December 10th, 2005, 01:48 PM
Why a backplate and wing
...Or Why I chose the equipment that I chose
By SeaJay Bayne
When I decided it was time to purchase my own buoyancy compensator, I began to explore some of the options... And it was positively mind-boggling! All of the manufacturers proclaimed that their "BC" was the best, and all of them promised to be the simplest, best functioning, and most rugged in design. With a price difference of several hundred dollars between the low end and high end BC, I began to wonder what the difference truly was, and how it would affect the enjoyment I got from scuba diving.
...And so my research began. I asked questions. I must have talked to hundreds of different divers, all of which proclaimed that the BC that they were diving was by far "the best." I researched Rodale's BC tests, where independent divers give analytical thoughts on different BC's on the market. Then I began to dive them. I personally tested countless BC's over a period of more than 130 dives before coming to the following conclusions... The result of many months of study. It is my hope that some of the information provided here will save other divers from having to bear the time and expense of all of the research.
A diver first has to ask himself/herself, "What is it, exactly, that I want a BC to do for me?" Certainly the answer to that question can be as diverse as the current lineup of BC's flooding the market. However, I found that most divers, myself included, placed two functions of a BC at the top of their list. They are:
In this article, I will address the second main function of a BC before the first, since I found this to be a less commonly addressed issue in a BC.
- A BC needs to hold everything together. Simply put, it must hold the tanks squarely on the diver's back, without shifting around and causing unnecessary movement. This includes the movement of tanks and weights, especially annoying in any case and downright painful in others.
- A BC needs to provide buoyancy. That is, it needs to enable the diver to wear the correct amount of weight in order to sink at all times, and it needs to enable the diver to float at the surface at all times. It needs to help the diver to achieve neutral buoyancy - the ability to be neither too heavy nor too light. Most of all, it needs to do that in such a fashion that it does not roll or pitch the diver in an unnatural position.
There are three main designs of BC's on the market today: The jacket-style BC, the back-inflate style BC, and the backplate and wing style BC.
If you were taught to dive any time in the past two decades, then you probably learned to dive in a jacket-style BC. These are by far the most common style of BC, and can be seen and rented in nearly any dive shop. They are common, accepted, and have a good record of safety. Typically, they come in several different sizes, and since they resemble a common life vest, are readily accepted by the new diver. They are also quickly and easily adjusted while on the diver, and provide the diver with a comforting "heads up" position while floating on the surface. However, many experienced divers shy away from jacket-style BC's, as they are notoriously bulky and cumbersome because of their "wraparound" air bladder. In the worst cases, divers claim that a fully inflated jacket-style BC "squeezes" them and prevents them from taking a full breath. While I personally have found some to be worse offenders of this than others, there are other reasons why a diver purchasing his/her own personal gear might want to steer clear of them.
Jacket-style BC's do a fine job of floating a diver vertically in the water because of the physical distance between their center of gravity and their center of buoyancy. Observe the simple sketch.
The black represents the diver's body, while the yellow represents the tank(s) that the diver typically carries on his/her back. The blue outline represents the air bladder, also called the "cell," and the red square represents the typical placement of weight on the diver, usually in weight pockets or a weight belt worn by the diver. The green dot represents the center of buoyancy or lift, while the orange dot represents the center of gravity. Notice that while the diver is vertical, the two are nearly in line with each other, and the diver can control whether he sinks or floats based on how much air he puts in the cell (or how much lift he creates). Obviously, this is a relatively effective system.
However, once underwater, most divers desire a prone or "horizontal" body position. There are many advantages to this position, including the ability to swim forward in an easy and efficient manner.
Unfortunately, jacket-style BC's do not allow a diver to easily stay in this position without constantly swimming or "sculling" with the hands and feet.
The reason is because the center of gravity and the center of buoyancy are so far apart. The forces at work - lift and gravity - tend to make this diver prone to becoming vertical constantly, reducing the diver's efficiency in the water. Reduced efficiency means that the diver has to work harder to stay correctly positioned, and therefore uses up his air or gas supply much faster than he would otherwise. Additionally, a diver who is constantly forced into a vertical position has the tendency to have his fins pointed downward, stirring up a soft muddy bottom and reducing visibility. In the worst case, an unskilled diver wearing jacket-style BC can do quite a bit of damage to the surrounding marine life, especially if he's diving over a delicate coral reef.
Back-inflate style BC's are a departure from the "norm" and are largely marketed as "more streamlined" or "more comfortable" because they remove the air cell from the chest of the diver. The advantages in doing this are numerous: The diver now has less material to push through the water when swimming. The diver cannot suffer from "squeeze," because there is no cell on the chest. With a reduced amount of material on the front of the diver, there is now a more comfortable "open" feeling associated with back-inflates, and there is more room to put more useful gear, such as lights and cameras.
Not all divers, however, are big fans of back-inflate style BC's. These are notorious for pushing the diver "face forward" at the surface, and therefore are not favored by many divers who spend a lot of time at the surface, such as instructors, students, or boat divers.
The following diagram shows this "face forward" phenomenon in action. At the surface (or while vertical) the diver's center of lift and center of gravity are poorly situated, and the resulting net force is a forward pitch.
Underwater, and in a horizontal position, the system does not work much better than a jacket-style BC. There is a large difference between the center of lift and the center of gravity.
To counteract this problem in both of these styles of BC, many of today's manufacturers are selling their BC's with "trim pockets" placed behind the diver and on either side of the tank. this is a dramatic improvement to the systems, and with the correct weighting, a diver's center of lift and center of gravity can be much closer to each other, reducing any unpleasant effects from what I have termed as "opinionated" BC's.
A radical departure from either of these two BC's is simultaneously the newest and oldest BC offered on the market today. Divers from decades ago will remember that there was a time when a "BC" consisted of a simple metal plate affixed to a scuba tank, which was simply strapped to the body of the diver. Sometimes a diver had an air cell attached, and sometimes he did not (in order to achieve simplicity, a diver would sometimes forgo a bulky air cell, which was commonly worn on the FRONT of the diver, in a "horse collar" style).
What makes this type of BC "new" is the concept of taking the cell and moving it behind the diver, around the tank rather than around the diver.
This has a very interesting effect on the center of gravity of the diver and the center of lift of the diver... It brings them very close together, to the point of being nearly the same. Experienced divers use tanks of different weights, placement of weights (the backplate itself weighing typically six pounds, and perfectly balanced on the back, bringing the center of gravity up and back on the diver) and a variety of accessories to balance and trim themselves skillfully, making a BC that is not only effective, but very trimmed and "unopinionated," no matter what position the diver chooses.
Backplates and wings have other advantages as well.
Since they are modular, a diver can completely customize the BC to include or exclude any feature he or she desires. They can choose from a variety of pockets, clips, D-rings, lights, and accessories; whatever they prefer in their own custom "rig." Backplates and wings (commonly referred to as "bp/wings") also typically utilize simple 2-inch (50mm) straps, proven to be comfortable, durable, streamlined, and easily replaced in the event of damage. They also pack up easily and simply, and clean off and dry out faster than BC's with thick, heavy padding.
It is argued by those divers who are opposed to bp/wings that they will "miss" the padding of a jacket-style or back-inflate style BC. While this may initially seem logical (especially if you simply try one on in a dive shop somewhere), almost always a diver dives with enough neoprene or insulation that it really is a non-issue above the water. Under the water, the lack of thick padding means that the rig feels stable and solid, and makes for a very enjoyable dive. In fact, of all of the BC's I tested, a bp/wing was by far the most secure and stable mount. The lack of thick padding also means that it does not absorb water (which would otherwise make for a very heavy rig when exiting the water).
There are other advantages as well... Typically, a bp/wing is considerably less expensive than the best jacket-style BC's or back-inflate style BC's, and there is much more flexibility in terms of the size of the cell. A diver who uses a bp/wing has the ability to choose large, powerful wings, should the dive require it, and then easily switch to smaller, simpler, less cumbersome wings when the situation allows it. A bp/wing also allows for a very easy switch between single tanks and double tanks. Lastly, since bp/wings are of such simple and rugged design (most of them consisting primarily of a stainless steel plate), the bp/wing can outlast any other BC on the market.
For these reasons, but especially because I liked the sheer trim and buoyancy performance of the bp/wing, I chose to purchase that rather than a jacket-style BC or a back-inflate style BC.
...And I would recommend it to other divers. Highly.
Last edited by Clare; December 10th, 2005 at 09:37 PM.