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CHAPTER 37

WEATHER ROUTING

PRINCIPLES OF WEATHER ROUTING

3700. Introduction

Ship weather routing develops an optimum track forocean voyages based on forecasts of weather, seaconditions, and a ship’s individual characteristics for aparticular transit. Within specified limits of weather and seaconditions, the term optimum is used to mean maximumsafety and crew comfort, minimum fuel consumption,minimum time underway, or any desired combination ofthese factors. The purpose of this chapter is to acquaint themariner with the basic philosophy and procedures of shipweather routing as an aid to understanding the routingagency’s recommendations.

The mariner’s first resources for route planning inrelation to weather are thePilot Chart Atlases, theSailingDirections (Planning Guides), and other climatologicalsources such as historical weather data tables. Thesepublications give climatic data, such as wind speed anddirection, wave height frequencies and ice limits, for themajor ocean basins of the world. They may recommendspecific routes based on probabilities, but not on specificconditions.

The ship routing agency, acting as an advisory service,attempts to avoid or reduce the effects of specific adverseweather and sea conditions by issuing initial routerecommendations prior to sailing. It recommends trackchanges while underway (diversions), and weatheradvisories to alert the commanding officer or master aboutapproaching unfavorable weather and sea conditions whichcannot be effectively avoided by a diversion. Adverseweather and sea conditions are defined as those conditionswhich will cause damage, significant speed reduction, ortime loss.

The initial route recommendation is based on a surveyof weather and sea forecasts between the point of departureand the destination. It takes into account the type of vessel,hull type, speed capability, safety considerations, cargo,and loading conditions. The vessel’s progress is continuallymonitored, and if adverse weather and sea conditions areforecast along the vessel’s current track, a recommendationfor a diversion or a weather advisory is transmitted. By thisprocess of initial route selection and continued monitoringof progress for possible changes in the forecast weather andsea conditions along a route, it is possible to maximize bothspeed and safety.

In providing for optimum sailing conditions, the

advisory service also attempts to reduce transit timeavoiding the adverse conditions which may be encounteon a shorter route, or if the forecasts permit, diverting toshorter track to take advantage of favorable weather andconditions. A significant advantage of weather routinaccrues when: (1) the passage is relatively long, abo1,500 miles or more; (2) the waters are navigationalunrestricted, so that there is a choice of routes; andweather is a factor in determining the route to be followe

Use of this advisory service in no way relieves thcommanding officer or master of responsibility for prudenseamanship and safe navigation. There is no intent byrouting agency to inhibit the exercise of professionjudgment and prerogatives of commanding officers amasters.

The techniques of ship routing and access to the advare increasingly less expensive, and are thus being mavailable to coastal vessels, smaller commercial craft, aeven yachts.

3701. Historical Perspective

The advent of extended range forecasting anddevelopment of selective climatology, along with powerfucomputer modeling techniques, have made ship routing systpossible. The ability to effectively advise ships to takadvantage of favorable weather was hampered previouslyforecast limitations and the lack of an effective communicatiosystem.

Development work in the area of data accumulation aclimatology has a long history. Benjamin Franklin, as depupostmaster general of the British Colonies in North Americproduced a chart of the Gulf Stream from information suppliby masters of New England whaling ships. This first mappingthe Gulf Stream helped improve the mail packet servibetween the British Colonies and England. In some passagesailing time was reduced by as much as 14 days over roupreviously sailed.

In the mid-19th century, Matthew Fontaine Maurcompiled large amounts of atmospheric and oceanographicfrom ships’ log books. For the first time, a climatology of oceaweather and currents of the world was available to the marinThis information was used by Maury to develop seasonarecommended routes for sailing ships and early steam powevessels in the latter half of the 19th century. In many casMaury’s charts were proved correct by the savings in tran

545

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time. Average transit time on the New York to California viaCape Horn route was reduced from 183 days to 139 days withthe use of his recommended seasonal routes.

In the 1950’s the concept of ship weather routing wasput into operation by several private meteorological groupsand by the U.S. Navy. By applying the available surface andupper air forecasts to transoceanic shipping, it was possibleto effectively avoid much heavy weather while generallysailing shorter routes than previously. The development ofcomputers, the internet and communications technology hasmade weather routing available to nearly everyone afloat.

3702. System Types

Optimum Track Ship Routing (OTSR), the ship rout-ing service of the U.S. Navy, utilizes short range andextended range forecasting techniques in route selectionand surveillance procedures. The short range dynamic fore-casts of 3 to 5 days are derived from meteorologicalequations. These forecasts are computed at least twice dailyfrom a data base of northern hemisphere surface and upperair observations, and include surface pressure, upper airconstant pressure heights, and the spectral wave values. Asignificant increase in data input, particularly from satelliteinformation over ocean areas, can extend the time periodfor which these forecasts are useful.

Selective climatology has been effective in predictingaverage conditions months in advance during such eventsas El Nino and La Nina. Such predictions do not representforecasting, but can indicate the likelihood of certain condi-tions prevailing.

For extended range forecasting, generally 3 to 14 days,a computer searches a library of historical northern hemi-sphere surface pressure and 500 hPa analyses for ananalogous weather pattern. This is an attempt at selectiveclimatology by matching the current weather pattern withpast weather patterns and providing a logical sequence-of-events forecast for the 10 to 14 day period following the dy-namic forecast. It is performed for both the Atlantic andPacific Oceans using climatological data for the entire peri-od of data stored in the computer. For longer ocean transits,monthly values of wind, seas, fog, and ocean currents areused to further extend the time range.

Aviation was first in applying the principle of mini-mum time tracks (MTT) to a changing wind field. But theproblem of finding an MTT for a specific flight is muchsimpler than for a transoceanic ship passage because an air-craft’s transit time is much shorter than a ship’s. Thus,marine minimum time tracks require significantly longerrange forecasts to develop an optimum route.

Automation has enabled ship routing agencies to de-velop realistic minimum time tracks. Computation ofminimum time tracks makes use of:

1. A navigation system to compute route distance,time enroute, estimated times of arrival (ETA’s),

and to provide 6 hourly DR synoptic positions fothe range of the dynamic forecasts for the shipcurrent track.

2. A surveillance system to survey wind, seas, foand ocean currents obtained from the dynamic aclimatological fields.

3. An environmental constraint system imposedpart of the route selection and surveillance proceConstraints are the upper limits of wind and seadesired for the transit. They are determined by thship’s loading, speed capability, and vulnerabilityThe constraint system is an important part of throute selection process and acts as a warnisystem when the weather and sea forecast alongpresent track exceeds predetermined limits.

4. Ship speed characteristics used to approximship’s speed of advance (SOA) while transiting thforecast sea states.

Criteria for route selection reflect a balance betweethe captain’s desired levels of speed, safety, comfort, aconsideration of operations such as fleet maneuvers, fiing, towing, etc.

Ship weather routing services are being offered bmany nations. These include Japan, United Kingdom, Rsia, Netherlands, Germany, and the United States. Alseveral private firms provide routing services to shippinindustry clients. Several PC-based software applicatiohave become available, making weather routing availato virtually everyone at sea.

There are two general types of routing services avaable. The first uses techniques similar to the Navy’s OTSsystem to forecast conditions and compute routing recomendations, which are then broadcast to the vessel. Tsecond assembles and processes weather and sea condata and transmits this to ships at sea for on-board proceing and generation of route recommendations. The formsystem allows for greater computer power to be appliedthe routing task because powerful computers are availaashore. The latter system allows greater flexibility to thship’s master in changing parameters, evaluating varioscenarios, selecting routes, and displaying data.

3703. Ship and Cargo Considerations

Ship and cargo characteristics have a significant inflence on the application of ship weather routing. Ship sizspeed capability, and type of cargo are important considations in the route selection process prior to sailing and tsurveillance procedure while underway. A ship’s characteistics identify its vulnerability to adverse conditions and itability to avoid them.

Generally, ships with higher speed capability and ligher loads will have shorter routes and be better ablemaintain near normal SOA’s than ships with lower speecapability or heavy cargoes. Some routes are unique

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cause of the type of ship or cargo. Avoiding one element ofweather to reduce pounding or rolling may be of prime im-portance. For example, a 20 knot ship with a heavy deckcargo may be severely hampered in its ability to maintain a20 knot SOA in any seas exceeding moderate head or beamseas because of the possibility of damage resulting from thedeck load’s characteristics. A similar ship with a stable car-go under the deck is not as vulnerable and may be able tomaintain the 20 knot SOA in conditions which would dras-tically slow the deck-loaded vessel. In towing operations, atug is more vulnerable to adverse weather and sea condi-tions, not only in consideration of the tow, but also becauseof its already limited speed capability. Its slow speed addsto the difficulty of avoiding adverse weather and seaconditions.

Ship performance curves (speed curves) are used to es-timate the ship’s SOA while transiting the forecast seastates. The curves indicate the effect of head, beam, and fol-lowing seas of various significant wave heights on theship’s speed. Figure 3703 is a performance curve preparedfor a commercial 18-knot vessel. Each vessel will have itsown performance curves, which vary widely according tohull type, length, beam, shape, power, and tonnage. Recom-mendations for sailing vessels must account for wind speed,wind angle, and vessel speed.

With the speed curves it is possible to determine justhow costly a diversion will be in terms of the requireddistance and time. A diversion may not be necessary wherethe duration of the adverse conditions is limited. In thiscase, it may be better to ride out the weather and seasknowing that a diversion, even if able to maintain thenormal SOA, will not overcome the increased distance and

time required.

At other times, the diversion track is less costlbecause it avoids an area of adverse weather andconditions, while being able to maintain normal SOA evethough the distance to destination is increased. Basedinput data for environmental conditions and shipbehavior, route selection and surveillance techniques sto achieve the optimum balance between time, distanand acceptable environmental and seakeeping conditioAlthough speed performance curves are an aid to the srouting agency, the response by mariners to deterioratweather and sea conditions is not uniform. Some reduspeed voluntarily or change heading sooner than othwhen unfavorable conditions are encountered. Certawaves with characteristics such that the ship’s bow astern are in successive crests and troughs present speproblems for the mariner. Being nearly equal to the shiplength, such wavelengths may induce very dangerostresses. The degree of hogging and sagging andassociated danger may be more apparent to the marinerto the ship routing agency. Therefore, adjustment in couand speed for a more favorable ride must be initiated by tcommanding officer or master when this situationencountered.

3704. Environmental Factors

Environmental factors of importance to ship weathrouting are those elements of the atmosphere and oceanmay produce a change in the status of a ship transit. In srouting, consideration is given to wind, seas, fog, ice, aocean currents. While all of the environmental factors a

Figure 3703. Performance curves for head, beam, and following seas.

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important for route selection and surveillance, optimumrouting is normally considered attained if the effects ofwind and seas can be optimized.

Wind: The effect of wind speed on ship performanceis difficult to determine. In light winds (less than 20-knots),ships lose speed in headwinds and gain speed slightly in fol-lowing winds. For higher wind speeds, ship speed isreduced in both head and following winds. This is due to theincreased wave action, which even in following seas resultsin increased drag from steering corrections, and indicatesthe importance of sea conditions in determining ship per-formance. In dealing with wind, it is also necessary to knowthe ship’s sail area. High winds will have a greater adverseeffect on a large, fully loaded container ship or car carrierthan a fully loaded tanker of similar length. This effect ismost noticeable when docking, but the effect of beamwinds over several days at sea can also be considerable. Forsailing vessels, the wind is critical and accurate forecastsare vital to a successful voyage.

Wave Height: Wave height is the major factoraffecting ship performance. Wave action is responsible forship motions which reduce propeller thrust and causeincreased drag from steering corrections. The relationshipof ship speed to wave direction and height is similar to thatof wind. Head seas reduce ship speed, while following seasincrease ship speed slightly to a certain point, beyondwhich they retard it. In heavy seas, exact performance maybe difficult to predict because of the adjustments to courseand speed for shiphandling and comfort. Although theeffect of sea and swell is much greater for large commercialvessels than is wind speed and direction, it is difficult toseparate the two in ship routing.

In an effort to provide a more detailed description ofthe actual and forecast sea state, the U.S. Navy Fleet Nu-merical Meteorology and Oceanography Center,Monterey, California, produces the Global Spectral OceanWave Model (GSOWM) for use by the U.S. Navy’s Opti-mum Track Ship Routing (OTSR) service. This modelprovides energy values from 12 different directions (30°sectors) and 15 frequency bands for wave periods from 6to 26 seconds with the total wave energy propagatedthroughout the grid system as a function of direction andfrequency. It is based on the analyzed and forecast plane-tary boundary layer model wind fields, and is producedfor both the Northern and Southern Hemispheres out to 72hours. For OTSR purposes, primary and secondary wavesare derived from the spectral wave program, where theprimary wave train has the principal energy (direction andfrequency), and the secondary has to be 20 percent of theprimary.

Fog: Fog, while not directly affecting ship perfor-mance, should be avoided as much as feasible, in order tomaintain normal speed in safe conditions. Extensive areasof fog during summertime can be avoided by selecting alower latitude route than one based solely upon wind andseas. Although the route may be longer, transit time may

be less due to not having to reduce speed in reduced vbility. In addition, crew fatigue due to increasedwatchkeeping vigilance can be reduced.

North Wall Effect: During the Northern Hemispherefall and winter, the waters to the north of the Gulf Streain the North Atlantic are at their coldest, while the GulStream itself remains at a constant relatively warm temperature. After passage of a strong cold front or behinddeveloping coastal low pressure system, Arctic airsometimes drawn off the Mid-Atlantic coast of the UniteStates and out over the warm waters of the Gulf Streamnortherly winds. This cold air is warmed as it passes ovthe Gulf Stream, resulting in rapid and intense deepeniof the low pressure system and higher than normal surfawinds. Higher waves and confused seas result from thewinds. When these winds oppose the northeast set ofcurrent, the result is increased wave heights and a shorting of the wave period. If the opposing current isufficiently strong, the waves will break. These phenomna are collectively called the “North Wall Effect,”referring to the region of most dramatic temperatuchange between the cold water to the north and the waGulf Stream water to the south. The most dangerouspect of this phenomenon is that the strong winds aextremely high, steep waves occur in a limited area amay develop without warning. Thus, a ship that is laboing in near-gale force northerly winds and rough seaproceeding on a northerly course, can suddenly encounstorm force winds and dangerously high breaking seNumerous ships have foundered off the North Americacoast in the approximate position of the Gulf StreamNorth Wall. A similar phenomenon occurs in the NortPacific near the Kuroshio Current and off the SoutheaAfrican coast near the Agulhas Current.

Ocean Currents: Ocean currents do not presentsignificant routing problem, but they can be a determininfactor in route selection and diversion. This is especiatrue when the points of departure and destination arerelatively low latitudes. The important considerations tbe evaluated are the difference in distance betweengreat-circle route and a route selected for optimucurrent, with the expected increase in SOA from thfollowing current, and the decreased probability ofdiversion for weather and seas at the lower latitude. Fexample, it has proven beneficial to remain equatorwaof approximately 22°N for westbound passages betweethe Canal Zone and southwest Pacific ports. Feastbound passages, if the maximum latitude on a grecircle track from the southwest Pacific to the Canal Zonis below 24°N, a route passing near the axis of thEquatorial Countercurrent is practical because tincreased distance is offset by favorable current. Directiand speed of ocean currents are more predictable twind and seas, but some variability can be expecteMajor ocean currents can be disrupted for several daysvery intense weather systems such as hurricanes and

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global phenomena such as El Nino.Ice: The problem of ice is twofold: floating ice (ice-

bergs) and deck ice. If possible, areas of icebergs or pack iceshould be avoided because of the difficulty of detection andthe potential for collision. Deck ice may be more difficult tocontend with from a ship routing point of view because it iscaused by freezing weather associated with a large weathersystem. While mostly a nuisance factor on large ships, itcauses significant problems with the stability of small ships.

Latitude: Generally, the higher the latitude of a route,even in the summer, the greater are the problems with theenvironment. Certain operations should benefit fromseasonal planning as well as optimum routing. Forexample, towing operations north of about 40° latitudeshould be avoided in non-summer months if possible.

3705. Synoptic Weather Considerations

A ship routing agency should direct its forecastingskills to avoiding or limiting the effect of weather and seasassociated with extratropical low pressure systems in themid and higher latitudes and the tropical systems in low lat-itude. Seasonal or monsoon weather is also a factor in routeselection and diversion in certain areas.

Despite the amount of attention and publicity given totropical cyclones, mid-latitude low pressure systemsgenerally present more difficult problems to a ship routingagency. This is primarily due to the fact that major shiptraffic is sailing in the latitudes of the migrating lowpressure systems, and the amount of potential exposure tointense weather systems, especially in winter, is muchgreater.

Low pressure systems weaker than gale intensity(winds less than 34 knots) are not a severe problem for mostships. However, a relatively weak system may generate pro-longed periods of rough seas which may hamper normalwork aboard ship. Ship weather routing can frequently limitrough conditions to short periods of time and provide morefavorable conditions for most of the transit. Relatively smallvessels, tugs with tows, low powered ships, yachts, andships with sensitive cargoes can be significantly affected byweather systems weaker than gale intensity. Using a routingagency can enhance both safety and efficiency.

Gales (winds 34 to 47 knots) and storms (winds greaterthan 48 knots) in the open sea can generate very rough orhigh seas, particularly when an adverse current such as theGulf Stream is involved. This can force a reduction in speedin order to gain a more comfortable and safe ride. Because ofthe extensive geographic area covered by a well developedlow pressure system, once ship’s speed is reduced the abilityto improve the ship’s situation is severely hampered. Thus,exposure to potential damage and danger is greatly increased.The vessel in such conditions may be forced to slow downjust when it is necessary to speed up to avoid even worseconditions.

A recommendation for a diversion by a routing agency

well in advance of the intense weather and associated swill limit the duration of exposure of the vessel. If effectiveship speed will not be reduced and satisfactory progress wbe maintained even though the remaining distancedestination is increased. Overall transit time is usuashorter than if no track change had been made and the shad remained in heavy weather. In some cases diversionsmade to avoid adverse weather conditions and shortentrack at the same time. Significant savings in time and cocan result.

In very intense low pressure systems, with high windand long duration over a long fetch, seas will be generaand propagated as swell over considerable distances. Eon a diversion, it is difficult to effectively avoid allunfavorable conditions. Generally, original routes fotransoceanic passages, issued by the U.S. Navy’s srouting service, are equatorward of the 10% frequenisoline for gale force winds for the month of transit, ainterpreted from the U.S. Navy’s Marine Climatic Atlas othe World. These are shown in Figure 3705a and Figu3705b for the Pacific. To avoid the area of significant gaactivity in the Atlantic from October to April, the latitude oftransit is generally in the lower thirties.

The areas, seasons, and the probability of developmof tropical cyclones are fairly well defined in climatologicapublications. In long range planning, considerable benecan be gained by limiting the exposure to the potenthazards of tropical systems.

In the North Pacific, avoid areas with the greateprobability of tropical cyclone formation. Avoiding existingtropical cyclones with a history of 24 hours or more of 6hourly warnings is in most cases relatively straightforwarHowever, when transiting the tropical cyclone generatinarea, the ship under routing may provide the first reportenvironmental conditions indicating that a new disturbanis developing. In the eastern North Pacific the generatiarea for a high percentage of tropical cyclones is relativecompact (Figure 3705c). Remain south of a line from la9°N, long. 90°W to lat. 14°N, long. 115°W. In the westernNorth Pacific it is advisable to hold north of 22°N when notropical systems are known to exist. See Figure 3705d.

In the Atlantic, sail near the axis of the Bermuda high onorthward to avoid the area of formation of tropical cycloneOf course, avoiding an existing tropical cyclone takeprecedence over avoiding a general area of potendevelopment.

It has proven equally beneficial to employ similaconsiderations for routing in the monsoon areas of tIndian Ocean and the South China Sea. Thisaccomplished by providing routes and diversions thgenerally avoid the areas of high frequency of gale forwinds and associated heavy seas, as much as feasible. Scan then remain in satisfactory conditions with limiteincreases in route distance.

Depending upon the points of departure and destinatithere are many combinations of routes that can be used w

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transiting the northern Indian Ocean (Arabian Sea, Bay ofBengal) and the South China Sea. For example, in theArabian Sea during the summer monsoon, routes to and fromthe Red Sea, the western Pacific, and the eastern IndianOcean should hold equatorward. Ships proceeding to thePersian Gulf during this period are held farther south andwest to put the heaviest seas on the quarter or stern whentransiting the Arabian Sea. Eastbound ships departing thePersian Gulf may proceed generally east southeast towardthe Indian sub-continent, then south, to pass north and east ofthe highest southwesterly seas in the Arabian Sea.Westbound ships out of the Persian Gulf for the Cape ofGood Hope appear to have little choice in routes unlessconsiderable distance is added to the transit by passing eastof the highest seas. In the winter monsoon, routes to or fromthe Red Sea for the western Pacific and the Indian Ocean areheld farther north in the Arabian Sea to avoid the highestseas. Ships proceeding to the Persian Gulf from the westernPacific and eastern Indian Ocean may hold more eastwardwhen proceeding north in the Arabian Sea. Ships departingthe Persian Gulf area will have considerably less difficultythan during the summer monsoon. Similar considerationscan be given when routing ships proceeding to and from theBay of Bengal.

In the South China Sea, transits via the PalawanPassage are recommended when strong, opposing wind andseas are forecast. This is especially true during the wintermonsoon. During periods when the major monsoon flow isslack, ships can use the shortest track as conditions permit.

3706. Special Weather and Environmental Consider-ations

In addition to the synoptic weather considerations inship weather routing, there are special environmentalproblems that can be avoided by following recommen-dations and advisories of ship routing agencies. Theseproblems generally cover a smaller geographic area and areseasonal in nature, but are still important to ship routing.

In the North Atlantic, because of heavy shippingtraffic, frequent poor visibility in rain or fog, and restrictednavigation, particularly east of Dover Strait, some marinersprefer to transit to or from the North Sea via Pentland Firth,passing north of the British Isles rather than via the EnglishChannel.

Weather routed ships generally avoid the area of densefog with low visibility in the vicinity of the Grand Banks offNewfoundland and the area east of Japan north of 35°N.Fishing vessels in these two areas provide an added hazard tosafe navigation. This condition exists primarily from Junethrough September. Arctic supply ships en route from theU.S. east coast to the Davis Strait-Baffin Bay area in thesummer frequently transit via Cabot Strait and the Strait ofBelle Isle, where navigation aids are available and icebergsare generally grounded.

Icebergs are a definite hazard in the North Atlantic from

late February through June, and occasionally later. Thazard of floating ice is frequently combined with restrictevisibility in fog. International Ice Patrol reports and warningare incorporated into the planning of routes to safely avodangerous iceberg areas. It is usually necessary to hold sof at least 45°N until well southeast of Newfoundland. TheU.S. Navy ship routing office at the Naval AtlanticMeteorology and Oceanography Center in Norfolk maintaia safety margin of at least 100 miles from icebergs reporby the International Ice Patrol. Also, in a severe winter, thDenmark Strait may be closed by ice.

In the northern hemisphere winter, a strong higpressure system moving southeast out of the RocMountains brings cold air down across Central America athe western Gulf of Mexico producing gale force winds in thGulf of Tehuantepec. This fall wind is similar to thepampero, mistral, and bora of other areas of the world. Aadjustment to ship’s track can successfully avoid the highseas associated with the “Tehuantepecer.” For tranbetween the Canal Zone and northwest Pacific ports, litadditional distance is required to avoid this area (in winteby remaining south of at least 12°N when crossing 97°W.While avoiding the highest seas, some unfavorable swconditions may be encountered south of this line. Shitransiting between the Panama Canal and North Americwest coast ports can stay close along the coast of the GuTehuantepec to avoid heavy seas during gale conditions,may still encounter high offshore winds.

In the summer, the semi-permanent high pressusystems over the world’s oceans produce strong equatorwflow along the west coasts of continents. This feature is mpronounced off the coast of California and Portugal in thNorthern Hemisphere and along Chile, western Australand southwest Africa in the Southern Hemisphere. Verough seas are generated and are considered a definite fain route selection or diversion when transiting these area

3707. Types of Recommendations and Advisories

An initial route recommendation is issued to a ship orrouting authority normally 48 to 72 hours prior to sailing, anthe process of surveillance begins. Surveillance iscontinuous process, maintained until the ship arrives atdestination. Initial route recommendations are a composrepresentation of experience, climatology, weather andstate forecasts, the vessel’s mission and operatioconcerns, and the vessel’s seagoing characteristics.planning route provides a best estimate of a realistic routea specific transit period. Such routes are provided whestimated dates of departure (EDD’s) are given to the routagency well in advance of departure, usually a weekseveral months. Long range planning routes are based mon seasonal and climatological expectations than the currweather situation. While planning routes are an attemptmake extended range (more than a week) or long range (mthan a month) forecasts, these recommendations are likel

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be revised near the time of departure to reflect the currentweather pattern. An initial route recommendation is moreclosely related to the current weather patterns by using thelatest dynamic forecasts than are the planning routerecommendations. These, too, are subject to revision prior tosailing, if weather and sea conditions warrant.

Adjustment of departure time is a recommendationfor delay in departure, or early departure if feasible, and isintended to avoid or significantly reduce the adverse weatherand seas forecast on the first portion of the route, if sailing onthe original EDD. The initial route is not revised, only thetiming of the ship’s transit through an area with currentlyunfavorable weather conditions. Adjusting the departuretime is an effective method of avoiding a potentiallyhazardous situation where there is no optimum route forsailing at the originally scheduled time. A go/no gorecommendation may be made to vessels engaged in specialmissions such as speed record attempts or heavy-liftvoyages.

A diversion is an underway adjustment in track and isintended to avoid or limit the effect of adverse weatherconditions forecast to be encountered along the ship’s currenttrack, or to take advantage of favorable conditions alonganother route. Ship’s speed is expected to be reduced by theencounter with the heavy weather. In most cases the distanceto destination is increased in attempting to avoid the adverseweather, but this is partially overcome by being able tomaintain a nearly normal SOA.

Adjustment of SOA is a recommendation for slowingor increasing the ship’s speed as much as practicable, in anattempt to avoid an adverse weather situation by adjustingthe timing of the encounter. This is also an effective meansof maintaining maximum ship operating efficiency, whilenot diverting from the present ship’s track. By adjusting theSOA, a major weather system can sometimes be avoidedwith no increase in distance. The development of fast ships(SOA greater than 30 knots) gives the ship routing agencythe potential to “make the ship’s weather” by adjusting theship’s speed and track for encounter with favorable weatherconditions.

Evasion is a recommendation to the vessel to takeindependent action to avoid, as much as possible, apotentially dangerous weather system. The ship routingmeteorologist may recommend a general direction for safeevasion but does not specify an exact track. The recommen-dation for evasion is an indication that the weather and seaconditions have deteriorated to a point where shiphandlingand safety are the primary considerations and progresstoward destination has been temporarily suspended, or is atleast of secondary consideration.

A weather advisory is a transmission sent to the shipadvising the commanding officer or master of expectedadverse conditions, their duration, and geographic extent. Itis initiated by the ship routing agency as a service and an aidto the ship. The best example of a situation for which aforecast is helpful is when the ship is currently in good

weather but adverse weather is expected within 24 hourswhich a diversion has not been recommended, or a diverswhere adverse weather conditions are still expected. Ttype of advisory may include a synoptic weather discussioand a wind, seas, or fog forecast.

The ability of the routing agency to achieve optimumconditions for the ship is aided by the commanding officermaster adjusting course and speed where necessary foefficient and safe ride. At times, the local sea conditions mdictate that the commanding officer or master take independaction.

3708. Southern Hemisphere Routing

Available data on which to base analyses and forecais generally very limited in the Southern Hemispheralthough this situation is improving with the increasinavailability of remotely sensed data. Weather and othenvironmental information obtained from satellitescontributing greatly to an improvement in southerhemisphere forecast products.

Passages south of the Cape of Good Hope and CHorn should be timed to avoid heavy weather as muchpossible, since intense and frequent low pressure systemscommon in these areas. In particular, near the southecoasts of Africa and South America, intense low pressusystems form in the lee of relatively high terrain near thcoasts of both continents. Winter transits south of Cape Hoare difficult, since the time required for transit is longer thathe typical interval between storms. Remaining equatorwaof about 35°S as much as practicable will limit exposure tadverse conditions. If the frequency of lows passing theareas is once every three or four days, the probabilityencountering heavy weather is high.

Tropical cyclones in the Southern Hemisphere presensignificant problem because of the sparse surface and upair observations from which forecasts can be made. Satellprovide the most reliable means by which to obtain accurapositions of tropical systems, and also give the firindication of tropical cyclone formation.

In the Southern Hemisphere, OTSR and other shweather routing services are available, but are hamperedsparse data reports from which reliable short and extendrange forecasts can be produced. Strong climatologiconsideration is usually given to any proposed southehemisphere transit, but satellite data is increasingavailable to enhance short and extended range forecaOTSR procedures for the Northern Hemisphere caninstituted in the Southern Hemisphere whenever justifiby basic data input and available forecast models.

3709. Communications

A vital part of a ship routing service is communicatiobetween the ship and the routing agency. Reports fromship show the progress and ability to proceed in existi

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conditions. Weather reports from the ship enrich the basicdata on which analyses are based and forecasts derived,assisting both the reporting ship and others in the vicinity.

Despite all efforts to achieve the best forecasts possible,the quality of forecasts does not always warrant maintainingthe route selected. In the U.S. Navy’s ship routing program,experience shows that one-third of the ships using OTSRreceive some operational or weather-dependent changewhile underway.

The routing agency needs reports of the ship’s positionand the ability to transmit recommendations for track changeor weather advisories to the ship. The ship needs both sendand receive capability for the required information.Information on seakeeping changes initiated by the ship isdesirable in a coordinated effort to provide optimum transitconditions. New satellite communications services aremaking possible the transmission of larger amounts of datathan possible through traditional radio messages, adevelopment which supports systems using on-boardanalysis to generate routes.

3710. Benefits

The benefits of ship weather routing services areprimarily in time and cost reductions and increased safety.The savings in operating costs are derived from reductions intransit time, heavy weather encounters, fuel consumption,cargo and hull damage, and more efficient scheduling ofdockside activities. The savings are further increased byfewer emergency repairs, more efficient use of personnel,improved topside working conditions, lower insurance ratesas preferred risks under weather routing, and ultimately,extended ship operating life.

An effective routing service maximizes safety by greatlyreducing the probability of severe or catastrophic damage tothe ship, and injury of crew members. The efficiency andhealth of the crew is also enhanced by avoiding heavyweather. This is especially important on modern, automatedships with reduced crews and smaller craft such as fishingvessels and yachts.

3711. Conclusion

The success of ship weather routing depends upon thevalidity of forecasts and the routing agency’s ability to

make appropriate route recommendations and diversioAnticipated improvements in a routing agency’recommendations will come from advancementsmeteorology, technology, and the application of ocewave forecast models. Advancements in mathematimeteorology, coupled with the continued applicationforecast computer models, will extend the time range aaccuracy of the dynamic and statistical forecasAdditionally, a better understanding of the problemencountered by the mariner and their implications whioffshore will assist the routing agency in makinappropriate recommendations.

Technological advancements in the areas of sateland automated communications and onboard ship resposystems will increase the amount and type of informationand from the ship with fewer delays. Mariners will havebetter quality of meteorological information, and thmeteorologists will have a better understanding of thproblems, constraints, and priorities of the vessel’s masteShip response and performance data included withship’s weather report will provide the routing agency witreal-time information with which to ascertain the actuastate of the ship. Being able to predict a ship’s responsemost weather and sea conditions will result in improverouting procedures.

Shipboard and anchored wave measuring deviccontribute to the development of ocean wave analysis aforecast models. Shipboard seakeeping instrumentation, winput of measured wave conditions and predetermined sresponse data for the particular hull, enables a mastercommanding officer to adjust course and speed for actconditions.

Modern ship designs, exotic cargoes, and sophisticatransport methods require individual attention to eaship’s areas of vulnerability. Any improvement in thdescription of sea conditions by ocean wave models wimprove the output from ship routing and seakeepinsystems.

Advanced planning of a proposed transit, combinewith the study of expected weather conditions, both befoand during the voyage, as is done by ship routing agencand careful on board attention to seakeeping (with instrmentation if available) provide the greatest opportunityachieve the goal of optimum environmental conditions focean transit.