SUBSTANCE KCAL/HOUR Air 2.3 Foam neoprene 4.6 Wool 8.0 Helium 12.2 Seawater 52.0 Diving Physics TABLE 3-2. EXAMPLES OF HEAT CONDUCTIVITY THROUGH A 1CM divers is from the sun–on the beach after the dive rather than in the water. Propulsion, Drag, and Trim Water is a dense medium and presents several forces on the diver moving through it. We have already discussed buoyancy, the upward force caused by the displaced water, and the downward force of the diver’s own weight. The diver is able to move forward by the force of the thrust of the kick. The fins are able to provide this thrust because they are effective against the denser medium of the water. Two other forces which affect the diver are drag, the force of resistance to your movement through the water, and lift, the upward or downward force that results from the resistance of the water when you swim with your body at an angle to your direction of movement. Drag and lift can be significant, especially in divers who are overweighted or have poorer diving skills. Drag is the result of the friction of the water. It acts Chapter 3- Diving Physics backwards in a direction opposite to your direction of travel. The thrust of your kick must overcome the resisting drag. The amount of drag is directly related to the cross-sectional area you present as you move through the water. The greater the cross section presented, the greater the force resisting your forward propulsion. If you swim with your body lined up with your direction of travel, with your arms against your body, and with no “danglies,” you present minimal surface area to the water, and drag is low. However, if you swim at a head up or head down angle in the water or wave your arms, or carry bulky equipment, you present a proportionally greater surface area, increasing the drag and the amount of energy you must expend to swim. Drag generally increases by the square of the area presented. Thus if a proper horizontal swimming position reduces your frontal area by half, your expended energy requirement will be reduced four-fold. Drag is also proportional to the square of your speed. Doubling your swimming speed increases resistance to your movement four-fold. Your body attitude in the water (head up, level, head down) is referred to as your trim and is usually a function of your buoyancy and weighting. An overweighted diver must to swim at an upward angle to overcome gravity and remain at the desired depth. The overweighted diver swimming at a head up angle is said to have a positive “angle of attack.” An underweighted diver who must swim at a head down angle to keep from bobbing to the surface is said to have a negative angle of attack. In addition to the energy needed to swim against the force of buoyancy or gravity as well as the extra work required to overcome the increased drag, the diver with a poor angle of attack is generating lift. The lift is an upward or downward force tending to push the diver off the intended direction of travel. Good diving habits and diving skills can do much to reduce the energy you expend in diving to counter undesirable forces you create. Diving neutrally buoyant, without excess weighting, and moving slowly with good streamlining will greatly enhance your diving comfort and safety. And you will be much less worn out at the end of the dive. The water environment introduces you to an entire- 101 THICK SLAB OF 1M2 OF MATERIAL Buoyancy Weight Lift Drag Thrust Direction of Travel FIGURE 3-14. THE FORCES ON A DIVER
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