Look around any dive boat these days, and you’ll see a great deal of variety when it comes to equipment configurations, ranging from traditional jacket-style BCDs to closed-circuit rebreathers. The reason is simple: Today’s divers have a wealth of choices that give them the ability to customize their gear configuration for maximum comfort, safety and utility. One option that is increasingly popular among more experienced divers is the backplate buoyancy system.
Backplate systems consist of three basic components: a rigid (usually metal) plate, a harness system and a back-inflation air cell. These components can be mixed and matched to fit a diver’s unique body shape or customized to the needs of the dive.
Florida cave diver Greg Flanagan has been credited with inventing the first backplate design in 1979. It was intended to prevent heavy twin cylinders from shifting during the dive, but the technical divers who adopted backplates found other advantages. In addition to rock-solid stability, simple harness systems left the diver’s chest unencumbered and, when incorporated with a back-inflating air cell, they provided a more horizontal swimming position while also providing ample lift without squeezing the diver. Of course, these are all benefits for recreational divers, too.
The important thing to consider when evaluating backplate systems is that they are endlessly customizable. No one size fits all divers, and no one configuration fits all dives. Thinking about a backplate system? Here are the basics to consider.
Typically made of stainless steel (although hard plastic and aluminum are options, too), the plate is designed to hold cylinders securely to the diver’s back. Holes and slots cut into the plate allow for a variety of tank bands, harnesses and attachment points for accessories. Stainless-steel plates, which provide about 6 pounds of negative buoyancy, are preferred by coldwater divers to help offset the positive buoyancy of drysuits and undergarments. Plastic and aluminum are often the preferred choice for traveling divers facing airline weight restrictions.
For divers considering a backplate system, it’s important to try on the plate for size. The base of the plate should be even with the diver’s waist and the top of the plate parallel with the top of the shoulder blades. This is a basic suggestion as various body shapes can alter the sizing. The most important thing is the diver’s comfort. The length of a standard plate works well for most men; however, a woman’s torso length is shorter compared with a man of equal height. A standard plate can be too long and press on the top of the buttocks, which is uncomfortable and interferes with the diver’s fin kick. Smaller-length plates, which are now on the market, address this issue. Typically, they are made of aluminum since a diver needing this size generally does not need added ballast.
The original backplate systems featured a harness made from a single piece of 2-inch webbing routed through the notches of the plate to form shoulder straps and ending in a belt cinched at the diver’s waist. Most divers also add a crotch strap to keep heavy tanks from shifting forward when swimming in a head-down position.
In the technical market, this is commonly referred to as a “Hogarthian Rig,” a reference to Bill Hogarth Main, the technical diver who popularized this simple and efficient configuration. Manufacturers also offer prebuilt harnesses that include options like padding, quick-release buckles and multiple adjustment points.
Back-Inflation Air Cell
The ability to swap air cells to fit the diver and meet the demands of a specific dive is one of the most useful features of backplate buoyancy systems. Often called wings, back-inflation air cells are available in a variety of sizes, with lift capacity ranging from less than 30 pounds to more than 100 pounds. They come in two different styles.
The original and still-popular horseshoe style extends from the top of the tank down along the sides, while newer “donut” styles wrap completely around the tank (or tanks). The advantage of the “donut” is the easy transfer of air from one side of the wing to the other in any swimming position. A diver using a horseshoe wing may find air shifting to only one side of the wing if he is descending and leaning to one side when adding air to offset negative buoyancy. The additional air will move only to the highest point of the wing. Restoring balance requires moving to a slightly upright position. Some horseshoe wings use bungee cords to help control air movement.
Divers new to the concept of backplates are often surprised to learn that the amount of lift is not the main feature to consider when selecting a wing. More important is the size of the wing. Single-tank divers generally need a wing with approximately 30 pounds of lift. A diver wearing a set of double 100-cubic-foot tanks will require a wing with approximately 60 pounds of lift. Even though the double tanks are not so negatively buoyant as to need this amount of lift, the width of these wings will position nicely along the edge of the tanks, enabling easier buoyancy control at depth and on the surface.
Individual body shape is the second feature to consider. A large, 6-foot-tall diver wearing double 80-cubic-foot tanks could use a wing with 60 pounds of lift and be very comfortable; however, an individual who stands 5 feet 2 inches wearing the same configuration may feel the gear is diving him!
The shorter diver will be dealing with two problems. The wing will extend too low (over the buttocks) and will shift the diver to a head-down position. The extra material will also create drag, leading to unnecessary work. If the wing is too short (above the kidneys), the diver will be in a head-up or standing position, causing him to drag his feet.
A properly sized wing should extend no further than the end of an individual diver’s torso. An overly large wing used on a single tank will wrap around the single tank when the diver is in the horizontal position. This “taco” effect will create a space for air that will not vent easily. Surface use of an oversized wing will force the diver into a face-down position if the wing is fully inflated. An undersized wing, on the other hand, may mean the diver is unable to ascend easily from depth or float comfortably on the surface.
Pros and Cons of Backplate Systems
So why use a backplate system? Maintaining trim and a horizontal swimming position is easier with back inflation and is the primary reason for selecting this style of dive gear. In a horizontal position, the diver’s fins are off the bottom, which reduces silting that can damage coral or other sessile marine organisms. The simple harness of a backplate system leaves the chest area unencumbered, and inflating the wing doesn’t create a feeling of squeeze. Women, in particular, have discovered that the versatility in plate sizes and the ability to rearrange straps to their torso shape allows for a better, custom fit.
The drawback in backplate systems is the tendency of back-inflation air cells to float the diver in the face-down position at the surface. This tendency is exaggerated if the diver is using a wing that is too long and/or is overinflated. Weights that are positioned in the front will also create this effect. Backplate divers will find swimming on their back is the most comfortable position at the surface.
© Alert Diver — Q2 Spring 2010