Maneuvering the Sally Maersk

Maneuvering the Sally MaerskM O R T E N N I E L S E NCenter for Human-Machine Interaction. Report CHMI-3-2000

Table of contentsIntroduction ...................................................................................................................3Steering..........................................................................................................................3Follow up steering.....................................................................................................4Auto Pilot ..................................................................................................................7Voyage Management System..................................................................................10The passage plan .................................................................................................10System modes......................................................................................................11Maneuvering characteristics........................................................................................12Propulsion....................................................................................................................13Acceleration.............................................................................................................13Deceleration.............................................................................................................16Thrusters..................................................................................................................172

MANEUVERING THE SALLY MÆRSKIntroductionBasically the movements of a seagoing vessel—its maneuvers—are determined bythe interaction between controllable and uncontrollable physical forces. Thus, fromthis perspective safe and timely maritime operations are a matter of applying thecontrollable forces on the uncontrollable forces in a way that results in the desiredmovements.Controlable forcesEngineRudderThrusterAnchorTugHawserUncontrolable forcesWindCurrentSwellSeaSuction effectBank effectFigure 1. The Controllable and uncontrollable forces involved in maneuvering a seagoing vessel.The present report presents the controllable forces available to the crew of theSally Mærsk during at-sea maneuvers. Focus will be on steering and propulsion, thatis, the rudder and its control systems and the propulsion system driving the vessel.SteeringSally Mærsk is fitted with a streamlines semi spade-type balanced rudder. Themassive 100m 2 rudder is mounted on a vertical rudder stock which is turned by fourelectro-hydraulic cylinders. The maximum rudder angle 'hard over' to port orstarboard is 35°.As seen from the perspective of the actor on the bridge of the vessel the steeringgear is an object to be controlled with the purpose of achieving certain movements ofthe vessel. In order to facilitate different modes of interaction with the steering gearan elaborate control system has been placed between the actor and the steering gear.The purpose of the following is to illustrate how the rudder is operated in the contextof maneuvering rather then explicating in detail the engineering details of the steeringsystem. To this end we will consider the physical steering gear (rudder blade, rudderstock, cylinders, etc.) the object of control, while the system in-between the humanactor and the actual steering gear will be understood as the control system. Our3

purpose is to explicate the control system in order to provide an understanding of thepossible ways of maneuvering the vessel.The steering control system fitted on the Sally Mærsk is a Sperry Adaptive DigitalGyropilot ADG 6000. The system facilitates five different modes of steering:A. Fully automatic. The voyage management system provides the auto pilot withcourse information.B. Auto pilot. The course to be followed is set by a human actor and the auto pilotfollows the given courseC. Follow up. The human actor turns the steering wheel to a rudder setting(rudder angle) and the control system moves the rudder to the desired position.D. Non follow up. The human actor moves the rudder via a direct link to thecylinders turning the rudder. The rudder only moves as long as the switch ispressed to either starboard or port.E. Telephone. An actor on the bridge communicate desired rudder setting viatelephone to an engineer who operates the rudder from the emergency stand.The emergency stand functions according to the principle of non follow up.The five different modes of steering represent different levels of automationavailable within the steering control system. To understand the use objective of thedifferent modes of maneuvering we must distinguish between the objectives ofrobustness and maneuverability. Not all modes have been implemented strictly inorder to provide better opportunities for maneuvering. This is the case for both thetelephone mode and non follow up. There simply is no motivation for these modes ifwe consider them exclusively in terms of maneuvering. They are however perfectlysensible as measures of robustness. The fully automatic mode, the auto pilot, andfollow up mirror the nature of different types of maneuvering while the lower levelsof automation are measures counteracting a total loss of steering if the higher levelsof automation fail. The counter measures to system failure can be traced even furtherto the mountings on the rudder stock allowing it to be turned by chains, and so forth.However, to understand how the actors maneuver the vessel we may concentrateexclusively on the three topmost levels of automation as these are the modes usedduring standard operations.The different modes of steering are performed by use of dedicated devices. In theabove we have already mentioned the vessel management system and the auto pilotbeing two such effectuators. In the below we will deal with these and further controlsubsystems in more detail. We will explain their basic functioning and illustrate howthey serve different types of maneuvering. We will start with the helm stand and theother effectuators that are in use during follow up maneuvering.Follow up steeringWhen operating in open water, say, when crossing the Atlantic Ocean, the vesselfollows the same course for long periods of time. Space is ample and the need forrash course changes seldom occurs. However, the conditions change radically whenoperating closer to a harbor where traffic density is high and the space (restricted bywater depth and traffic regulations) is limited. In the latter case maneuvering is mostoften performed manually, for example, by a helmsman steering the vessel manuallyfrom the helm stand.4

Figure 2. Helm standThe rudder moves when the wheel on the helm stand is turned. Yet the wheel canbe turned significantly faster than the hydraulic pumps can turn the rudder. Thus,when changing the setting of the rudder from, say, amidships to hard a port it willtake a moment before the actual rudder angle equals the desired rudder angle set onthe helm stand.The helm stand holds a number of indicators. The desired rudder angle (the angleset by the helmsman) is engraved on a scale just above the base of the axle holdingthe wheel, and it is also displayed on the electronic helm order indicator. The rounddisplay at the center of the stand is a gyrocompass indicating the present course. Tothe left of the compass is a rudder indicator (actual rudder angle), and to its right is arate of turn indicator (displaying rate of turn measured in degrees per minute).The helmsman is in hands on control of the wheel on the helm stand, while thesettings to be implemented are decided by the master or the pilot and communicatedorally to the helmsman. The helmsman acts on two basic type of commands: ruddercommands and course commands. Rudder commands have the form 'ten degreesstarboard' while a course command could be 'go to two two five' (two hundred andtwenty five degrees). The rudder command is implemented by turning the wheel onthe helm stand to starboard until the helm order indicator reads 10°.Course commands are more complex. They demand a higher level of skill on thepart of the helmsman and they demand his attention for longer periods of time: firstthe helmsman changes the course (indicated on the compass) according to thecommand and then he performs continuos adjustments (counteracting wind, swell,current, etc.) to the rudder setting in order to maintain the commanded course.5

Figure 3. Displays on the helm stand: gyrocompass at the center, rudder angle indicator to its left, rateof turn indicator to is right. Helm order indicator is positioned below the compass.Finally, the helmsman may be asked to perform a specific rate of turn, say, 'keepher at ten degrees rate of turn'. Normally, a rate of turn command will be given whena turn is already in progress and the objective is to have the helmsman maintain thepresent rate of turn. He does so by means of the wheel and the rate of turn indicator.Figure 4. Dial for setting the rudder angle.The bridge holds three other actuators used for manual steering of the vessel theyall look like one shown in figure 4. One is positioned on the central console on thecenter bridge, and the other two are located on the consoles in the starboard and portbridge wing. The steering control in the bridge wings are used exclusively by themaster during mooring maneuvers, and the actuator on the center bridge is used bythe master and officers during manual steering in coastal waters.6

Auto PilotThe auto pilot is capable of maintaining a heading which is fed to it manually by thenavigating crew or automatically by the voyage management system. The latter casewill be dealt with in the section concerned with the voyage management system, inthe present section we will describe the auto pilot as it functions when the course isentered manually.Figure 5. Auto pilotThe auto pilot's interface is composed of a left, right, and mid section. At the topof the center section (figure 5) is the rate of turn indicator reading the vessels presentrate of turn to starboard or port. Below the rate of turn indicator is the heading display(gyrocompass repeater) reading the present heading. The mode display reading themode settings is located to the left of the heading display, and to its right is the courseorder dial (used for setting the desired course) and the course order display (readingthe ordered course).Figure 6. The top section of the auto pilot: the rate of turn indicator at the top, below it is the headingindicator (gyrocompass repeater). To the left of the heading display is the mode display reading themode setting, and to the right of the heading display is the course order dial (used for setting theordered course) and the course order display.The below five figures (7-11) show the components positioned in a vertical line atthe right side of the auto pilot. System (figure 7) displays the setting of the interactive7

system display (figure 11). The labels PORT and STARBOARD in figure 11 refer to thetwo set of hydraulic pumps turning the rudder.Figure 7. Figure 8. Figure 9. Figure 10. Figure 11.Elsewhere, the actor can determine if one or both sets of pumps are on; if only oneset is on then the setting of the system dial (figure 11) determines which set of pumps(starboard or port) should be activated. Figure 8 illustrates a set of dimmers whichallow the actor to adjust the light INTENSITY on the individual auto pilot displays(intensity is turned down during nighttime operations). The Sally Mærsk is fitted withtwo gyrocompasses. Figure 9 shows the dial determining which COMPASS is used asthe source of course information (the other gyrocompass functions as a backup unit).The MODE dial (figure 10) plays an important role within the steering control system:the actor changes the steering mode (degree of automation) by turning this dial. Themodes NFU, HELM, AUTO (auto pilot), and NAV (fully automatic) where mentioned atthe beginning of this chapter. The RMT (remote) setting changes control of steeringfrom the console on the central bridge to the steering console in port or starboardbridge wing. The procedure of switching between steering consoles is a two-stepoperation to ensure that only one steering console is active, and that the activeconsole is manned: first the mode dial is set to RMT, but the actual change of controllocation is not effectuated before the ACCEPT button in the port or starboard bridgewing has been pushed.Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18.The units on the right side of the auto pilot all have to do with parameters for thetrack following performed by the auto pilot. In order to perform satisfactory the autopilot needs information on the present WEATHER condition (figure 15) and the LOADcondition of the vessel (figure 18). The auto pilot is a means of maintaining a certaincourse automatically but it is also an effective means of optimizing the fuelconsumption while en route. The setting of the RUDDER dial (figure 16) limits themaximum rudder angle resulting in smooth curves. The TURN RATE dial has an8

identical function but the limitation is now expressed in a maximum turn rate allowedduring track following.The present setting of the dials are somewhat misleading because the pictureswhere taken just after having entered a harbor: the different parameters are set tohigher values in coastal waters in order to ensure responsive auto pilot maneuvering.On open water the parameters are set to more restrictive values (e.g. rudder 5, turnrate 10), thus ensuring a minimum degree of braking effect resulting from any ruddersetting other than amidships (neutral). The SPEED dial (figure 17) is always at maxbecause the speed of the vessel is never regulated from the auto pilot (see the chapteron propulsion). The AUTO/NAV dial (figure 13) is the central parameter involved inreducing the fuel consumption on open water: the AUTO/NAV dial allow the actor tospecify a certain tolerance within the track following function of the auto pilot.Setting the AUTO/NAV dial to 10° means that the auto pilot will not perform a coursecorrection until the vessel is more than 10° off the ordered course. Combine thisfeature with limitations on rudder angle and rate of turn and the result is a smoothtrack with few rudder alterations. The steering parameters can however become torestrictive in certain cases, say, if the vessel enters an area with strong cross current orwind which means that the auto pilot is unable to perform the desired course. In suchcases the auto pilot will sound an alarm and the little lamp at the top of the AUTO/NAVsection (figure 13) will switch on. It is then up to the navigating officer to ensure thatappropriate parameter tuning is achieved before the vessel comes too far off course.The ALARM section (figure 12) allows the actor to test the alarm function of the autopilot and to turn of active alarms.Compared to the contemporary navigational aids found in integrated bridgesystems the auto pilot features a relatively simple functionality. It receives a limitednumber of inputs and is capable of relatively simple control functions. Still, to thepresent day, the auto pilot remains on of the most valued maneuvering aids.Figure 19. The auto pilot performs its calculations based on input concerning speed, heading, andtuning parameters. Output is provided to the visual displays reading the tuning parameters, alarms, etc.,and to the engine speed control system, and the steering gear (the effectuators activating the pumpsturning the rudder)In the following we shall turn to the core unit within the integrated bridge system:the state-of-the-art voyage management system.9

Voyage Management SystemThe voyage management system is at the top of the automation hierarchy andcompared to the next level down—the auto pilot—the voyage management system isa significantly more complex and flexible system. Steering and speed control are atthe core of both systems—yet the voyage management system works on input from awider selection of sensors and is capable of more refined steering control.Figure 20. Inputs and outputs to the voyage management system.The voyage management system consists of three major units: the voyagemanagement system unit, the conning station (including conning display), and theplaning station. For now we need only notice that the calculations performed by thevoyage management system can be used to steer the vessel because the voyagemanagement system is hooked up to the auto pilot. The voyage management systemcan override the track following performed by the auto pilot. In order to understandthe essence and utility of this particular automation function we need to consider thenavigational aspects in the use of the voyage management system.The passage planThe Sally Mærsk never leaves a harbor without a detailed plan of where to go andhow to get there safely and on time. One does not just head out from A in the generaldirection of B. Every step of the voyage is carefully planned before initiating thevoyage. The document specifying the route to be sailed between two subsequent portsis referred to as the passage plan. The passage plan is provided by the master andcarefully reviewed on board by the navigating crew prior to departure.During the voyage the passage plan (figure 21) is available to the navigators inthree formats: alphanumeric (digital and hard a copy of an Excel spreadsheet,graphical representation in the sea charts, and plotted as a graphical superimpositionon the digital sea charts displayed by the voyage management system.The basic content of a track plan is a list of way points WP defining certainactions—most commonly the position where a course change is to take place. Thecolumns labeled LATITUDE and LONGITUDE specify the geographical position of theway point, the position at which to take action.10

Passage plan: ROTTERDAM DEPARTUREWP Latitude Longitude RL/GL Course Dist. Dist. rem. Notesnm nm52 51°57,80N 004°05,27E RL 83° 1,5 6,3 ROT: 2xspeed53 51°58,58N 004°05.56E RL 12,9° 0,8 5,5 ROT: 2xspeed54 51°59,78N 004°0165E RL 296,4° 2,7 2,855 52°00,80N 003°57,50E RL 291,7° 2,8 0 Mass PilotTotal distance: 7,7 nm.Charts:132-122Radar chart: ROTRadar route: DEPROT2Admiralty list: NP 286/1Figure 21. Passage planPilot information:Mass Pilot / Mass ApproachVHF Channel: 02/01Tide information:See table NP. 201Local phenomena:Consult pilotHave in mind that positions ofbuoys might differ betweenthe chart and the actualpositions.Buoys may be extinguished ormissing—ask Pilot.The column marked COURSE specifies the course to be sailed from the specifiedway point onwards; that is, the course to be sailed in order to arrive at the subsequentway point. Distance (DIST) lists the distance of the present course segment—thedistance to the next course change, and distance remaining (DIST. REM.) shows theremaining distance to be sailed within the present part of the passage plan. Thecategory NOTES can hold different sorts of information of relevance to the navigation.In the presented plan 1 the first two notes specify that the course change—or rather theturn that will take the vessel from one course segment onto the next—should beperformed at a turn rate (measured in degrees per minute) of two times the speed ofthe vessel: if the vessel travels at 10 knots the turn should be performed at a rate of20° per minute.The first column in the lower part of the passage plan shows a list of the sea chartsto be used during the passage; the totality of sea charts needed for a voyage isreferred to as the chart portfolio. The on board radars are capable of superimposingsketched charts of lanes and course lines (called 'nav lines') onto the radar image:ROT and DEPROT2 are the file names of the respective radar chart and route to beloaded while leaving the port of Rotterdam. PILOT INFORMATION lists the name of thetwo pilot services of the Rotterdam harbor, and VHF CHANNELS lists the channels onwhich the pilots can be contacted on the VHF radio. TIDE TABLES play an importantrole in determining the time frame within which the sufficient water depth allowingthe vessel to enter/exit a harbor will be available. Tides should allow for no less thanone meter of water under the keel during navigation in and around the harbor area.While the vessel is moored (made fast) at the quay, the acceptable limit is at 30centimeters. The category labeled LOCAL PHENOMENA may list any peculiarities orspecifics of the present harbor. The third column is another set of general remarks tobe taken into account during the approach.System modesThe voyage plan is keyed into the voyage management system via the planing station.The way point positions can be entered into the voyage management system in twodifferent ways: they can be established by pointing to the position in one of the1The format of the shown passage plan is in accordance with the passage plan used on board the SallyMærsk—only the contents is fictive.11

systems digital sea charts or the way point position can be entered manually byreference to the master's spreadsheet version. On the Sally Mærsk the latter is thecase. The reason is that the vessel is in line service and thus calls on a fixed set ofharbors during a round trip. This normally means that only moderate changes need tobe made to the passage plans in-between round trips—and the standard passage planfor a specific round trip can thus be reused.In terms of automation the track plan is at the heart of the voyage managementsystem. In theory the coupling between track plan data and tuning parameters, sensorinput, radar targets, etc. may lead to the conclusion that the voyage managementsystem is capable of sailing the ship from port A to port B. However, reality tells adifferent story.Even though the voyage management system is far more sophisticated than theauto pilot both systems are—at the end of the day—automatic track followingdevices. The voyage management system—in contrast to the auto pilot—is capable offollowing a series of subsequent course segments; yet non of the systems have anycapabilities in regard to automating traffic issues: traffic is represented within thevoyage management system for the purpose of visual presentation but the system hasno model for the actual handling of traffic. As a consequence, track following isperformed manually in restricted waters with dense traffic. Automatic trackfollowing—performed by means of the auto pilot or the voyage management systemis only engaged in open water with ample space and little traffic.The fully automatic track following performed by the voyage management systemis refered to as the NAV MODE. But most often the voyage management system isrunning in the so called ADVISORY MODE. The two modes are identical with respect tothe calculations performed by the voyage management system. However, inADVISORY MODE the calculations are only displayed on the monitor—while the link tothe lower levels of the steering and propulsion control systems remains blocked. Inother words, the advise given by the voyage management system will only take effectif they are implemented by the navigating officer. The actual information provided bythe voyage management system will be discussed in more detail in the report dealingwith navigation issues.Maneuvering characteristicsThe rudder fitted on the Sally Mærsk is a conventional spade-type rudder—a passivedevice in the sense that it functions by guiding forces that are not the product of therudder itself. The rudder only functions if there is a certain magnitude of waterpressure on the rudder blade.Intuitively, one may assume that the rudder achieves its guiding effect on themovements of the vessel by pushing the water hitting the rudder blade to starboard orport. But in fact, the push only accounts for one third of the rudder effect. Theremaining comes from the vacuum and consequent suction effect created on the backside of the rudder blade—the side of the rudder facing away from the water pressure.In the case of the Sally Mærsk, the rudder demands a water pressure equivalent topressure available when the vessel is moving ahead at speeds of three knots or greaterspeeds. Below this magnitude of water pressure, the rudder has no significant effect on themaneuverability of the vessel.12

Figure 22: Turning circle for Sally Mærsk at 94 RPM in the loaded condition.The above figure shows the maneuvering characteristics of the Sally Mærsk duringa 360° turn as measured during test runs. Measurements were performed during tworuns at 94 RPM with the rudder hard a port and a starboard respectively. During thestarboard turn, the vessel advances 750 m. while transferring 800 m. The entire 360°starboard maneuver took 215 sec. while the 90° turn a starboard took 100 sec. Thebelow figure illustrates the test results for a 360° turn performed at 50 RPM.Figure 23: Turning circle for the Sally Mærsk at 50 RPM in the loaded condition.As can be seen from the two figures, the vessel uses less time but covers moreground when turning at greater speeds.PropulsionIn the following we will look at the controllable forces driving the vessel through thewater. We will consider the functioning and use of the main engine propulsion systemand the thrusters, and we will present the data illustrating the actual maneuveringcapabilities of the Sally Mærsk. We will start by introducing the main enginepropulsion system.AccelerationThe Sally Mærsk is fitted with one Mitsui-B&W 12k 90 MC two-stroke, singleacting, turbo-charged crosshead marine diesel engine with direct reversing—capable13

of producing 74640 BHP at 94 RPM. In short: 12 cylinders, 74640 horse power and acruise speed of 24 knots.A direct reversing engine like the one fitted on the Sally Mærsk has no clutch andno gear: one revolution of the engine equals one revolution of the main propeller.Accelerations are thus achieved by increasing the amount of fuel being pumped intothe cylinders. While en route (also referred to as 'sea passage') only very few andminor changes are made to the RPM of the main engine. However, well in timebefore a harbor approach, the operational mode of the main engine will be switchedto 'maneuvering' allowing for rapid and frequent changes to the production ofpropulsion. The below table lists the approximate correlation between RPM andspeed.RPM and SpeedEngine order RPM Speedloaded conditionSpeedballast condition85% MCR AHEAD 94 23.6 knots 24.6 knotsFULL AHEAD 65 16.7 knots 17.3 knotsHALF AHEAD 50 12.8 knots 13.6 knotsSLOW AHEAD 35 8.9 knots 9.6 knotsDEAD SLOW AHEAD 25 6.3 knots 6.9 knotsFigure 24. RPM and speed.The number of revolutions per minute (RPM) of the main engine is controlleddirectly from the bridge. Three operational modes are available:1. Automatic speed control via the voyage management system2. Automatic speed control via the auto pilot3. Automatic speed control via the load program4. Manual speed control via the engine telegraph5. Manual speed control via the emergency engine telegraph6. Manual speed control via telephone connection to an engineer in the enginecontrol roomThe voyage management system and the auto pilot represent the two highest levelsin the speed control system (which runs on top of the engine control system). Boththe voyage management system and the auto pilot are capable of controlling thevessels speed (via altering the RPM of the main engine) but in practice speed isalways controlled via hands on control of the main engine telegraph.Accelerations up to speeds of about six knots are performed fully manually, whilethe remaining acceleration up to cruise speed is handled automatically by the socalled'load program'. In case the RPM of the main engine is increased at a too fastrate, the risk of inflicting serious thermal damage to the engine is imminent.14

Figure 25. Main engine telegraph.Basically, the load program functions as an automatic interceptor preventingoverheating of the cylinder linings by limiting acceleration to a rate of one RPM per30 seconds. The desired RPM is set on the main engine telegraph or on the loadprogram display, and once the desired RPM has been reached the load program willmaintain it.In case of engine telegraph malfunction speed commands can be communicated tothe engine control room via the emergency telegraph or via the closed circuitemergency telephone.Figure 26. From right to left: emergency telephone, emergency telegraph, and the display of the loadprogram.The emergency telegraph (at the center of figure 24) on the bridge is linked to asimilar display located in the engine control room. The back-lit buttons on theemergency telegraph represent speed settings (slow ahead, half ahead, slow astern,15

stop engine, etc.). When a button is pushed on the bridge (indicating a command) thebutton will start blinking on the bridge and in the engine control room. The engineerin the engine control room confirms the command by pressing the button: theblinking stops and the now constantly lit button indicates the most speed command.The load program display is shown at the right hand side of figure 24. The twonumerical readings represent the desired and the actual RPM of the main engine. Thebuttons at the bottom of the display for fine tuning the RPM and for overriding theload program (the latter used in cases of emergency only).In terms of acceleration it is important to notice that it takes more than horsepower to get the vessel moving: it also takes water under the keel, and preferably lotsof it. Forceful accelerations in shallow waters will lead to increased turbulence at thepropeller, a higher stern wave, bad fuel economy, and only limited acceleration. As arule of thumb the vessel needs a water depth below the keel equaling twice its draughtin order to accelerate freely.DecelerationThe massive inertia inherent to the vessel becomes strikingly clear should the needfor a crash astern maneuver (emergency braking) occur. The below table lists the timeand distance needed to arrest the ahead movement of the vessel at various speedsduring loaded and ballast condition.Time and distance to stopLoaded condition Ballast conditionTime Distance Time Distance85% MCR ahead 960 sec 6100 m. 540 sec. 3240 m.HALF ahead 410 sec. 1600 m. 260 sec. 1000 m.Figure 27: Time and distance to stop the vessel.The long stopping distances are not solely the product of inertia, but can also beatributed to the de facto powers of braking available given the nature of the directreversing engine.The first step in reducing the speed of the vessel is to cut off the fuel supply to themain engine. This is done by setting the main engine telegraph in the 'Stop' position.Hereafter no power will be produced, yet the pistons will still move as a consequenceof the water pressure on the propeller: The rotations of the propeller will beforwarded to the pistons via the drive shaft and the connecting rods. Active brakingwould take to stop and reverse the pistons. However, the propeller has a diameter of10 meters, and when travelling at full speed, the forces produced by the water turningthe propeller simply prevent active braking. That is, active braking at full speed ispossible (via the engine telegraph) but at the risk of severely damaging or tearingloose the main propulsion plant. Therefore, it is not recommended to go astern untilthe vessel is at a speed below six knots.Between full speed and six knots, the only safe way of reducing the vessel’s speedactively is by means of the rudder. The rudder blade has a surface area of more than100 m2, and by putting the rudder at the maximum angle (35°) to either starboard orport, a noticeable braking effect is achieved. When the vessel starts turning, therudder is moved from hard a port to hard a starboard or vice versa.16

Figure 28: The crash astern test.Finally, the crew can decide to let go the anchors in order to arrest the vessel. Inpractice, however, this is only feasible at low speed—or else, at best, they risk rippingthe anchor chain or tearing the winches clean off the mooring deck! Yet, if this iswhat it takes to avoid a collision or grounding, these types of damages can certainlybe considered minute.Uneventful deceleration is performed over a relatively long time span. Most oftenthe deceleration from full to maneuvering speed (six knots) is handled automaticallyby the load program that cuts the engine’s revolutions at a rate of two RPM perminute. Decelerations from six knots and down are performed manually by means ofthe main engine telegraph (but preferably at a moderate rate). The load program canbe overridden in cases of emergency.ThrustersThe Sally Mærsk is fitted with three thrusters. One Brunvol thruster with a maximumlateral thrust of 30 t. at the bow and two 15 t. thrusters at the stern. The bow thrusteris fitted 22.23 m. abaft the front of the bulbous bow at a height of 4.69 m. above thekeel. The stern thrusters are fitted 28.58 m. and 33.42 m. forward of the transom at aheight of 2.66 m. above the keel.17

Figure 29. Bow thruster (not Sally Mærsk's thruster)A thruster is a device combined of a pipe crossing the vessel's hull and a propellerat the center of the pipe capable of pumping water from one side of the vessel to theother, thus producing a vacuum on the one side of the vessel and a lateral push orthrust on the opposite side. The main purpose of the thrusters is to provide a positiveside thrust at a 90° angle to port or starboard side of the vessel when steering byrudder alone is insufficient at low speed.EfficiencyLoaded conditionBowthrusterSternthrustersof bow- and stern thrustersBallast conditionBowthrusterSternthrustersSpeedaheadinknotsEfficiency in percent100 100 100 100 075 88 50 85 350 75 0 70 625 63 0 55 90 50 0 40 12Figure 30: Efficiency of bow- and stern thrusters.As it can be seen from the table, the thrusters are most efficient at low speed. At aspeed of zero knots ahead, the thrusters provide a maximum of 60 t. While at 12knots the effect is down to some 25 t. in total.18

Figure 31: Thruster controls.The thrusters are operated from the bridge via the thruster controls. The upperhandle is for controlling the bow thruster and the low handle controls the sternthruster. Thrust is produced by turning the handle to either starboard or port.The primary use of the thrusters is in and around the harbor area where the vesselis progressing at slow speed with limited powers of steering on the rudder. Thrustersare used for turning the vessel around its axis and for sideward movements.19