Two precautions

TM-55-1930-209-14P-17 Water Purification Barges (NSN 1930-01-234-2165) Miscellaneous Equipment Dayroom Workshop Access and Sanitation Systems Manual
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Your chart, and a quick calculation, tells you that if you maintain your course and speed, you will reach another fathom curve and the water will shoal to about 20-24 feet in about 10 minutes, and soon thereafter become dangerously shallow. You proceed cautiously, set your depth alarms, watching your depthfinder closely, and as soon as you see your depth changing from around 30 ft. to 23 ft., you alter course at once. The harbor you want is up the coast and as long as you follow this fathom curve, i.e., steer to stay within 20 to 24 ft. of water, you'll reach your harbor entrance safely. Simply adjust the shallow alarm to sound at a depth between two to four ft. shallower than the bottom contour you wish to follow. (see "Use the Shallow Alarm" section from the Operating Instructions in this manual). Then set the deep alarm to sound at a set point which is two to four ft. deeper than that contour line. Naturally, the actual set points of the alarms will vary with the sea conditions and wave heights. Then, steer as close a compass course as you are able; if the alarm sounds, note the depth from the LCD readout and steer shallower or deeper as required to follow the contour and quiet the alarm. Two precautions Before going on to position finding, two important factors must be emphasized again. Your Aqua-Probe digital, like all depthfinders, measures depth from the bottom of your transducer-not from the surface of the water, and not from the bottom of your keel. This difference, which is often a couple of feet or more; must be remembered and factored into your readings. The second factor is seasonal change on lakes and rivers, or tides in coastal areas. Since your digital depthfinder makes no allowance for the season of the year (thaws, rains, etc.), or tides that change water depths every hour, these must be factored in as well. Your chart gives the reference plane-or "datum"-of its charted soundings. Any basic book on piloting explains how to use tables to calculate the height of tide at any given hour at any given location. It is then a simple matter to modify the "depth of water" readings given on your Aqua-Prove digital so that they correspond to the "depths of soundings" given on your chart. Position by line of soundings Suppose visibility is bad and you're navigating by "dead reckoning." You know your compass course, estimated speed over the bottom, and are making allowances for current and/or wind. How can your digital depthfinder tell you almost precisely where you are? The procedure is to establish a line of position by soundings. You study your chart to find promising bottom terrain ahead-that is, you look for fathom curves that lie ahead of your estimated course or "track." You then maintain course and speed and make a series of soundings (corrected for tide) at regularly timed intervals-say 5 minutes apart. You record six or more of these soundings and plot them on a transparent sheet (clear plastic or tracing paper) with the distance between each sounding matching boat speed and scale of chart. You then move the sheet around on your chart until your plots are aligned with the chart soundings. Assuming they closely agree, your last sounding along the line of position can be considered your "fix". It goes without saying that soundings on a flat bottom are worthless in determining position, and that the more pronounced the unevenness of the bottom, the more reliable your "fix." It is also obvious that you have to be reasonably sure of your speed over the ground, and that this position finding method should be planned a bit in advance and practiced in non-emergency situations-i.e., with knowledge of exactly where you are. Negative information Your digital depthfinder can tell you where you are; and, it can tell you where you are not. Suppose, again, that you're navigating by "dead reckoning" and you have an "estimated position." If your digital depthfinder agrees with the charted depth at this position you may or may not be right where you think you are (depending on whether or not there are other areas of equal depth in the vicinity). If, on the other hand, 17

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