NAVIGATIONAL HINTS 



it is often impractical to definitely establish whether the cause was 

 external to the vessel. Therefore, only the most probable of these 

 reports are shown on the charts subject to later verification or 

 disproval. 



Since magnetic force diminishes rapidly with distance, a mag- 

 netic center in the visible land would have to be of unprecedented 

 intensity to affect the compass of a vessel 1 2 mile from it. Mariners 

 may note a temporary deflection of the compass when very close to 

 another vessel, a large mass of iron or steel or when passing over a 

 wreck in shallow water. The influence radius in such cases, however, 

 will be very small. If the compass continues to show erratic behav- 

 ior the cause is probably within the vessel itself. In most cases, the 

 trouble is attributable to some source of artificial disturbing influence, 

 such as swinging booms, change in location of iron or steel gear near 

 the compass, or defective electrical wiring in the bridge area. The 

 ordinary phenomena of static electricity will not cause any noticeable 

 deflection of the compass. Severe magnetic storms, often associ- 

 ated with sun spots or auroral displays, cause no more than a degree 

 or so of deflection. Flashes of lightning, however, have been 

 known to derange many compasses to the point of requiring complete 

 readjustment. 



Regardless of how well the gyro is operating or how well adjusted 

 the compass may be, routine observations, to detect abnormal devi- 

 ation, should be made once every watch and after a course change 

 of 15° or more. 



RADIO DIRECTION FINDERS 



Due to the reliance placed on radio bearings in fixing a vessel's 

 position, especially during periods of low visibility when celestial 

 observations are unobtainable, the shipboard radio direction finder 

 deserves the same care and consideration the navigator gives his 

 sextant and the compass. Like these instruments, the direction 

 finder has certain errors which can be minimized greatly by the skill 

 and sound judgment of the operator. In order to obtain the maximum 

 built-in potential of the equipment, it is essential that the navigator 

 take every opportunity available to use the direction finder in good 

 weather when results can be checked by other means. By so doing, 

 the reliability of fixes obtained during adverse conditions can be more 

 accurately ascertained. 



Discussions on radio direction finders often refer to the effect of 

 coastal refraction (land effect) stating that errors may be expected 

 when radio bearings are taken by ships so located that the line of 

 sight to the radiobeacon passes over land or along the shore. Exten- 

 sive observations, however, seem to indicate that when the vessel is 

 well off the shore this error is negligible. Bearings secured entirely 

 over water areas are, of course, more desirable as any question of the 

 coastal refraction error is thus eliminated. 



Radio bearings taken on commercial entertainment broadcasting 

 stations, on the other hand, should be viewed with extreme caution. 

 The mariner must consider that the operating frequencies of such 

 stations (550 to 1600 kcs), as compared with marine or aeronautical 

 radiobeacons which operate well below 550 kcs, will require materially 

 different calibration curves or compensation adjustment. Several 

 other factors affecting reliability of bearings taken on such stations 

 are: — the published position of the station may be that of the studio 

 and not the transmitter site; the position coordinates have not been 

 adjusted to the datum of the nautical chart being used; the trans- 

 mitter may be located well inland, causing excessive coastal refraction 

 error; and that the station may be synchronized with other stations, 

 making identification of the transmitter impossible. 



The navigator, considering all the foregoing limitations, may 

 still be able to use certain commercial stations to good advantage. 

 This can only be determined by carefully checking positions obtained 

 from an individual station when the vessel's position is accurately 

 known by other means. 



The radio direction finder, like the compass, should be checked 

 for deviation after changes in its surroundings have taken place and 

 on a routine basis. It should be remembered that the compensated 

 sets are just as vulnerable to changes in the position of ship's gear as 

 are the non-compensated sets. 



Each year the Oceanographic Office receives many inquiries 

 asking why a certain aeronautical radiobeacon or light is not charted 

 when it can be heard or seen for many miles at sea. 



The Office welcomes all such inquiries and thoroughly investi- 

 gates each one. The mariner should bear in mind, however, that 

 these aids are placed for the maximum use of aircraft and not for 



surface vessels, a factor which must be carefully considered before 

 placing these aids on a nautical chart. Many aeronautical radio 

 aids and lights are moved from one area to another as seasonal wind 

 and weather patterns change. The fact that such a relocation has 

 taken place is not always made known immediately to maritime 

 interests. The aeronautical radio aids listed in H. 0. Publications 

 117A and 117B meet or surpass what the Oceanographic Office 

 considers the minimum requirements for safe surface use. The 

 aeronautical lights listed in the various volumes of Light Lists also 

 meet this criterion. 



SOUND SIGNAL CAUTION 



The whistle, horn and bell serve as the principle means of com- 

 munications for vessels to indicate or learn presence and intent or fact. 

 The transitory nature sound transmitted by these devices has a 

 significant bearing on their reliability as a navigational aid and a 

 communications link. As now used, the various coded signals 

 indicate not only presence but type of vessel (such as tug with tow, 

 sail, or power-driven) and nature of the vessel's activity (such as 

 underway, at anchor, backing down, or approaching a bend). There 

 is a great deal of evidence to indicate that the failure to correctly 

 hear or respond to sound signals is a major contributing factor in 

 ship collisions. 



A study of the testimony given following numerous collisions, 

 resulting in damage in excess of one million dollars, occurring in good 

 visibility under inland rules, reveals that the significant factor 

 bringing about many of these collisions was the watch officer's belief 

 that he had heard a signal other than that actually sounded. Inland 

 rules require not only establishment of intent, but also agreement by the 

 vessel signalled. With such a built-in safety factor in the rules, 

 collision must then be the result of either human failure or 

 overconfidence. 



Let us look at the cause of one collision; where the main ingre- 

 dient responsible has been experienced by almost every watch officer. 

 A vessel approaching another desired to take the starboard side of 

 the channel for a port-to-port passing situation and so indicated this 

 by sounding one short blast. The blast, however, amounted to little 

 more than a wisp of steam and a rather sick gasping cough barely 

 audible on the vessel's own forecastle head. The watch officer, real- 

 izing that the approaching vessel could not possibly have heard the 

 signal, sounded another short blast. This second blast was very 

 clear and audible. Unfortunately, the watch officer on the signalled 

 vessel observed the wisp of steam from the first attempt, but due to 

 noise on deck had concluded that he had just not heard it. Then, 

 both seeing and hearing the second short blast, he assumed that the 

 other vessel had sounded two short blasts, answered in kind, and 

 altered his course into a costly collision. 



Overconfidence in the old saying "seeing is believing" certainly 

 spoiled his day! The officer initiating the signals also lacked good 

 judgment in sounding the second blast so soon after his first unsuc- 

 cessful attempt. There appears to be a definite reluctance on the 

 part of many watch officers to sound the danger signal, as required 

 by the inland rules, when the intentions of the other vessel are in 

 doubt. This reluctance probably stems from the desire not to unduly 

 alarm the master, but the sounding of four or more short rapid blasts 

 to indicate uncertainty is much less alarming than maneuvering a 

 vessel on assumption and guesswork. 



RADAR LANDFALL 



One of the more hazardous situations confronting the mariner 

 involves the approach to land during poor visibility, especially after 

 several days of overcast weather conditions when sights of doubtful 

 value have been obtained and uncertain currents encountered. 



The situation is further aggravated when the approach course 

 makes a small angle with the coastline; where, due to depth of water, 

 soundings are of little avail, and there are offshore shoals and reefs to 

 be avoided. The mariner's main objective in this situation will be to 

 identify, without any doubt, some feature and determine the vessel's 

 position relative to it. 



Charts are constructed with the emphasis placed on depicting 

 the most prominent visual features for idlntification, such as con- 

 spicuous spires, domes, tanks, towers and so on. While these objects 

 may be excellent visual landmarks, they may be extremely poor radar 

 contacts, particularly if previous radar experience in the area is 

 lacking. 



356-691 O - 69 - 



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