Chapter 7- Navigation for Divers Navigation for Divers 201 more enjoyable. Not only that, if anything of note is discovered on a dive, you are confident that you can relocate it because you employed good navigational techniques throughout. At times it is easier to find a small reef or wreck by following a bearing from a known point on a larger reef than it is to try to descend directly onto the smaller site. NAVIGATIONAL PROBLEMS Two problems you will encounter in underwater navigation are obstacles and as previously noted, currents. An obstacle will interrupt your dive pattern or distance measuring. A current will affect your speed when swimming with or against the water movement and will cause leeway, which is side-slippage in the direction of a current when you swim across a current. The error produced by leeway can seriously affect a dive pattern or even a heading. An obstacle can be circumnavigated by modifying your dive pattern. By swimming perpendicular to your intended course while measuring the distance from your course until you are clear of an obstacle, you can then swim past the obstacle and swim a reciprocal course back to the original intended line as depicted in the illustration. This is one situation where effective distance measuring, such as kick cycles, is very useful. There are several ways in which to handle the effects of leeway underwater. First of all, navigate close to the bottom where the water movement is at its minimum. Next, navigate from object to object along a heading. If visibility is poor or no objects are available for visual reference, angle yourself in the direction of the current to compensate for the leeway. Practice and experience will enable you to make reasonable estimations on how much to compensate for side-slippage in a current (figure 7-10). It is possible to measure the strength of a current and to calculate complex vectors to navigate accurately in currents using trigonometry and other means. One method used to measure a current is to measure the time required for a floating object to travel a known distance. For example, if you measure the time in minutes that it takes for an object to travel 50 m (165 ft) and multiply the result by 32, this gives the time it would take an object to travel 1.6 km (1 mi). If the measured time was two minutes: 2 min X 32 = 64 min 64 min X 1 hour = 1.06 hr 60 min 1.6 km (1 mi) = 1.5 kph (.94 mph) 1.06 hr Another method is to measure the time it takes an object to travel 30 m (100 ft). This distance traveled in 1 minute equals 1800 m (6000 ft) traveled in one hour, or about one knot (i.e., one nautical mile per hour). To swim to a fixed object across a current, you need to know the speed of the diver and the speed of the current. If the diver’s speed and the current are the same, then the ratio of the two is 1:1. If the diver is twice as fast as the current, then the ratio is 2:1. For example, if the diver swims at 1.6 kph (1 mph), he travels 0.4 km (0.25 mi) in a period of 15 minutes. If the diver swims perpendicular to a current which is also traveling at 1.6 kph (1 mph), he must start swimming upcurrent 0.4 km (0.25 mi) of his desired destination. USE OF CHARTS Charts can be useful for diving navigation. Charts contain information on depths, formations, wrecks, bottom composition, landmarks, bearings, distances, and more. Because divers typically cover very small distances in relation to those on charts, and since corrections for variation are required when using a chart, these coastal “maps” may not allow divers to navigate as accurately as they desire. Charts establish position and measure distance by means of latitude and longitude. You are probably familiar with the grid-like lines on a globe that extend around it horizontally at the equator and vertically from the poles. Latitude is the angular distance in degrees north or south of the equator, while longitude is the angular distance in degrees east or west of Greenwich, England. A line of longitude is also referred to as a
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