Piloting involves navigating a vessel in restricted waters and fixing its position as precisely as possible at frequent intervals. More so than in other phases of navigation, proper preparation and attention to detail are important. This chapter will discuss a piloting methodology designed to ensure that procedures are carried out safely and efficiently. These procedures will vary from vessel to vessel according to the skills and composition of the piloting team. It is the responsibility of the navigator to choose the procedures applicable to his own situation, to train the piloting team in their execution, and to ensure that duties are carried out properly.
These procedures are written primarily from the perspective of the military navigator, with some notes included where civilian procedures might differ. This set of procedures is designed to minimize the chance of error and maximize safety of the ship.
The military navigation team will nearly always consist of several more people than are available to the civilian navigator. Therefore, the civilian navigator must streamline these procedures, eliminating certain steps, doing only what is essential to keep his ship in safe water.
The navigation of civilian vessels will therefore proceed differently than for military vessels. For example, while the military navigator might have bearing takers stationed at the gyro repeaters on the bridge wings for taking simultaneous bearings, the civilian navigator must often take and plot them himself. While the military navigator will have a bearing book and someone to record entries for each fix, the civilian navigator will simply plot the bearings on the chart as they are taken and not record them at all.
If the ship is equipped with an ECDIS, it is reasonable for the navigator to simply monitor the progress of the ship along the chosen track, visually ensuring that the ship is proceeding as desired, checking the compass, sounder and other indicators only occasionally. If a pilot is aboard, as is often the case in the most restricted of waters, his judgement can generally be relied upon explicitly, further easing the workload. But should the ECDIS fail, the navigator will have to rely on his skill in the manual and time-tested procedures discussed in this chapter.
While an ECDIS is the legal equivalent of a paper chart and can be used as the primary plot, an ECS, (non-ECDIS compliant electronic chart system) cannot be so used. An ECS may be considered as an additional resource used to ensure safe navigation, but cannot be relied upon for performing all the routine tasks associated with piloting. The individual navigator, with knowledge of his vessel, his crew, and the capabilities they possess, must make a professional judgement as to how the ECS can support his efforts to keep his ship in safe water. The navigator should always remember that reliance on any single navigation system courts disaster. An ECS does not relieve the navigator of maintaining a proper and legal plot on a paper chart.
The navigator’s job begins well before getting under-way.Much advance preparation is necessary to ensure asafe and efficient voyage. The following steps arerepresentative:
Ensure the plotting station(s) have the followinginstruments:
Once the navigator verifies the above equipment is in place,he tapes down the charts on the chart table. If more than onechart is required for the transit, tape the charts in a stack such thatthe plotter works from the top to the bottom of the stack. Thisminimizes the time required to shift the chart during the transit.If the plotter is using a PMP, align the arm of the PMP with anymeridian of longitude on the chart. While holding the PMP armstationary, adjust the PMP to read 000.0°T. This procedurecalibrates the PMP to the chart in use. Perform this alignmentevery time the piloting team shifts charts.
Be careful not to fold under any important informationwhen folding the chart on the chart table. Ensure the chart’sdistance scale, the entire track, and all important warninginformation are visible.
Energize and test all electronic navigation equipment,if not already in operation. This includes the radar and theGPS receiver. Energize and test the fathometer. Ensure theentire electronic navigation suite is operating properly priorto entering restricted waters.
Mark the Minimum Depth Contour: Determine theminimum depth of water in which the vessel can safelyoperate and outline that depth contour on the chart. Dothis step before doing any other harbor navigationplanning. Highlight this outline in a bright color so thatit clearly stands out. Carefully examine the area insidethe contour and mark the isolated shoals less than theminimum depth which fall inside the marked contour.Determine the minimum depth in which the vessel canoperate as follows:
Minimum Depth = Ship’s Draft – Height of Tide +Safety Margin + Squat.
(See Article 804 and Article 818.)
Remember that often the fathometer’s transducer is notlocated at the section of the hull that extends the furthestbelow the waterline. Therefore, the indicated depth ofwater is that below the fathometer transducer, not thedepth of water below the vessel’s deepest draft.
Figure 802a. Advance and transfer.
Example: Figure 802b illustrates using advance and transferto determine a turn bearing. A ship proceeding on course 100° is to turn60° to the left to come on a range which will guide it up a channel.For a 60° turn and the amount of rudder used, the advance is 920 yardsand the transfer is 350 yards..
Required: The bearing of flagpole “FP.” when therudder is put over.
Solution:
Answer: Bearing 058°.
Figure 802b. Allowing for advance and transfer.
Figure 802c. The slide bar technique.
Figure 802d. A danger bearing, hatched on the dangerous side, labeled with the appropriate bearing.
Ensure the following records are assembled andpersonnel assigned to maintain them:
Determining the tidal and current conditions of the portis crucial. This process is covered in depth in Chapter 9. Inorder to anticipate early or late transit, plot a graph of thetidal range for the 24-hour period centered on the scheduledtime of arrival or departure. Depending on a vessel’s draftand the harbor’s depth, some vessels may be able to transitonly at high tide. If this is this case, it is critically importantto determine the time and range of the tide correctly.
The magnitude and direction of the current will givethe navigator some idea of theset and drift the vessel willexperience during the transit. This will allow him to plan inadvance for any potential current effects in the vicinity ofnavigational hazards.
While printed tide tables can be used for predicting andplotting tides, it is far more efficient to use a computer withappropriate software, or the internet, to compute tides andprint out the graphs. These graphs can be posted on thebridge at the chart table for ready reference, and copiesmade for others involved in the piloting process. NOAAtide tables for the U.S. are available at the following site:
Always remember that tide tables give predicted data, but thatactual conditions may be quite different due to weather orother natural phenomena.
The navigator should obtain a weather report coveringthe route which he intends to transit. This will allow him toprepare for any adverse weather by stationing extralookouts, adjusting speed for poor visibility, and preparingfor radar navigation. If the weather is thick, considerstanding off the harbor until it clears.
The navigator can receive weather information anynumber of ways. Military vessels may receive weatherreports from their parent squadrons prior to coming intoport. Marine band radio carries continuous weather reports.Many vessels are equipped with weather facsimilemachines. Some navigators carry cellular phones to reachshoreside personnel and harbor control; these can also beused to get weather reports from NOAA weather stations. Ifthe ship is using a weather routing service for the voyage, itshould provide forecasts when asked. Finally, if the vesselhas an internet connection, this is an ideal source of weatherdata. NOAA weather data can be obtained at:
However he obtains theinformation, the navigator should have a good idea of theweather before entering piloting waters.
Assemble the entire navigation team for a piloting briefprior to entering or leaving port. The vessel’s captain andnavigator should conduct the briefing. All navigation andbridge personnel should attend. The pilot, if he is already onboard, should also attend. If the pilot is not onboard whenthe ship’s company is briefed, the navigator shouldimmediately brief him when he embarks. The pilot mustknow the ship’s maneuvering characteristics beforeentering restricted waters. The briefing should cover, as aminimum, the following:
The navigator should always accomplish the followingevolutions prior to piloting:
Report the magnitude and direction of the gyro error tothe navigator and captain. The direction of the error isdetermined by the relative magnitude of the gyro readingand the value against which it is compared. When thecompass is least, the error is east. Conversely, when thecompass is best, the error is west. See Chapter 6.
The vessel’s planned estimated time of arrival (ETA) atits mooring determines the vessel’s course and speed to theharbor entrance. Arriving at the mooring site on time may beimportant in a busy port which operates its port services on atight schedule. Therefore, it is important to plan the arrivalaccurately. Take the desired time of arrival at the mooring andsubtract from that the time it will take to navigate to it from theentrance. The resulting time is when you must arrive at theharbor entrance. Next, measure the distance between thevessel’s present location and the harbor entrance. Determinethe speed of advance (SOA) the vessel will use to make thetransit to the harbor. Use the distance to the harbor and theSOA to calculate what time to leave the present position tomake the mooring ETA, or what speed must be made good toarrive on time.
Consider these factors which might affect this decision:
At the appropriate time, station the piloting team. Allowplenty of time to acclimate to the navigational situation andif at night, to the darkness. The number and type of personnelavailable for the piloting team depend on the vessel. A Navywarship, for example, has more people available for pilotingthan a merchant ship. Therefore, more than one of the jobslisted below may have to be filled by a single person. Thepiloting team should consist of:
Plot Supervisors: On many military ships, the pilotingteam will consist of two plots: the primary plot and thesecondary plot. The navigator should designate the typeof navigation that will be employed on the primary plot.All other fix sources should be plotted on the secondaryplot. The navigator can function as the primary plotsupervisor. A senior, experienced individual should beemployed as a secondary plot supervisor. The navigatorshould frequently compare the positions plotted on bothplots as a check on the primary plot.
There are three major reasons for maintaining aprimary and secondary plot. First, as mentioned above, thesecondary fix sources provide a good check on theaccuracy of visual piloting. Large discrepancies betweenvisual and radar positions may point out a problem withthe visual fixes that the navigator might not otherwisesuspect. Secondly, the navigator often must change theprimary means of navigation during the transit. He mayinitially designate visual bearings as the primary fixmethod only to have a sudden storm or fog obscure thevisual NAVAIDS. If he shifts the primary fix means toradar, he has a track history of the correlation betweenradar and visual fixes. Finally, the piloting team often mustshift charts several times during the transit. When the oldchart is taken off the plotting table and before the new chartis secured, there is a period of time when no chart is in use.Maintaining a secondary plot eliminates this complication.Ensure the secondary plot is not shifted prior to getting thenew primary plot chart down on the chart table. In thiscase, there will always be a chart available on which topilot. Do not consider the primary chart shifted until thenew chart is properly secured and the plotter hastransferred the last fix from the original chart onto the newchart.
The piloting team must make the transition from coastalnavigation to piloting smoothly as the vessel approachesrestricted waters. There is no rigid demarcation betweencoastal navigation and piloting. Often visual NAVAIDS arevisible miles from shore where Loran and GPS are easier touse. The navigator should take advantage of this overlapwhen approaching the harbor. Plotting Loran, GPS, andvisual fixes concurrently ensures that the piloting team hascorrectly identified NAVAIDS and that the different types ofsystems are in agreement. Once the vessel is close enough tothe shore such that sufficient NAVAIDS (at least three withsufficient bearing spread) become visible, the navigatorshould order visual bearings only for the primary plot andshift all other fixes to the secondary plot, unless the decisionhas been made to proceed with ECDIS as the primarysystem.
Take advantage of the coastal navigation and pilotingoverlap to shorten the fix interval gradually. The navigatormust use his judgment in adjusting fix intervals. If the shipis steaming inbound directly towards the shore, set a fixinterval such that two fix intervals lie between the vesseland the nearest danger. Upon entering restricted waters, thepiloting team should be plotting visual fixes at three minuteintervals.
Commercial vessels with GPS and/or Loran C,planning the harbor transit with a pilot, will approach acoast differently. The transition from ocean to coastal toharbor approach navigation will proceed as visual aids andradar targets appear and are plotted. With GPS or ECDISoperating and a waypoint set at the pilot station, only a fewfixes are necessary to verify that the GPS position iscorrect. Once the pilot is aboard, the captain/pilot team mayelect to navigate visually, depending on the situation.
Safe navigation while piloting requires frequent fixingof the ship’s position. If ECDIS is the primary navigationsystem in use, this process is automatic, and the role of thenavigator is to monitor the progress of the vessel,cross-check the position occasionally, and be alert for anyindication that the system is not operating optimally.
If an ECS is in use, it should be considered only asupplement to the paper navigation plot, which legally muststill be maintained. As long as the manual plot and the ECSplot are in agreement, the ECS is a valuable tool whichshows the navigator where the ship is at any instant, not twoor three minutes ago when the last fix was taken. It cannotlegally take the place of the paper chart and the manual plot,but it can provide an additional measure of assurance thatthe ship is in safe water and alert the navigator to adeveloping dangerous situation before the next round ofbearings or ranges.
The next several articles will discuss the three majormanual methods used to fix a ship’s position when piloting:crossing lines of position, copying satellite or Loran data, oradvancing a single line of position. Using one method doesnot exclude using other methods. The navigator must obtainas much information as possible and employ as many ofthese methods as necessary.
While the intersection of two LOP’s constitutes a fixunder one definition, and only an estimated position byanother, the prudent navigator will always use at least threeLOP’s if they are available, so that an error is apparent ifthey don’t meet in a point. Some of the most commonlyused methods of obtaining LOP’s are discussed below:
Figure 811a. A fix by two bearing lines.
Figure 811b. A fix by two radar ranges.
Figure 811c. Principle of stadimeter operation.
Figure 811d. A fix by range and bearing of a single object.
Figure 811e. A fix by a range and distance.
When only one NAVAID is available from which to obtain bearings,use a technique known as therunning fix.Use the following method:
Figure 812a represents a ship proceeding on course020°, speed 15 knots. At 1505, the plotter plots an LOPto a lighthouse bearing 310°. The ship can be at any pointon this 1505 LOP. Some possible points are representedas points A, B, C, D, and E in Figure 812a. Ten minutes laterthe ship will have traveled 2.5 miles in direction 020°. If theship was at A at 1505, it will be at A' at 1515. However, if theposition at 1505 was B, the position at 1515 will be B'. Asimilar relationship exists between C and C', D and D', E andE'. Thus, if any point on the original LOP is moved a distanceequal to the distance run in the direction of the motion, a linethrough this point parallel to the original line of positionrepresents all possible positions of the ship at the later time.This process is calledadvancing a line of position. Moving aline back to an earlier time is calledretiring a line of position.
Figure 812a. Advancing a line of position.
When advancing a line of position, account for coursechanges, speed changes, and set and drift between the twobearing lines. Three methods of advancing an LOP are discussed below:
Method 1: See Figure 812b. To advance the 1924 LOP to1942, first apply the best estimate of set and drift to the 1942DR position and label the resulting position point B. Then,measure the distance between the dead reckoning position at1924 (point A) and point B. Advance the LOP a distance equalto the distance between points A and B. Note that LOP A'B' isin the same direction as line AB.
Figure 812b. Advancing a line of position with a change incourse and speed, allowing for set and drift.
Method 2: See Figure 812c. Advance the NAVAIDSposition on the chart for the course and distance traveled by thevessel and draw the line of position from the NAVAIDS
Figure 812c. Advancing a circle of position.
Method 3: See Figure 812d. To advance the 1505 LOPto 1527, first draw a correction line from the 1505 DRposition to the 1505 LOP. Next, apply a set and driftcorrection to the 1527 DR position. This results in a 1527estimated position (EP). Then, draw from the 1527 EP acorrection line of the same length and direction as the onedrawn from the 1505 DR to the 1505 LOP. Finally, parallelthe 1505 bearing to the end of the correction line as shown.
Figure 812d. Advancing a line of position by its relation to the dead reckoning.
Label an advanced line of position with both the timeof observation and the time to which the line is adjusted.
Figure 812e through Figure 812g demonstrate threerunning fixes. Figure 812e illustrates the case of obtaininga running fix with no change in course or speedbetween taking two bearings on the same NAVAID.
Figure 812e. A running fix by two bearings on the same object.
Figure 812f illustrates a running fix with changes in avessel’s course and speed between taking two bearingson two different objects.
Figure 812f. A running fix with a change of course and speed between observations on separate landmarks.
Finally, Figure 812g illustrates a running fix obtained by advancing range circles ofposition using the second method discussed above.
Figure 812g. A running fix by two circles of position.
The previous section discussed the methods for fixingthe ship’s position. This section discusses integrating themanual fix methods discussed above, and the use of thefathometer, into a piloting procedure. The navigator mustdevelop his piloting procedure to meet severalrequirements. He must obtain enough information to fix theposition of the vessel without question. He must also plotand evaluate this information. Finally, he must relay hisevaluations and recommendations to the vessel’s conningofficer. This section examines some considerations toensure the navigator accomplishes all these requirementsquickly and effectively. Of course, if ECDIS is the primaryplot, manual methods as discussed here are for backup use.
The preferred piloting fix is taken from visual bearingsfrom charted fixed NAVAIDS. Plot visual bearings on theprimary plot and plot all other fixes on the secondary plot. Ifpoor visibility obscures visual NAVAIDS, shift to radarpiloting on the primary plot. If neither visual or radar pilotingis available, consider standing off until the visibility improves.
The interval between fixes in restricted waters shouldusually not exceed three minutes. Setting the fix interval atthree minutes optimizes the navigator’s ability to assimilateand evaluate all available information. He must relate it tocharted navigational hazards and to his vessel’s intended track.It should take a well trained plotting team no more than 30seconds to measure, record, and plot three bearings to threeseparate NAVAIDS. The navigator should spend the majorityof the fix interval time interpreting the information, evaluatingthe navigational situation, and making recommendations to theconning officer.
If three minutes goes by without a fix, inform thecaptain and try to plot a fix as soon as possible. If the delaywas caused by a loss of visibility, shift to radar piloting. Ifthe delay was caused by plotting error, take another fix. Ifthe navigator cannot get a fix down on the plot for severalmore minutes, consider slowing or stopping the ship untilits position can be fixed. Never continue a passage throughrestricted waters if the vessel’s position is uncertain.
The secondary plot supervisor should maintain thesame fix interval as the primary plot. Usually, this means heshould plot a radar fix every three minutes. He should plotother fix sources (Loran and GPS fixes, for example) at aninterval sufficient for making meaningful comparisonsbetween fix sources. Every third fix interval, he should passa radar fix to the primary plot for comparison with the visualfix. He should inform the navigator how well all the fixsources plotted on the secondary plot are tracking.
Following a cyclic routine ensures the timely and efficient processing of data and forms a smoothly functioning piloting team. It quickly yields the informationwhich the navigator needs to make informed recommendationsto the conning officer and captain.
Repeat this routine at each fix interval beginning whenthe ship gets underway until it clears the harbor (outbound)or when the ship enters the harbor until it is moored (inbound).
The routine consists of the following steps:
Approximately 30 seconds before the time to turn,train the alidade on the turn bearing NAVAID. Watch thebearing of the NAVAID approach the turn bearing. About1° away from the turn bearing, announce to the conningofficer: “Stand by to turn.” Slightly before the turn bearingis indicated, report to the conning officer: “Mark the turn.”Make this report slightly before the bearing is reachedbecause it takes the conning officer a finite amount of timeto acknowledge the report and order the helmsman to putover the rudder. Additionally, it takes a finite amount oftime for the helmsman to turn the rudder and for the ship tostart to turn. If the navigator waits until the turn bearing isindicated to report the turn, the ship will turn too late.
Once the ship is steady on the new course, immediatelytake another fix to evaluate the vessel’s position in relationto the track. If the ship is not on the track after the turn,calculate and recommend a course to the conning officer toregain the track.
Use the fathometer to determine whether the depth ofwater under the keel is sufficient to prevent the ship fromgrounding and to check the actual water depth with thecharted water depth at the fix position. The navigator mustcompare the charted sounding at every fix position with thefathometer reading and report to the captain any discrepancies.Taking continuous soundings in restricted waters is mandatory.
See the discussion of calculating the warning and dangersoundings in Article 802. If the warning sounding is received,then slow the ship, fix the ship’s position more frequently, andproceed with extreme caution. Ascertain immediately wherethe ship is in the channel; if the minimum expected soundingwas noted correctly, the warning sounding indicates the vesselmay be leaving the channel and standing into shoal water.Notify the vessel’s captain and conning officer immediately.
If the danger sounding is received, take immediate actionto get the vessel back to deep water. Reverse the engines andstop the vessel’s forward movement. Turn in the direction ofthe deepest water before the vessel loses steerageway.Consider dropping the anchor to prevent the ship from driftingaground. The danger sounding indicates that the ship has leftthe channel and is standing into immediate danger. It requiresimmediate corrective action by the ship’s conning officer,navigator, and captain to avoid disaster.
Many underwater features are poorly surveyed. If afathometer trace of a distinct underwater feature can beobtained along with accurate position information, send thefathometer trace and related navigational data to NIMA forentry into the Digital Bathymetric Data Base.
Most U.S. Navy vessels receive instructions in theirmovement orders regarding the choice of anchorage.
Merchant ships are often directed to specific anchorages byharbor authorities. However, lacking specific guidance, themariner should choose his anchoring positions using thefollowing criteria:
It is usually best to follow an established procedure toensure an accurate positioning of the anchor, even whenanchoring in an open roadstead. The following procedure isrepresentative. See Figure 817.
Figure 817. Anchoring.
Locate the selected anchoring position on the chart.Consider limitations of land, current, shoals, and othervessels when determining the direction of approach. Whereconditions permit, make the approach heading into the current.Close observation of any other anchored vessels willprovide clues as to which way the ship will lie to her anchor.If wind and current are strong and from differentdirections, ships will lie to their anchors according to thebalance between these two forces and the draft and trim ofeach ship. Different ships may lie at different headings inthe same anchorage depending on the balance of forcesaffecting them.
Approach from a direction with a prominent NAVAID,preferably a range, available dead ahead to serve as a steeringguide. If practicable, use a straight approach of at least1200 yards to permit the vessel to steady on the requiredcourse. Draw in the approach track, allowing for advanceand transfer during any turns. In Figure 817, the chimneywas selected as this steering bearing. A turn range may alsobe used if a radar-prominent object can be found directlyahead or astern.
Next, draw a circle with the selected position of theanchor as the center, and with a radius equal to thedistance between the hawsepipe and pelorus, alidade, orperiscope used for measuring bearings. This circle ismarked “A” in Figure 817. The intersection of this circleand the approach track is the position of the vessel’sbearing-measuring instrument at the moment of letting theanchor go. Select a NAVAID which will be on the beamwhen the vessel is at the point of letting go the anchor. ThisNAVAID is marked “FS” in Figure 817. Determine whatthe bearing to that object will be when the ship is at the droppoint and measure this bearing to the nearest 0.1°T. Labelthis bearing as the letting go bearing.
During the approach to the anchorage, plot fixes at frequentintervals. The navigator must advise the conningofficer of any tendency of the vessel to drift from thedesired track. The navigator must frequently report to theconning officer the distance to go, permitting adjustment ofthe speed so that the vessel will be dead in the water or havevery slight sternway when the anchor is let go. To aid indetermining the distance to the drop point, draw and label anumber of range arcs as shown in Figure 817 representingdistances to go to the drop point.
At the moment of letting the anchor go, take a fix andplot the vessel’s exact position on the chart. This isimportant in the construction of the swing and drag circlesdiscussed below. To draw these circles accurately,determine the position of the vessel at the time of letting gothe anchor as accurately as possible.
Veer the anchor chain to a length equal to five to seventimes the depth of water at the anchorage. The exact amountto veer is a function of both vessel type and severity ofweather expected at the anchorage. When calculating thescope of anchor chain to veer, take into account themaximum height of tide.
Once the ship is anchored, construct two separatecircles around the ship’s position when the anchor wasdropped. These circles are called the swing circle and thedrag circle. Use the swing circle to check for navigationalhazards and use the drag circle to ensure the anchor isholding.
The swing circle’s radius is equal to the sum of theship’s length and the scope of the anchor chain released.This represents the maximum arc through which a ship canswing while riding at anchor if the anchor holds. Examinethis swing circle carefully for navigational hazards,interfering contacts, and other anchored shipping. Use thelowest height of tide expected during the anchoring periodwhen checking inside the swing circle for shoal water.
The drag circle’s radius equals the sum of the hawsepipeto pelorus distance and the scope of the chain released. Anybearing taken to check on the position of the ship should, ifthe anchor is holding, fall within the drag circle. If a fix fallsoutside of that circle, then the anchor is dragging. If thevessel has a GPS or Loran system with an off-station alarm,set the alarm at the drag circle radius, or slightly more.
In some cases, the difference between the radii of theswing and drag circles will be so small that, for a givenchart scale, there will be no difference between the circleswhen plotted. If that is the case, plot only the swing circleand treat that circle as both a swing and a drag circle. On theother hand, if there is an appreciable difference in radiibetween the circles when plotted, plot both on the chart.Which method to use falls within the sound judgment of thenavigator.
When determining if the anchor is holding or dragging,the most crucial period is immediately after anchoring.Fixes should be taken frequently, at least every threeminutes, for the first thirty minutes after anchoring. Thenavigator should carefully evaluate each fix to determine ifthe anchor is holding. If the anchor is holding, the navigatorcan then increase the fix interval. What interval to set fallswithin the judgment of the navigator, but the intervalshould not exceed 30 minutes. If an ECDIS, Loran, or GPSis available, use its off-station alarm feature for anadditional safety factor.
A ship moving through shallow water experiencespronounced effects from the proximity of the nearbybottom. Similarly, a ship in a channel will be affected by theproximity of the sides of the channel. These effects caneasily cause errors in piloting which lead to grounding. Theeffects are known assquat,bank cushion, andbank suction.They are more fully explained in texts onshiphandling, but certain navigational aspects are discussedbelow.
Squat is caused by the interaction of the hull of theship, the bottom, and the water between. As a ship movesthrough shallow water, some of the water it displacesrushes under the vessel to rise again at the stern. This causesa venturi effect, decreasing upward pressure on the hull.Squat makes the ship sink deeper in the water than normaland slows the vessel. The faster the ship moves throughshallow water, the greater is this effect; groundings on bothcharted and uncharted shoals and rocks have occurredbecause of this phenomenon, when at reduced speed theship could have safely cleared the dangers. Whennavigating in shallow water, the navigator must reducespeed to avoid squat. If bow and stern waves nearly perpendicularthe direction of travel are noticed, and the vesselslows with no change in shaft speed, squat is occurring.Immediately slow the ship to counter it. Squatting occurs indeep water also, but is more pronounced and dangerous inshoal water. The large waves generated by a squatting shipalso endanger shore facilities and other craft.
Bank cushion is the effect on a ship approaching asteep underwater bank at an oblique angle. As water isforced into the narrowing gap between the ship’s bow andthe shore, it tends to rise or pile up on the landward side,causing the ship to sheer away from the bank.
Bank suction occurs at the stern of a ship in a narrowchannel. Water rushing past the ship on the landward sideexerts less force than water on the opposite or open waterside. This effect can actually be seen as a difference in draftreadings from one side of the vessel to the other, and issimilar to the venturi effect seen in squat. The stern of theship is forced toward the bank. If the ship gets too close to thebank, it can be forced sideways into it. The same effectoccurs between two vessels passing close to each other.
These effects increase as speed increases. Therefore, inshallow water and narrow channels, navigators shoulddecrease speed to minimize these effects. Skilled pilots mayuse these effects to advantage in particular situations, butthe average mariner’s best choice is slow speed and carefulattention to piloting.
Current affects the accuracy of a running fix. Consider,for example, the situation of an unknown headcurrent. In Figure 819a, a ship is proceeding along acoast, on course 250 ° speed 12 knots. At 0920 light Abears 190°, and at 0930 it bears 143°. If the earlier bearingline is advanced a distance of 2 miles (10 minutesat 12 knots) in the direction of the course, the runningfix is as shown by the solid lines. However, if there is ahead current of 2 knots, the ship is making good a speedof only 10 knots, and in 10 minutes will travel a distanceof only 1 2/3 miles. If the first bearing line isadvanced this distance, as shown by the broken line, theactual position of the ship is at B. This actual positionis nearer the shore than the running fix actually plotted.A following current, conversely, would show a positiontoo far from the shore from which the bearing wasmeasured.
Figure 819a. Effect of a head current on a running fix.
If the navigator assumes a following current whenadvancing his LOP, the resulting running fix will plotfurther from the NAVAID than the vessel’s actual position.Conversely, if he assumes a head current, therunning fix will plot closer to the NAVAID than thevessel’s actual position. To ensure a margin of safetywhen plotting running fix bearings to a NAVAID onshore, always assume the current slows a vessel’s speedover ground. This will cause the running fix to plotcloser to the shore than the ship’s actual position.
When taking the second running fix bearing from adifferent object, maximize the speed estimate if the secondobject is on the same side and farther forward, oron the opposite side and farther aft, than the first objectwas when observed.
All of these situations assume that danger is on thesame side as the object observed first. If there is either ahead or following current, a series of running fixes basedupon a number of bearings of the same object will plot in astraight line parallel to the course line, as shown in Figure819b. The plotted line will be too close to the object observedif there is a head current and too far out if there is afollowing current. The existence of the current will not beapparent unless the actual speed over the ground is known.The position of the plotted line relative to the dead reckoningcourse line is not a reliable guide.
Figure 819b. A number of running fixes with a following current.
A current oblique to a vessel’s course will also result in anincorrect running fix position. An oblique current can bedetected by observing and plotting several bearings of thesame object. The running fix obtained by advancing onebearing line to the time of the next one will not agree with therunning fix obtained by advancing an earlier line. See Figure820a. If bearings A, B, and C are observed at five-minuteintervals, the running fix obtained by advancing B to the timeof C will not be the same as that obtained by advancing A tothe time of C, as shown in Figure 820a.
Figure 820a. Detecting the existence of an oblique current,by a series of running fixes.
Whatever the current, the navigator can determine thedirection of the track made good (assuming constantcurrent and constant course and speed). Observe and plotthree bearings of a charted object O. See Figure 820b.Through O draw XY in any direction. Using a convenientscale, determine points A and B so that OA and OB areproportional to the time intervals between the first andsecond bearings and the second and third bearings, respectively.From A and B draw lines parallel to the secondbearing line, intersecting the first and third bearing lines atC and D, respectively. The direction of the line from C andD is the track made good.
The distance of the line CD in Figure 820b from thetrack is in error by an amount proportional to the ratio of thespeed made good to the speed assumed for the solution. If agood fix (not a running fix) is obtained at some time beforethe first bearing for the running fix, and the current has notchanged, the track can be determined by drawing a linefrom the fix, in the direction of the track made good. Theintersection of the track with any of the bearing lines is anactual position.
Figure 820b. Determining the track made good.
Geometrical relationships can define a running fix. InFigure 821, the navigator takes a bearing on NAVAID D. Thebearing is expressed as degrees right or left of course. Later, atB, he takes a second bearing to D; similarly, he takes a bearingat C, when the landmark is broad on the beam. The navigatorknows the angles at A, B, and C and the distance run betweenpoints. The various triangles can be solved using Table 18.
From this table, the navigator can calculate the lengths ofsegments AD, BD, and CD. He knows the range and bearing;he can then plot an LOP. He can then advance these LOP’s tothe time of taking the CD bearing to plot a running fix.Enter the table with the difference between the courseand first bearing (angle BAD in Figure 821) along the topof the table and the difference between the course and secondbearing (angle CBD) at the left of the table. For eachpair of angles listed, two numbers are given. To find thedistance from the landmark at the time of the second bearing(BD), multiply the distance run between bearings (in nauticalmiles) by the first number from Table 18. To find thedistance when the object is abeam (CD), multiply the distancerun between A and B by the second number from thetable. If the run between bearings is exactly 1 mile, thetabulated values are the distances sought.
Figure 821. Triangles involved in a Table 18 running fix.
Example: A ship is steaming on course 050°, speed 15 knots. At1130 a lighthouse bears 024°, and at 1140 it bears 359°.
Required:
Solution:
Answer: (1) D 2.6 mi., (2) D 2.0 mi.
This method yields accurate results only if the helmsmanhas steered a steady course and the navigator uses thevessel’s speed over ground.
Piloting requires a thorough familiarity with principlesinvolved, constant alertness, and judgment. A study ofgroundings reveals that the cause of most is a failure to useor interpret available information. Among the morecommon errors are:
Some of the errors listed above are mechanical andsome are matters of judgment. Conscientiously applyingthe principles and procedures of this chapter will go a longway towards eliminating many of the mechanical errors.However, the navigator must guard against the feeling thatin following a checklist he has eliminated all sources oferror. A navigator’s judgment is just as important as hischecklists.
When measuring bearings from two NAVAIDS, thefix error resulting from an error held constant for bothobservations is minimized if the angle of intersection of thebearings is 90°. If the observer in Figure 823a is located atpoint T and the bearings of a beacon and cupola are observedand plotted without error, the intersection of thebearing lines lies on the circumference of a circle passingthrough the beacon, cupola, and the observer. With constanterror, the angular difference between the bearings of thebeacon and the cupola is not affected. Thus, the angleformed at point F by the bearing lines plotted with constanterror is equal to the angle formed at point T by the bearinglines plotted without error. From geometry it is known thatangles having their apexes on the circumference of a circleand that are subtended by the same chord are equal. Sincethe angles at points T and F are equal and the angles aresubtended by the same chord, the intersection at point F lies onthe circumference of a circle passing through the beacon,cupola, and the observer.
Figure 823a. Two-bearing plot.
Assuming only constant error in the plot, the directionof displacement of the two-bearing fix from the position ofthe observer is in accordance with the sign (or direction) ofthe constant error. However, a third bearing is required todetermine the direction of the constant error.
Assuming only constant error in the plot, the two-bearingfix lies on the circumference of the circle passingthrough the two charted objects observed and the observer.The fix error, the length of the chord FT in Figure 823b,depends on the magnitude of the constant error ∈, the distancebetween the charted objects, and the cosecant of the angleof cut, angle θ. In Figure 823b,
where ∈ is the magnitude of the constant error, BC isthe length of the chord BC, and θ is the angle of the LOP’sintersection.
Figure 823b. Two-bearing plot with constant error.
Figure 823c. Error of two-bearing plot.
Since the fix error is a function of the cosecant of theangle of intersection, it is least when the angle of intersectionis 90°. As illustrated in Figure 823c, the error increasesin accordance with the cosecant function as the angle ofintersection decreases. The increase in the error becomesquite rapid after the angle of intersection has decreased tobelow about 30°. With an angle of intersection of 30°, thefix error is about twice that at 90°.
If several fixes obtained by bearings on three objectsproduce triangles of error of about the same size, theremight be a constant error in observing or plotting thebearings. If applying of a constant error to all bearingsresults in a pinpoint fix, apply such a correction to allsubsequent fixes. Figure 824 illustrates this technique.The solid lines indicate the original plot, and the brokenlines indicate each line of position moved 3° in aclockwise direction.
Figure 824. Adjusting a fix for constant error.
Employ this procedure carefully. Attempt to find andeliminate the error source. The error may be in thegyro-compass, the repeater, or the bearing transmission system.Compare the resulting fix positions with a satellite position,a radar position, or the charted sounding. A high degree ofcorrelation between these three independent positioningsystems and an “adjusted” visual fix is further confirmationof a constant bearing error.
Civilian piloting training has traditionally been afunction of both maritime academies and on-the-jobexperience. The latter is usually more valuable, becausethere is no substitute for experience in developing judgment.Military piloting training consists of advancedcorrespondence courses and formal classroom instructioncombined with duties on the bridge. U.S. Navy Quartermastersfrequently attend Ship’s Piloting and Navigation(SPAN) trainers as a routine segment of shoreside training.Military vessels in general have a much clearer definition ofresponsibilities, as well as more people to carry them out,than civilian ships, so training is generally more thoroughand targeted to specific skills.
Computer technology has made possible thedevelopment of computerized ship simulators, whichallow piloting experience to be gained without riskingaccidents at sea and without incurring underway expenses.Simulators range from simple micro-computer-basedsoftware to a completely equipped ship’s bridge with radar,engine controls, 360° horizon views, programmable seamotions, and the capability to simulate almost any navigationalsituation.
A different type of simulator consists of scale modelsof ships. The models, actually small craft of about 20-30feet, have hull forms and power-to-weight ratios similar tovarious types of ships, primarily supertankers, and theoperator pilots the vessel from a position such that his viewis from the craft’s “bridge.” These are primarily used intraining pilots and masters in docking maneuvers withexceptionally large vessels.
The first computer ship simulators came into use in thelate 1970s. Several years later the U.S. Coast Guard beganaccepting a limited amount of simulator time as “sea time”for licensing purposes. They can simulate virtually anyconditions encountered at sea or in piloting waters,including land, aids to navigation, ice, wind, fog, snow,rain, and lightning. The system can also be programmed tosimulate hydrodynamic effects such as shallow water,passing vessels, current, and tugs.
Virtually any type of vessel can be simulated,including tankers, bulkers, container ships, tugs and barges,yachts, and military vessels. Similarly, any givennavigational situation can be modeled, including passage throughany chosen harbor, river, or passage, convoy operations,meeting and passing situations at sea and in harbors.
Simulators are used not only to train mariners, but alsoto test feasibility of port and harbor plans and visual aids tonavigation system designs. This allows pilots to “navigate”simulated ships through simulated harbors beforeconstruction begins to test the adequacy of channels,turning basins, aids to navigation, and other factors.
A full-capability simulator consists of a ship’s bridgewhich may have motion and noise/vibration inputs, aprogrammable visual display system which projects asimulated picture of the area surrounding the vessel in bothdaylight and night modes, image generators for the variousinputs to the scenario such as video images and radar, acentral data processor, a human factors monitoring systemwhich may record and videotape bridge activities for lateranalysis, and a control station where instructors control theentire scenario.
Some simulators are part-task in nature, providing specifictraining in only one aspect of navigation such as radarnavigation, collision avoidance, or night navigation.
While there is no substitute for on-the-job training,simulators are extremely cost effective systems which canbe run for a fraction of the cost of an actual vessel. Further,they permit trainees to learn from mistakes with no possibilityof an accident, they can model an infinite variety ofscenarios, and they permit replay and reassessment of eachmaneuver.