CHAPTER 26EMERGENCYNAVIGATIONINTRODUCTION2600.Planning For Emergency Navigationmethod.He should be ableto construct aplotting sheet withaprotractorand improvisea sextant.Forthenavigatorpre-With a complete set of emergency equipment, emer-pared with such knowledge the situation is never hopelessgency navigation differs little from traditional shipboardSome method of navigation is aways available.This wasrecently proven by a sailor who circumnavigated the earthnavigationroutine.Increasingrelianceoncomplexelec-using no instruments ofanykind,notevena compass.Basictronic systems has changed the perspectiveof emergencynavigation.Todayitismorelikelythatanavigatorwill suf-knowledgecansuffice.ferfailureofelectronicdevicesandbe left with littlemoreThemodern ship's regular navigationgear consists ofthanasextant withwhichtonavigatethanthathewill bemanycomplexelectronicsystems.Thoughtheymaypossesforced to navigate a lifeboat. In the event offailure or de-a limited backup power supply,most depend on an uninter-structionofelectronicsystems,navigationalequipmentandrupted supplyof electrical power.Thefailureof thatpowermethods may need tobe improvised.Theofficer who regu-due to hostile action, fire, or breakdown can instantly ren-larly navigates by blindly“filling in the blanks"or readingder the unprepared navigator helpless. This discussion isthe coordinatesfrom“black boxes"will notbeprepared tointended to provide the navigator withthe informationusebasicprinciplestoimprovisesolutionsinanneeded to navigate a vessel in the absence of the regularemergencysuite ofnavigation gear.Training and preparation for a nav-For offshore voyaging, the professional navigator mustigation emergency are essential. This should consist ofbecome thoroughlyfamiliar withthe theory of celestialregularpractice inthetechniques discussedherein whilethenavigation.He should be ableto identifythemost usefulregular navigation routine is in effect, so that confidence instars andknowhowto solvehis sights by any widely usedemergencyprocedures is establishedBASICTECHNIOUESOFEMERGENCYNAVIGATION2601.Emergency Navigation Kitseasons should be included.Plotting sheets areuseful but not essential if charts are available.The navigator should assemble akit containing equip-Universal plottingsheetsmaybepreferred,partic-ment for emergency navigation. Even with no expectationularly if the latitude coverage is large. Includeofdanger, it is good practiceto have sucha kitpermanentlymaneuvering boards andgraph paper.3. Plotting equipment. Pencils, erasers, a straight-locatedinthechartroomoronthebridgesothatitcanbequickly broken out if needed.It can be used on the bridgeedge, protractor or plotter, dividers and compasses,intheeventofdestructionorfailureofregularnavigationand aknife orpencil sharpenershould beincluded.systems, or taken to a lifeboat if the“abandon ship"call isAruleris also useful.made4.Timepiece.A good watch is needed iflongitude is toIf practical.full navigational equipment shouldbeprobe determined astronomically.It should be water-vided inthe emergencykit.As many as possible of theproof or kept in a waterproof container whichitems in thefollowing list should be includedpermits reading and winding ofthe watch if necessary without exposing it to the elements.The1.A notebook or journal suitable for use as a deck logoptimumtimepieceisa quartzcrystalchronometerbut any high-quality digital wristwatch will suffice ifandforperformingcomputationsit is synchronized with the ship's chronometer. A2Charts and plotting sheets. A pilot chart is ex-portable radio capable ofreceiving time signals,to-cellent for emergency use. It can be used forgether with a good wristwatch, will also suffice.plottingand asasourceof informationoncom-5.Sextant.A marine sextant should be included.Ifthispass variation, shipping lanes, currents, winds,is impractical,an inexpensive plastic sextantwill suf-andweather.Chartsforbothsummerandwinter379
379 CHAPTER 26 EMERGENCY NAVIGATION INTRODUCTION 2600. Planning For Emergency Navigation With a complete set of emergency equipment, emergency navigation differs little from traditional shipboard navigation routine. Increasing reliance on complex electronic systems has changed the perspective of emergency navigation. Today it is more likely that a navigator will suffer failure of electronic devices and be left with little more than a sextant with which to navigate than that he will be forced to navigate a lifeboat. In the event of failure or destruction of electronic systems, navigational equipment and methods may need to be improvised. The officer who regularly navigates by blindly “filling in the blanks” or reading the coordinates from “black boxes” will not be prepared to use basic principles to improvise solutions in an emergency. For offshore voyaging, the professional navigator must become thoroughly familiar with the theory of celestial navigation. He should be able to identify the most useful stars and know how to solve his sights by any widely used method. He should be able to construct a plotting sheet with a protractor and improvise a sextant. For the navigator prepared with such knowledge the situation is never hopeless. Some method of navigation is always available. This was recently proven by a sailor who circumnavigated the earth using no instruments of any kind, not even a compass. Basic knowledge can suffice. The modern ship’s regular navigation gear consists of many complex electronic systems. Though they may posses a limited backup power supply, most depend on an uninterrupted supply of electrical power. The failure of that power due to hostile action, fire, or breakdown can instantly render the unprepared navigator helpless. This discussion is intended to provide the navigator with the information needed to navigate a vessel in the absence of the regular suite of navigation gear. Training and preparation for a navigation emergency are essential. This should consist of regular practice in the techniques discussed herein while the regular navigation routine is in effect, so that confidence in emergency procedures is established. BASIC TECHNIQUES OF EMERGENCY NAVIGATION 2601. Emergency Navigation Kit The navigator should assemble a kit containing equipment for emergency navigation. Even with no expectation of danger, it is good practice to have such a kit permanently located in the chart room or on the bridge so that it can be quickly broken out if needed. It can be used on the bridge in the event of destruction or failure of regular navigation systems, or taken to a lifeboat if the “abandon ship” call is made. If practical, full navigational equipment should be provided in the emergency kit. As many as possible of the items in the following list should be included. 1. A notebook or journal suitable for use as a deck log and for performing computations. 2. Charts and plotting sheets. A pilot chart is excellent for emergency use. It can be used for plotting and as a source of information on compass variation, shipping lanes, currents, winds, and weather. Charts for both summer and winter seasons should be included. Plotting sheets are useful but not essential if charts are available. Universal plotting sheets may be preferred, particularly if the latitude coverage is large. Include maneuvering boards and graph paper. 3. Plotting equipment. Pencils, erasers, a straightedge, protractor or plotter, dividers and compasses, and a knife or pencil sharpener should be included. A ruler is also useful. 4. Timepiece. A good watch is needed if longitude is to be determined astronomically. It should be waterproof or kept in a waterproof container which permits reading and winding of the watch if necessary without exposing it to the elements. The optimum timepiece is a quartz crystal chronometer, but any high-quality digital wristwatch will suffice if it is synchronized with the ship’s chronometer. A portable radio capable of receiving time signals, together with a good wristwatch, will also suffice. 5. Sextant. A marine sextant should be included. If this is impractical, an inexpensive plastic sextant will suf-
380EMERGENCYNAVIGATIONupdated with a DR position will be adequate. But whenfice.Several types are availablecommercially.Theemergencysextantshouldbeusedperiodicallyinac-conflicting information or information ofquestionablereli-tualdailynavigationsoitslimitationsandcapabilitiesability is received, the navigator must determine an MPParefully understood.Plastic sextants havebeen usedWhen completepositional information is lacking,orsafely onextensive ocean voyages.Do not hesitate towhentheavailable information is questionable,the mostuse them in an emergency.probable position mightbedetermined fromtheintersec-tionof a single line of position anda DR, from a line of6.Almanac.A currentNautical Almanac containssoundings, from lines of position which are somewhat in-ephemeral data and concise sight reduction tables.Anothervear'salmanaccanbeusedforstarsandconsistent, or from a dead reckoning position with athe sun without serious error by emergency stan-correction for current or wind.Continuea dead reckoningdards.Someform of long-term almanacmight beplotfromonefixtoanotherbecausetheDRplotoftenpro-vides the best estimate of theMPP.copied or pasted in the notebook7.Tables.Someformof tablewill be neededfor re-A series of estimated positions may not be consistentducing celestial observations.The Nauticalbecause of thecontinual revisionof the estimateas addi-Almanac produced by the U.S.Naval Observatorytional information is received.However,it isgood practicetoplotallMPP's,and sometimestomaintaina separateEPcontains detailed procedures forcalculator sight re-duction and a compact sight reduction table.plot based upon thebest estimateoftrack and speed madegood over the ground.This could indicate whether the8Compass.Each lifeboat must carry a magneticpresent course is a safe one.See Chapter 23.compass.For shipboard use,makea deviationtablefor each compass with magnetic material in its nor-2603.Plotting Sheetsmal place.The accuracy of each table should becheckedperiodicallyIf plotting sheets are not availablea Mercator plotting9.Flashlight.A flashlight isrequired in each lifeboat.Check the batteries periodically and include extrasheet can be constructed through either of two alternativemethods based upon agraphical solution ofthe secantofthebatteriesandbulbsinthekitlatitude,which approximates the expansion of latitude10.Portableradio.Atransmitting-receiving set ap-provedbytheFederalCommunicationsFirstmethod (Figure 2603a)Commissionforemergencyusecanestablishcom-munications with rescueauthorities.A smallStep one.Drawa series of equally spacedverticalportable radio may be used as a radio directionlines at any spacing desired.These arefinder or for receiving time signals.the meridians; label them at any desired11.An Emergency Position Indicating Radiobeaconinterval, suchas1,2',5',10',30,1etc.(EPIRB)is essential.When activated, this deviceSteptwo.Drawand label a horizontal line throughemits a signal which will be picked up by theCOSPAS/SARSAT satellitesystem and automati-thecenter of the sheetto represent theparallel of the mid-latitude of the area.cally relayed to a ground station. It is then routedStep three.Through any convenientpoint,suchasdirectlyto rescueauthorities.Thelocation of thedistress can be determined very accurately.De-the intersection of the central meridianand theparallel ofthemid-latitude,drawpendingon thetypeofEPIRB,thesignalmayevenidentify the individual vessel in distress, thus al-a line making an angle with the horizon-lowingrescuerstodeterminehowmanypeoplearetal equal to the mid-latitude. In Figurein danger, the type of emergency gear they may2603athisangleis35°have,and otherfacts to aid in therescue.BecauseStep four.Draw in and label additional parallels.of this system,thenavigator must question thewis-The length of the oblique line betweendom of navigating away from the scene of themeridians is the perpendicular distancedistress.It may well be easier for rescue forces tobetween parallels,as shown by the bro-find him if heremains inoneplace.SeeChapter28ken arc.The number of minutes of arcThe Global MaritimeDistress and Safety Systembetween parallels is the same as that be-(GMDSS).tweenthemeridiansStep five.Graduate the oblique line into conve-2602.MostProbablePositionnient units. If I' is selected, this scaleservesasbothalatitudeandmilescale.ItInthe eventoffailureof primaryelectronic navigationcan also be used as a longitude scale bymeasuring horizontallyfrom ameridiansystems,thenavigatormayneed to establishthemostprobable position (MPP)of the vessel.Usually there isinstead of obliquelyalong the lineThe meridians may be shown at the desired interval and theusuallylittledoubtas to theposition.Themost recent fix
380 EMERGENCY NAVIGATION fice. Several types are available commercially. The emergency sextant should be used periodically in actual daily navigation so its limitations and capabilities are fully understood. Plastic sextants have been used safely on extensive ocean voyages. Do not hesitate to use them in an emergency. 6. Almanac. A current Nautical Almanac contains ephemeral data and concise sight reduction tables. Another year’s almanac can be used for stars and the sun without serious error by emergency standards. Some form of long-term almanac might be copied or pasted in the notebook. 7. Tables. Some form of table will be needed for reducing celestial observations. The Nautical Almanac produced by the U. S. Naval Observatory contains detailed procedures for calculator sight reduction and a compact sight reduction table. 8. Compass. Each lifeboat must carry a magnetic compass. For shipboard use, make a deviation table for each compass with magnetic material in its normal place. The accuracy of each table should be checked periodically. 9. Flashlight. A flashlight is required in each lifeboat. Check the batteries periodically and include extra batteries and bulbs in the kit. 10. Portable radio. A transmitting-receiving set approved by the Federal Communications Commission for emergency use can establish communications with rescue authorities. A small portable radio may be used as a radio direction finder or for receiving time signals. 11. An Emergency Position Indicating Radiobeacon (EPIRB) is essential. When activated, this device emits a signal which will be picked up by the COSPAS/SARSAT satellite system and automatically relayed to a ground station. It is then routed directly to rescue authorities. The location of the distress can be determined very accurately. Depending on the type of EPIRB, the signal may even identify the individual vessel in distress, thus allowing rescuers to determine how many people are in danger, the type of emergency gear they may have, and other facts to aid in the rescue. Because of this system, the navigator must question the wisdom of navigating away from the scene of the distress. It may well be easier for rescue forces to find him if he remains in one place. See Chapter 28, The Global Maritime Distress and Safety System (GMDSS). 2602. Most Probable Position In the event of failure of primary electronic navigation systems, the navigator may need to establish the most probable position (MPP) of the vessel. Usually there is usually little doubt as to the position. The most recent fix updated with a DR position will be adequate. But when conflicting information or information of questionable reliability is received, the navigator must determine an MPP. When complete positional information is lacking, or when the available information is questionable, the most probable position might be determined from the intersection of a single line of position and a DR, from a line of soundings, from lines of position which are somewhat inconsistent, or from a dead reckoning position with a correction for current or wind. Continue a dead reckoning plot from one fix to another because the DR plot often provides the best estimate of the MPP. A series of estimated positions may not be consistent because of the continual revision of the estimate as additional information is received. However, it is good practice to plot all MPP’s, and sometimes to maintain a separate EP plot based upon the best estimate of track and speed made good over the ground. This could indicate whether the present course is a safe one. See Chapter 23. 2603. Plotting Sheets If plotting sheets are not available, a Mercator plotting sheet can be constructed through either of two alternative methods based upon a graphical solution of the secant of the latitude, which approximates the expansion of latitude. First method (Figure 2603a): Step one. Draw a series of equally spaced vertical lines at any spacing desired. These are the meridians; label them at any desired interval, such as 1', 2', 5', 10', 30', 1°, etc. Step two. Draw and label a horizontal line through the center of the sheet to represent the parallel of the mid-latitude of the area. Step three. Through any convenient point, such as the intersection of the central meridian and the parallel of the mid-latitude, draw a line making an angle with the horizontal equal to the mid-latitude. In Figure 2603a this angle is 35°. Step four. Draw in and label additional parallels. The length of the oblique line between meridians is the perpendicular distance between parallels, as shown by the broken arc. The number of minutes of arc between parallels is the same as that between the meridians. Step five. Graduate the oblique line into convenient units. If 1' is selected, this scale serves as both a latitude and mile scale. It can also be used as a longitude scale by measuring horizontally from a meridian instead of obliquely along the line. The meridians may be shown at the desired interval and the
381EMERGENCYNAVIGATION*tsfrfes2WrE(Steps)36*N36*MA5)Tsrep新(Step2)35N35'Nrdsidors)ds)34N(Stepa)34°W15E1sofe1sewaft14gfeFigure2603a.Small area plotting sheet with selected longitude scalemid-parallel may be printed and graduated in units of lon-Step four.Draw in and label the meridians.Thegitude.In using the sheet it is necessary only to label thefirstisavertical linethroughthecenterofmeridians and draw the oblique line. From it determine thethe circle. The second is a vertical lineinterval usedtodraw in and label additional parallels.Ifthethrough the intersection of the obliqueline and the circle.Additional meridianscentral meridian is graduated, the oblique line need notbe.aredrawnthesamedistanceapartastheSecond method (Figure 2603b)firsttwo.Step five.Graduate the oblique line into conve-Step one. At the center of the sheet draw a circlenient units. If I'is selected, this scalewith a radius equal to 1o (or any otherserves as a latitude and mile scale.It canconvenient unit) of latitude at the desiredalso be used as a longitude scale by mea-scale.If a sheet with a compass rose issuring horizontally from a meridian,available, as in Figure 2603b, the com-instead of obliquelyalong the line.pass rose can beused as thecircle and willprove useful for measuring directions.ItIn the second method, theparallels may be shown atneed not limit the scale of the chart, as anthe desired interval, and the central meridian may be printedand graduated in units of latitude. In using the sheet it isadditional concentriccirclecanbedrawn,and desired graduations extended to it.necessary only to label the parallels, draw the oblique line,Step two.Drawhorizontal lines throughthe centerand from itdetermine the interval and draw in and label ad-of the circle and tangent at the top andditional meridians. If the central meridian is graduated, asbottom. These are parallels of latitude;shown in Figure 2603b, the oblique line need not be.label them accordingly,at the selected in-The same result is produced by either method. The firstterval (as every 1°, 30, etc.)method, starting with the selection ofthe longitude scale, isStep three.From the center of the circle draw aparticularly useful when the longitude limits of the plottinglinemaking an anglewith the horizontalsheet determine the scale. When the latitude coverage isequal to the mid-latitude. In Figuremore important, the second method may bepreferable.In2603bthisangleis40°eithermethod a central compassrosemightbeprinted
EMERGENCY NAVIGATION 381 mid-parallel may be printed and graduated in units of longitude. In using the sheet it is necessary only to label the meridians and draw the oblique line. From it determine the interval used to draw in and label additional parallels. If the central meridian is graduated, the oblique line need not be. Second method (Figure 2603b). Step one. At the center of the sheet draw a circle with a radius equal to 1° (or any other convenient unit) of latitude at the desired scale. If a sheet with a compass rose is available, as in Figure 2603b, the compass rose can be used as the circle and will prove useful for measuring directions. It need not limit the scale of the chart, as an additional concentric circle can be drawn, and desired graduations extended to it. Step two. Draw horizontal lines through the center of the circle and tangent at the top and bottom. These are parallels of latitude; label them accordingly, at the selected interval (as every 1°, 30’, etc.). Step three. From the center of the circle draw a line making an angle with the horizontal equal to the mid-latitude. In Figure 2603b this angle is 40°. Step four. Draw in and label the meridians. The first is a vertical line through the center of the circle. The second is a vertical line through the intersection of the oblique line and the circle. Additional meridians are drawn the same distance apart as the first two. Step five. Graduate the oblique line into convenient units. If 1’ is selected, this scale serves as a latitude and mile scale. It can also be used as a longitude scale by measuring horizontally from a meridian, instead of obliquely along the line. In the second method, the parallels may be shown at the desired interval, and the central meridian may be printed and graduated in units of latitude. In using the sheet it is necessary only to label the parallels, draw the oblique line, and from it determine the interval and draw in and label additional meridians. If the central meridian is graduated, as shown in Figure 2603b, the oblique line need not be. The same result is produced by either method. The first method, starting with the selection of the longitude scale, is particularly useful when the longitude limits of the plotting sheet determine the scale. When the latitude coverage is more important, the second method may be preferable. In either method a central compass rose might be printed. Figure 2603a. Small area plotting sheet with selected longitude scale
382EMERGENCYNAVIGATION60626rw59w58wE(Step 2)4fN4r'N10350TTTS20-0eul83RtepES?(Step 2)40N40°N3(tdans)1国(r dous)( dais)(tdais)mainiaa10S民lE店8ulw:.20E20C(Step 2)Fe39'NO'NE58w62w60y61°w59wFigure2603b.SmallareaplottingsheetwithselectedlatitudescaleBothmethods usea constantrelationshipoflatitudetosheet. If this proves too difficult, or if an independent check islongitudeover the entire sheet and both fail to allowfor thedesired,someform ofmathematical reckoning maybe useful.ellipticity ofthe earth.For practical navigation these are notTable 2604,a simplified traverse table, can be usedfor this pur-importantconsiderations.pose. This is a critical-type table, various factors being given forlimitingvalues of certainangles.Tofindthedifferenceor2604.DeadReckoningchange of latitude in minutes, enter thetable with course anglereckoned fromnorth or southtowardthe east orwest.MultiplyOfthevarioustypesofnavigation,deadreckoningaloneisthe distance run, in miles, by the factor. To find the departure inalways available in someform.In an emergency it is of moremiles, enter the table with the complement of the course angle.thanaverageimportance.Withelectronicsystemsoutofservice,Multiply the distance run in miles by the factor.To convert de-keepaclosecheckon speed,direction,and distancemadegoodparture to difference oflongitude in minutes, enter the table withCarefullyevaluatetheeffects ofwind and current.Long voyagmid-latitude and divide the departure by thefactor.es with accurate landfalls have been successfully completed bythismethodalone.ThisisnotmeanttominimizetheimportanceExample:Avesseltravels26milesoncourse205°ofother methods ofdetermining position.However,dead reckfromLat.41°44N,Long.56°21WoningpositionsmaybemoreaccuratethanthosedeterminedbyRequired:Latitudeand longitudeofthepointofarrival.other methods.If the means of determining direction and dis-tance (the elements of dead reckoning)are accurate, it maybeSolution: The course angle is 205°-1800= S25°W, andbestto adjustthedeadreckoningonlyaftera confirmedfixthecomplement is900-25°=65°.ThefactorscorrespondingPlotting canbedone directlyon apilot chart orplottingto these angles are 0.9and 0.4,respectively.The diference of75018314149566369818790Angle1.00.90.50.30.2Factor0.80.70.60.40.10.0Table 2604. Simplified traverse table
382 EMERGENCY NAVIGATION Both methods use a constant relationship of latitude to longitude over the entire sheet and both fail to allow for the ellipticity of the earth. For practical navigation these are not important considerations. 2604. Dead Reckoning Of the various types of navigation, dead reckoning alone is always available in some form. In an emergency it is of more than average importance. With electronic systems out of service, keep a close check on speed, direction, and distance made good. Carefully evaluate the effects of wind and current. Long voyages with accurate landfalls have been successfully completed by this method alone. This is not meant to minimize the importance of other methods of determining position. However, dead reckoning positions may be more accurate than those determined by other methods. If the means of determining direction and distance (the elements of dead reckoning) are accurate, it may be best to adjust the dead reckoning only after a confirmed fix. Plotting can be done directly on a pilot chart or plotting sheet. If this proves too difficult, or if an independent check is desired, some form of mathematical reckoning may be useful. Table 2604, a simplified traverse table, can be used for this purpose. This is a critical-type table, various factors being given for limiting values of certain angles. To find the difference or change of latitude in minutes, enter the table with course angle, reckoned from north or south toward the east or west. Multiply the distance run, in miles, by the factor. To find the departure in miles, enter the table with the complement of the course angle. Multiply the distance run in miles by the factor. To convert departure to difference of longitude in minutes, enter the table with mid-latitude and divide the departure by the factor. Example: A vessel travels 26 miles on course 205°, from Lat. 41°44’N, Long. 56°21’W. Required: Latitude and longitude of the point of arrival. Solution: The course angle is 205° - 180° = S25°W, and the complement is 90° - 25° = 65°. The factors corresponding to these angles are 0.9 and 0.4, respectively. The difference of Figure 2603b. Small area plotting sheet with selected latitude scale. Angle 0 18 31 41 49 56 63 69 75 81 87 90 Factor 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Table 2604. Simplified traverse table
EMERGENCYNAVIGATION383latitudeis26×0.9=23'(tothenearestminute)andthedepar-Meridian transit:Any celestial body bears due northtureis26x0.4=10mi.Sincethecourseisinthesouthwesternor southatmeridiantransit,eitherupperorlower.This isthequadrant,intheNorthernHemisphere,thelatitudeofthepointmomentofmaximum(orminimum)altitudeof thebodyof arrival is 41°44'N-23'= 41°21 N.The factor correspond-However, since the altitude at this time is nearly constanting to the mid-latitude 41°32N is 0.7.The difference ofduringa considerablechangeof azimuth,the instantofme-longitude is10+0.7=14:The longitude of the pointofarrivalridian transit may be difficult to determine. If time and anis5621W+14=56°35Walmanac are available, and the longitude isknown,the timeAnswer:Lat.41°21 N,Long.56°35W.oftransitcan be computed.It can alsobegraphed as a curveongraphpaper and thetimeofmeridiantransitdeterminedwith sufficient accuracyfor emergencypurposes.2605.Deck LogBody on prime vertical:If any method is available forAt the beginning of anavigation emergency a naviga-determiningwhen abodyis on theprime vertical (dueeastorwest),thecompassazimuthatthistimecanbeobserved.Tabletionlogshouldbestarted.Thedateandtimeofthecasualtyshould bethefirstentry,followed bynavigational informa-20, Meridian Angle and Altitude ofa Body on the Prime Ver-tionsuchas ship'sposition,status ofallnavigationsystems,tical Circle provides this information Any body on thethe decisions made, and the reasons for themcelestial equator (declination oo)is on the prime vertical at theThebestdetermination of theposition of the casualtytime of rising or setting.Forthe sun this occurs at the time ofthe equinoxes.The star Mintaka (Orionis),the leading star ofshould be recorded,followedbya full account of courses,distances,positions,winds,currents,and leeway.Noim-Orion'sbelt,hasadeclinationofapproximately0.3°sandcanportant navigational information should be left to memorybe consideredon thecelestial equator.For an observer neartheequator,such a body is always nearly east or west.Because ofif itcanberecorded.refractionanddip,theazimuthshould benoted when thecen-2606.Directionter of the sun ora star is a littlemorethan one sun diameter(half adegree)abovethehorizon.Themoon should beob-servedwhen its upper limb is on the horizon.Direction is one of the elements ofdead reckoning.Adeviationtableforeach compass,including lifeboat comBody at rising or setting: Except for themoon, the az-passes, should already have been determined.In the eventimuthangleofabodyisalmostthesameatrising as atof destruction or failure of the gyrocompass and bridgesetting,exceptthattheformer is toward theeast and thelat-magnetic compass, lifeboat compasses can be used.tertoward the west.Iftheazimuth is measured both at risingIf analmanac,accurateGreenwich time,and theneces-and setting,true south (or north)is midwaybetween the twosarytablesareavailable,the azimuthofanycelestial body canobservedvalues,and the difference between this value and180°(or0000)isthecompasserror.Thus,ifthecompassaz-becomputedandthisvaluecomparedwithanazimuthmea-imuth of a body is073°at rising,and277°at setting,truesured bythe compass.If it isdifficult toobservethe compassazimuth,selectabodydead aheadandnotethecompasshead-073°+277°= 175 by compass, and thesouth(180°)ising.The difference between the computed and observed2azimuths is compass error on thatheading.This is of more im+compass error is 5°E.This method may be in error ifthe ves-mediate value than deviation, but if thelatter is desired, it cansel is moving rapidly in a north or south direction. If thebedeterminedbyapplyingvariationtothecompass error.declination and latitudeareknown,thetrueazimuth of anySeveral unique astronomical situations occur,permit-body atrising or setting can bedetermined bymeans ofadi-tingdeterminationofazimuthwithout computation:agram on theplaneof the celestial meridianor byPolaris: Polaris is always within 2°of true north for ob-computation.Forthispurpose,thebody(exceptthemoon)servers between the equator and latitude 60°N.When this starshould be considered as rising or setting when its center is ais directly above or belowthe celestial pole, its azimuth is ex-littlemore than one sun diameter (halfa degree)above theactly north at any latitude. This occurs approximately when thehorizon,because of refraction anddip.trailing star of either Cassiopeia or the Big Dipper (Alkaid) isFinding directionbytherelationship of the suntothedirectly above or directly below Polaris (Figure 2611). Whenhands of a watch is sometimes advocated, butthelimita-a line through the trailing stars and Polaris is horizontal,thetions of this method prevent its practical use at seamaximumcorrection should beapplied.Belowlatitude50othiscanbeconsidered1°;andbetween50°and65°,2IfCas-A simple technique can be usedfor determining devia-siopeia is to the right of Polaris,theazimuth is 001°(or0020)tion.An object that will float but not driftrapidlybefore theand ifto theleft.359°(or358°).Thesouthcelestial pole islo-wind is thrown overboard.Thevessel is then steered steadilycated approximatelyat the intersection of a line through thein theoppositedirectiontothatdesired.Atadistanceof per-longer axis of the Southern Cross with a linefrom the north-hapshalfa mile,ormore ifthefloating objectis still clearlyernmost star of Triangulum Australe perpendicular to the linein view,the vessel is turned around in the smallest practicaljoiningtheothertwo stars ofthetriangle.Noconspicuousstarradius, and headed back toward the floating object. Themarksthisspot(SeestarchartsinChapter15)magnetic course ismidwaybetweenthe coursetowardthe
EMERGENCY NAVIGATION 383 latitude is 26 × 0.9 = 23’ (to the nearest minute) and the departure is 26 × 0.4 = 10 mi. Since the course is in the southwestern quadrant, in the Northern Hemisphere, the latitude of the point of arrival is 41°44’ N -23’ = 41°21’N. The factor corresponding to the mid-latitude 41°32’N is 0.7. The difference of longitude is 10 ÷ 0.7 = 14’. The longitude of the point of arrival is 56°21’W + 14 = 56°35’W. Answer: Lat. 41°21’N, Long. 56°35’W. 2605. Deck Log At the beginning of a navigation emergency a navigation log should be started. The date and time of the casualty should be the first entry, followed by navigational information such as ship’s position, status of all navigation systems, the decisions made, and the reasons for them. The best determination of the position of the casualty should be recorded, followed by a full account of courses, distances, positions, winds, currents, and leeway. No important navigational information should be left to memory if it can be recorded. 2606. Direction Direction is one of the elements of dead reckoning. A deviation table for each compass, including lifeboat compasses, should already have been determined. In the event of destruction or failure of the gyrocompass and bridge magnetic compass, lifeboat compasses can be used. If an almanac, accurate Greenwich time, and the necessary tables are available, the azimuth of any celestial body can be computed and this value compared with an azimuth measured by the compass. If it is difficult to observe the compass azimuth, select a body dead ahead and note the compass heading. The difference between the computed and observed azimuths is compass error on that heading. This is of more immediate value than deviation, but if the latter is desired, it can be determined by applying variation to the compass error. Several unique astronomical situations occur, permitting determination of azimuth without computation: Polaris: Polaris is always within 2° of true north for observers between the equator and latitude 60°N. When this star is directly above or below the celestial pole, its azimuth is exactly north at any latitude. This occurs approximately when the trailing star of either Cassiopeia or the Big Dipper (Alkaid) is directly above or directly below Polaris (Figure 2611). When a line through the trailing stars and Polaris is horizontal, the maximum correction should be applied. Below latitude 50° this can be considered 1°; and between 50° and 65°, 2°. If Cassiopeia is to the right of Polaris, the azimuth is 001° (or 002°), and if to the left, 359° (or 358°). The south celestial pole is located approximately at the intersection of a line through the longer axis of the Southern Cross with a line from the northernmost star of Triangulum Australe perpendicular to the line joining the other two stars of the triangle. No conspicuous star marks this spot (See star charts in Chapter 15). Meridian transit: Any celestial body bears due north or south at meridian transit, either upper or lower. This is the moment of maximum (or minimum) altitude of the body. However, since the altitude at this time is nearly constant during a considerable change of azimuth, the instant of meridian transit may be difficult to determine. If time and an almanac are available, and the longitude is known, the time of transit can be computed. It can also be graphed as a curve on graph paper and the time of meridian transit determined with sufficient accuracy for emergency purposes. Body on prime vertical: If any method is available for determining when a body is on the prime vertical (due east or west), the compass azimuth at this time can be observed. Table 20, Meridian Angle and Altitude of a Body on the Prime Vertical Circle provides this information. Any body on the celestial equator (declination 0°) is on the prime vertical at the time of rising or setting. For the sun this occurs at the time of the equinoxes. The star Mintaka (δ Orionis), the leading star of Orion’s belt, has a declination of approximately 0.3°S and can be considered on the celestial equator. For an observer near the equator, such a body is always nearly east or west. Because of refraction and dip, the azimuth should be noted when the center of the sun or a star is a little more than one sun diameter (half a degree) above the horizon. The moon should be observed when its upper limb is on the horizon. Body at rising or setting: Except for the moon, the azimuth angle of a body is almost the same at rising as at setting, except that the former is toward the east and the latter toward the west. If the azimuth is measured both at rising and setting, true south (or north) is midway between the two observed values, and the difference between this value and 180° (or 000°) is the compass error. Thus, if the compass azimuth of a body is 073° at rising, and 277° at setting, true south (180°) is by compass, and the compass error is 5°E. This method may be in error if the vessel is moving rapidly in a north or south direction. If the declination and latitude are known, the true azimuth of any body at rising or setting can be determined by means of a diagram on the plane of the celestial meridian or by computation. For this purpose, the body (except the moon) should be considered as rising or setting when its center is a little more than one sun diameter (half a degree) above the horizon, because of refraction and dip. Finding direction by the relationship of the sun to the hands of a watch is sometimes advocated, but the limitations of this method prevent its practical use at sea. A simple technique can be used for determining deviation. An object that will float but not drift rapidly before the wind is thrown overboard. The vessel is then steered steadily in the opposite direction to that desired. At a distance of perhaps half a mile, or more if the floating object is still clearly in view, the vessel is turned around in the smallest practical radius, and headed back toward the floating object. The magnetic course is midway between the course toward the 073° + 277° 2 - 175 =