NAUI Master Scuba Diver 196 Navigation for Divers pass toward the object, then rotate the bezel until zero degrees is aligned with the needle. The bearing at the point of the lubber line is the bearing to the object. Determining a bearing with a card direct reading compass also involves aiming the lubber line toward an object. The reading on the card at the point of the lubber line is the bearing to the object. To establish a bearing to an object with an indirect reading compass, simply point the lubber line toward the object and the bearing to the object will be indicated by the compass needle. The bearing can be set simply by aligning the index marks with the needle. The accuracy of any type of diving compass is improved with the use of a compass board. This is a board on which the compass is mounted and which extends the length of the lubber line. Another type of compass that is useful for above-water navigation by divers is the hand bearing compass. This is a special compass designed for taking accurate bearings. This compass is useful primarily for dive site relocation. A maneuvering board is an item of navigational equipment that is useful when precise navigation is required, as in underwater mapping. The diver plots direction and distance on the board and can then use the board to determine the bearing to the point of origin at any time. An underwater timer or dive watch is a useful piece of navigational equipment. When time and speed are known, distance can be calculated, or when time and distance are known, speed can be determined. Measured lines are frequently used by diving navigators, especially when distances must be measured exactly. The lines are usually marked at regular intervals and are stored on a reel for ease of use and to prevent tangling (figure 7-3). Underwater navigation is similar to celestial navigation because you must also take depth into consideration, which is the equivalent of altitude for an aviator. Underwater navigation, therefore, requires the use of a depth gauge. Finally, diving navigation can be enhanced by the use of charts, which are the aquatic equivalent of a map. A chart depicts the coast and the offshore bottom depths. It will also include features such as islands, shipwrecks, and submerged objects. Measuring Distance Underwater Divers must be able to measure distance reasonably well under water in order to navigate accurately. Distance measuring may be accomplished in a number of ways, ranging from rough approximations to extremely precise measurements. Approximations of distance can be achieved with cylinder pressure readings at a constant depth. If a dive team uses 35 bar (500 psi) of air while swimming in a given direction, the return distance will be about the same for 35 bar (500 psi) of air, provided the depth and swimming speed remain constant. Currents can interfere with this process. Kick cycles can also be used to approximate distance. The cycle of a kick is defined as one complete kick of both legs and is usually counted when one leg reaches the top of the kicking motion. A diver needs to know the distance traveled with each kick cycle when swimming at normal speed. This can be determined by counting the number of kick cycles required to swim a known distance and then dividing the number of kick cycles into the distance. For example, if 30 kick cycles are required to swim 30 m (100 ft), the distance covered with each kick cycle is about 1 m (3.3 ft). Time and speed can also be used to measure distance. You should know how many seconds are required to swim a given distance under water at a constant depth and an easily maintained speed. This information can then be used in conjunction with time to determine a distance covered. If 80 seconds are required to swim 30 m (100 ft), the following formula may be used to determine your swimming speed: D = S 30 m (100 ft) T 80 sec. = 0.4 m (1.25 ft) per sec. Where S = Speed, T = Time, and D = Distance. When speed and time are known, distance may be calculated. For example, if you swim for 140 seconds at normal speed, the distance may be determined with the
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