361 HYDROGRAPHIC MANUAL PaGE 276 



361. Danger of Surveying Shoals 



The survey of dangers and shoals is one of the most important phases of hydro- 

 graphic surveying, but it is also by far the most dangerous, and where this constitutes 

 a considerable portion of the work the hydrographer must beware of an overconfidence 

 which may lead him to take unnecessary risks with the survey vessel. Whether a 

 search is being made, a least depth being determined, or the existence of a reported 

 shoal being disproved, there is always the danger that the survey vessel may strike or 

 ground and be damaged or sunk with possible loss of life. 



In the survey of dangers and shoals, then, the hydrographer must always be alert 

 to the potential danger. Areas where shoals are known or suspected to exist should be 

 surveyed from launches to where a junction can be made safely from the survey ship. 

 The ship should not be endangered just because a launch is not available at the time. 

 Whatever vessel is used to survey in the vicmity of a potential danger, it must be run 

 at a conservative slow speed whenever there is risk of striking bottom or grounding. 



Safe passing distances cannot be prescribed for survey ships, or launches, when 

 surveying in the vicinity of a danger, because of the great variety of conditions and 

 circumstances which may arise. Each case must be decided on its own merits by the 

 Commanding Officer, or the hydrographer, who may be expected to have gained suf- 

 ficient experience to exercise his own judgment. In general, shoals must be surveyed 

 and developed, but property and life should not be unnecessarily risked to do so. No 

 vessel should ever be endangered when the duty can be adequately performed from a 

 smaller one; no potential danger should be approached or surveyed from a ship when 

 a launch can be used; and no launch should be risked when a whaleboat or pulling 

 boat can be used. 



When surveying a potentially dangerous area from a lamich the same precautions 

 should be taken as when surveying in the vicinity of one from a ship (see 1581). 



When surveying in the vicinity of a known danger or shoal, the hydrographer 

 must beware of overconfidence arising from the fact that he has a position of it plotted 

 on his sheet and that a previous depth was obtained on it. He must not endanger his 

 vessel by surveying too close to a danger, trusting to the fact that the position of the 

 vessel plots at what is apparently a safe distance from it. Any one of a number of 

 factors, or a combination of them, may endanger the survey vessel, although the 

 hydrographer believes he is keeping well clear of the danger. 



Before approaching or surveying in the vicinity of a potential danger, the hydrog- 

 rapher should review all the facts and make a careful study of all depth curves and all 

 previous soundings in the vicinity and take into account all possibilities. Some of 

 the factors that may contribute to a vessel being endangered when in the vicinity of a 

 known shoal are the following: 



(a) Although the danger is visible, other submerged dajigers may be in the immediate vicinity. 

 (6) Although a danger has been previously reported as visible, or indicated by breakers, tide rips, 

 or kelp, it may not be visible at the time approached, owing to a difference in tide, current, or sea. 



(c) There is likely to be less water than the least depth previously reported. 



(d) Abnormal currents are likely to be found in the vicinity of a shoal, which may set the survey 

 vessel toward the danger between fixed positions. 



(e) The known position may be in error; the danger may have been charted from a report, from 

 a reconnaissance survey, or from a survey controlled by less accurate methods than are now available. 



(/) The position may have been transferred wrong, due to an error in plotting, a change in 

 datum, or a misinterpretation of the data. 



