Buoyancy Compensator (diving) - Orientation in The Water

Orientation in The Water

The vertical-horizontal orientation, or trim, of the submerged diver is influenced by the BC and by other buoyancy and weight components and contributed to by the diver's body, clothing and equipment. The diver typically wishes to be positioned nearly horizontally (prone) while under water, to be able to see and swim usefully, but more nearly vertical and perhaps partly supine, to be able to breathe without a regulator when on the surface.

The orientation of a static and stable object in water, such as a diver, is determined by its center of buoyancy and its center of mass. At equilibrium, they will be lined up under gravity with the center of buoyancy vertically on a line with, (and preferably above) the center of mass. The diver's overall buoyancy and center of buoyancy can routinely be adjusted by altering the volume of the gas in the BC, lungs and diving suit. The diver's mass on a typical dive does not generally change by what seems like much (see above—a typical dive-resort "aluminum 80" tank at 200 bar contains about 2.8 kg (~6 lbs) of air or nitrox, of which about 2.3 kg (~5 lbs) is typically used in a dive, although any air spaces such as in the BC and in diving suits will expand and shrink with depth pressure. Large changes in buoyancy are of course possible if the weight belt is jettisoned, or a heavy object is picked up.

Generally, the diver has minimal control of the relative position of the center of buoyancy in the BC during a dive, but only its quantity. However the diver can change the buoyancy center by control of his equipment setup, which includes its configuration and weighting locations, which ultimately influence where his effective BC lift is positioned relative to his Center of Gravity.

Traditionally, weight belts or weight systems are worn with the weights on, or close to, the waist and are arranged with a quick release mechanism to allow them to be quickly jettisoned to provide extra buoyancy in an emergency, such belts have the advantage of being able to be shifted fore and aft so as to change the diver's center of mass. BCD systems that integrate the weights into the BCD, can provide added comfort so long as the BCD does not have to be removed from the body of the diver, for example in an underwater emergency such as an entanglement. When a weight integrated BCD is removed on the surface a diver wearing no weight-belt and any type of foamed neoprene wetsuit will remain very buoyant.

By inflating the BC at the surface, the conscious diver may be able to easily float face-up, depending on his equipment configuration choices. A fatigued or unconscious diver can be made to float face up on the surface by adjustment of their buoyancy and weights so the buoyancy raises the top and front of the diver's body and the weights act on the lower and back of the body. An inflated Horsecollar always provides this orientation, but an inflated Vest, and all styles of BP/w generally float the diver face-down because the center of buoyancy is too far from the diver's front. This floating orientation is generally considered undesirable and can be minimized pre-dive by relocation of some of the weights (perhaps of quick release type) further to the rear (such as in pockets close to the diver's cylinder), and avoiding the use of aluminum tanks and using higher density cylinders (typically steel), which similarly moves the center of mass to be further behind the diver, and thus, behind the center of buoyancy. The BC type can also be selected with this factor in mind, selecting a style that moves the center of buoyancy forward, as this accomplishes the same net effect. Any or all of these options can be utilized to trim the system out to its desired characteristics and many factors can contribute, such as the number and position of diving cylinders, the type of diving suit, the position and size of stage cylinders, the size and shape of the diver's body and the wearing of ankle weights, or additional dive equipment. Each of these influence a diver's preferred orientation in the water (horizontal) and on the surface (vertical) to some degree.