are still leg heavy and some of their weight is being held up by the bottom. This technique should never be used, as it ingrains similar bad habits to teaching on the knees. In order for neutral buoyancy and trim to be taught, the center of mass must match the center of displacement.
Future Blog Posts
Coming up next in this blog series is information on getting students comfortable with breathing underwater properly through a set of exercises in chest deep water with just a mask and snorkel, finning exercises at the surface, and skin-diving skills. I will also cover one method for properly weighting students. The third and final blog post will cover how I weight my students properly and distribute that weight. Upon completion of this series, you will have the tools you need to teach your open water students neutrally buoyant and trimmed. My hope is that if you are not teaching your open water courses completely neutrally buoyant and trimmed, you will try these techniques. I strongly believe you will see some amazing results with more skilled and confident divers.
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