INTRODUCTION 



A primary function of the U. S. Naval Oceanographic Office is 

 the improvement of its products used for navigating safely the vessels 

 of the United States Navy, the mercantile marine, and others engaged 

 in water-borne endeavors. The fulfillment of this function is a con- 

 tinuing and ever expanding task as new knowledge of the worlds of 

 inner and outer space is acquired. 



The era of modern navigation is generally considered to have 

 been ushered in by Captain James Cook of the British Royal Navy 

 during his three historic voyages of discovery into the Pacific Ocean 

 between 1768 and 1779. This new era fostered the first steps in the 

 transition of navigation from an art to a science. Today, with nuclear- 

 powered vessels laden with highly sophisticated electronic navigation 

 equipment and with experimental positioning satellites orbiting 

 brightly in outer space, one might be tempted to say the transition is 

 all but complete. Yet, we are all aware that the toll taken by 

 groundings, collisions, and other marine casualties continues to rise 

 in spite of all our electronic gadgetry. It thus appears that some of 

 the old art, the seamen's sense, is still very much needed by twentieth 

 century mariners. 



There are many excellent works on navigation principles and 

 techniques, such as the American Practical Navigator (Bowditch), 

 where the navigator can find complete descriptions and solutions to 

 about every known system of navigation. All too often, however, 

 many of the little hints making up part of the art are buried so deeply 

 within the text that they are overlooked. This article explores a few 

 of these hints in an effort to stir new interest in them or even, perhaps, 

 bring them to light for the first time. 



DEPTHS-SHOALS-CURRENTS 



The most important features of any chart are the soundings and 

 depth curves by which the main characteristics of the bottom con- 

 figuration are represented. The origin of the hand lead, the oldest 

 known means of measuring depths, is lost in antiquity, but mention 

 of soundings have been found in Egyptian records dating back several 

 millenniums before Christ. An interesting account of the use of 

 soundings and the danger they foretold is found in the twenty-seventh 

 chapter of Acts of the Apostles. 



The present day mariner faces many of the same problems of 

 his ancient predecessors when navigating off a strange and relatively 

 unsurveyed coast. As the lead or echo sounder can only give the 

 depth under or near the keel, there is no way to forecast the depth 

 ahead for any substantial distance. 



NAVIGATIONAL 

 HINTS 



By J. N. Spinning 



Maritime Safely Division 



U. S. Naval Oceanographic Office 



With this point in mind, the navigator should carefully examine 

 each chart he uses. If there are no fathom lines shown on the chart, 

 it is probably due to insufficient data or a highly irregular bottom 

 relief. A coast so shown should be given a wide berth. The same 

 caution is to be exercised in areas where only isolated soundings, 

 especially when those marked "doubtful sounding" or those enclosed 

 in a dotted ring are shown. Invariably, the rule to be followed is: — 

 consider a coast to be foul unless it is shown to be clear. The only posi- 

 tive way of determining that every pinnacle and obstruction has been 

 found in an area is for survey vessels to wire-drag the area to a 

 predetermined depth. Unfortunately, very few areas of the world 

 have been surveyed so carefully. 



What constitutes a safe sounding will vary with different vessels, 

 so that no hard and fast rule can be laid down. Generally, however, 

 when only .scattered sounding data are shown on the chart, partic- 

 ularly along rocky coastlines, 10 fathoms should be regarded as a 

 caution against the possible near approach to shallower water. When 

 operating off a coast known to be well surveyed or when navigating 

 inshore waters, as a useful aid , sketch in red ink or pencil a depth 

 curve on the chart somewhat greater than the maximum possible draft 

 of the vessel. This will provide the navigator with a meaningful 

 danger line. The 10-fathom curve, if charted, can be traced with 

 blue ink or pencil, or sketched in, and serve as a warning line. 



Charts made from surveys conducted by the Oceanographic 

 Office are reduced to a plane of reference, with due regard to tides, 

 which present the hydrography in its least favorable aspect. The 

 datum planes most often used are mean low water, mean low water 

 springs, and mean lower low water. The datum of charts based upon 

 those of other nations is that of the original authority. 



Even with the charted soundings reduced to the lowest practi- 

 cable plane of reference for the area, it should be remembered that 

 local conditions will at times cause the actual water level to be lower 

 than the chart datum. 



A change in wave formation is often an indication of shoaling as 

 waves close up and heighten when running from deep to shoal water. 

 A deeply laden vessel, especially during heavy weather, should, when 

 possible, avoid transiting areas of abrupt changes in depths, as the 

 seas running from the deeper water will follow the bottom rise and 

 become sharper. 



In tidal estuaries, without marked irregularities of bottom, the 

 maximum current velocity will occur at about half tide. The surface 

 current is usually greater than that near the bottom, a condition 

 which may enter into the navigation calculations of light and deep 

 draft vessels. 



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