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The stability and static trim of a scuba diver affect the convenience and safety of the diver both at the surface and under water during the dive. Underwater trim is at approximately neutral buoyancy, but surface trim may be at significant positive buoyancy. Surface supplied divers may also find trim important if they operate at neutral buoyancy. ==Surface trim== When the buoyancy compensator of a scuba diver is inflated at the surface to provide positive buoyancy, the positions of the centre of buoyancy and centre of gravity of the diver are generally different. The vertical and horizontal separation of these centroids will determine the static trim of the diver at the surface. The diver can usually overcome the trimming moment of buoyancy, but this requires constant directed effort, albeit usually not a great deal of effort. This allows a conscious diver to adjust trim to suit the circumstances such as the choice between swimming face down or face up, or remaining vertical for best field of view or visibility. The position of the diver's centre of gravity is determined by the distribution of weight, which is in most cases determined by the actual equipment in use. Trim of the centre of gravity is usually achieved by selection of cylinder material and positioning of main ballast weights and trim weights. Aluminium cylinders are generally less negatively buoyant than steel cylinders of equivalent capacity, and high pressure cylinders more negative than low pressure cylinders of the same material. Similarly, much of the diver's buoyancy is determined by the equipment in use, particularly the buoyancy compensator, which can significantly influence centre of buoyancy shifts as it is inflated and deflated. Stable trim implies that the centre of buoyancy is directly above the centre of gravity. Any horizontal offset will generate a moment which will rotate the diver until the equilibrium condition is restored. Several cases are possible for an upright diver at the surface. This attitude is stable only when the centre of buoyancy is nearer the head then the centre of gravity, and on the same vertical line. Otherwise the diver will tend to rotate forwards or backwards until the centre of buoyancy is directly above the centre of gravity. In almost all cases the centre of buoyancy is nearer the head than the centre of gravity, and buoyancy compensators are all designed to provide this as the default condition, as an inverted diver floating at the surface is at risk of drowning. The lateral offset of centre of buoyancy from centre of gravity is generally insignificant unless the diver has been weighted asymmetrically between the sides, which is unusual, and seldom intentional, though it can occur when weights are ditched or lost from one side only. The offset in the forward/backward axis is quite frequently significant, and is usually the dominant factor in determining static trim attitude. At the surface, it is generally undesirable to be trimmed strongly face down, but it is useful to be able to trim face down at will. Vertical trim is acceptable providing it can be overcome for swimming There can be a conflict between the requirements for good surface trim and large reserve of buoyancy, particularly with back inflation systems, where a large volume is more likely to move the centre of buoyancy further back than the centre of gravity, and moving the centre of gravity further back by shifting weights may compromise trim stability at neutral buoyancy. There is also a conflict between the requirements for a lifejacket, which must keep the wearer face up and afloat even when unconscious, and a buoyancy compensator, which should allow the diver freedom to control trim attitude both underwater and floating at the surface. Diving buoyancy compensators are generally labelled with a warning that they are not a substitute for life jackets. Ditching weights at the surface will move the centre of gravity slightly further back if the weights were carried forward of the centre of gravity, and in almost all cases will result in a stable condition where the diver is more horizontal in the water. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diver trim」の詳細全文を読む スポンサード リンク
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