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CHAPTER 38

WEATHER ROUTING

PRINCIPLES OF WEATHER ROUTING

3800. Introduction

Ship weather routing develops an optimum track forocean voyages based on forecasts of weather, sea condi-tions, and a ship’s individual characteristics for a particulartransit. Within specified limits of weather and sea condi-tions, the term optimum is used to mean maximum safetyand crew comfort, minimum fuel consumption, minimumtime underway, or any desired combination of these factors.The purpose of this chapter is to acquaint the mariner withthe basic philosophy and procedures of ship weather rout-ing as an aid to understanding the routing agency’srecommendations.

The mariner’s first resources for route planning in rela-tion to weather are the Pilot Chart Atlases and the SailingDirections (Planning Guides). These publications give cli-matic data, such as wave height frequencies and ice limits,for the major ocean basins of the world. They 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 route recom-mendations prior to sailing, recommendations for trackchanges while underway (diversions), and weather adviso-ries 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 hull type, speedcapability, cargo, and loading conditions. The ship’sprogress is continually monitored, and, if adverse weatherand sea conditions are forecast along the ship’s currenttrack, a recommendation for a diversion or weather adviso-ry is transmitted to the ship. By this process of initial routeselection and continued monitoring of the ship’s progressfor possible changes in the forecast weather and sea condi-tions along a route, it is possible to maximize the ship’sspeed and safety.

In providing optimum sailing conditions, the advisoryservice also attempts to reduce transit time by avoiding theadverse conditions which may be encountered on a shorter

route, or if the forecasts permit, diverting to a shorter trackto take advantage of favorable weather and sea conditions.The greatest potential advantage for this ship weather rout-ing exists when: (1) the passage is relatively long, about1,500 miles or more; (2) the waters are navigationally unre-stricted, so that there is a choice of routes; and (3) weatheris a factor in determining the route to be followed.

Use of this advisory service in no way relieves thecommanding officer or master of responsibility for prudentseamanship and safe navigation. There is no intent by therouting agency to inhibit the exercise of professional judg-ment and prerogatives of commanding officers andmasters.

3801. Historical Perspective

The advent of extended range forecasting and the develop-ment of selective climatology, along with powerful computermodeling techniques, have made ship routing systems possible.The ability to effectively advise ships to take advantage of favor-able weather was hampered previously by forecast limitationsand the lack of an effective communications system.

Development work in the area of data accumulation andclimatology has a long history. Benjamin Franklin, as deputypostmaster general of the British Colonies in North America,produced a chart of the Gulf Stream from information suppliedby masters of New England whaling ships. This first mapping ofthe Gulf Stream helped improve the mail packet service be-tween the British Colonies and England. In some passages thesailing time was reduced by as much as 14 days over routes pre-viously sailed. In the mid-19th century, Matthew FontaineMaury compiled large amounts of atmospheric and oceano-graphic data from ships’ log books. For the first time, aclimatology of ocean weather and currents of the world wasavailable to the mariner. This information was used by Maury todevelop seasonally recommended routes for sailing ships andearly steam powered vessels in the latter half of the 19th century.In many cases, Maury’s charts were proved correct by the sav-ings in transit time. Average transit time on the New York toCalifornia via Cape Horn route was reduced from 183 days to139 days with the 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 generally

540 WEATHER ROUTING

sailing shorter routes than previously.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 twice daily from adata base of northern hemisphere surface and upper air ob-servations, and include surface pressure, upper air constantpressure heights, and the spectral wave values. A signifi-cant increase in data input, particularly from satelliteinformation over ocean areas, can extend the time periodfor which these forecasts are useful.

For extended range forecasting, generally 3 to 14 days,a computer searches a library of historical northern hemi-sphere surface pressure and 500 millibar 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 forthe range of the dynamic forecasts for the ship’scurrent track.

2. A surveillance system to survey wind, seas, fog,and ocean currents obtained from the dynamic andclimatological fields.

3. An environmental constraint system imposed aspart of the route selection and surveillance process.Constraints are the upper limits of wind and seasdesired for the transit. They are determined by theship’s loading, speed capability, and vulnerability.The constraint system is an important part of theroute selection process and acts as a warning sys-tem when the weather and sea forecast along thepresent track exceeds predetermined limits.

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

Ship weather routing services are being offered bymany nations. These include Japan, United Kingdom, Rus-sia, Netherlands, Germany, and the United States. Also,several private firms provide routing services to shippingindustry clients.

There are two general types of commercial ship routingservices. The first uses techniques similar to the Navy’sOTSR system to forecast conditions and compute routingrecommendations. The second assembles and processesweather and sea condition data and transmits this to ships atsea for on-board processing and generation of route recom-mendations. The former system allows for greatercomputer power to be applied to the routing task becausepowerful computers are available ashore. The latter systemallows greater flexibility to the ship’s master in changingparameters, selecting routes, and displaying data.

3802. Ship And Cargo Considerations

Ship and cargo characteristics have a significant influ-ence on the application of ship weather routing. Ship size,speed capability, and type of cargo are important consider-ations in the route selection process prior to sailing and thesurveillance procedure while underway. A ship’s character-istics identify its vulnerability to adverse conditions and itsability to avoid them.

Generally, ships with higher speed capability and lesscargo encumbrances will have shorter routes and be betterable to maintain near normal SOA’s than ships with lowerspeed capability or cargoes. Some routes are unique be-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 3802 is a performance curve preparedfor an 18 knot vessel.

With the speed curves it is possible to determine justhow costly a diversion will be in terms of the required dis-tance and time. A diversion may not be necessary where theduration of the adverse conditions is limited. In this case, itmay be better to ride out the weather and seas knowing that

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a diversion, even if able to maintain the normal SOA, willnot overcome the increased distance and time required.

At other times, the diversion track is less costly be-cause it avoids an area of adverse weather and seaconditions, while being able to maintain normal SOA eventhough the distance to destination is increased. Based on in-put data for environmental conditions and ship’s behavior,route selection and surveillance techniques seek to achievethe optimum balance between time, distance, and accept-able environmental and seakeeping conditions. Althoughspeed performance curves are an aid to the ship routingagency, the response by mariners to deteriorating weatherand sea conditions is not uniform. Some reduce speed vol-untarily or change heading sooner than others whenunfavorable conditions are encountered. Certain waveswith characteristics such that the ship’s bow and stern are insuccessive crests and troughs present special problems forthe mariner. Being nearly equal to the ship’s length, suchwavelengths may induce very dangerous stresses. The de-gree of hogging and sagging and the associated danger maybe more apparent to the mariner than to the ship routingagency. Therefore, adjustment in course and speed for amore favorable ride may be initiated by the commandingofficer or master when this situation is encountered.

3803. Environmental Factors

Environmental factors of importance to ship weatherrouting are those elements of the atmosphere and ocean thatmay produce a change in the status of a ship transit. In shiprouting, consideration is given to wind, seas, fog, ice, andocean currents. While all of the environmental factors are

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.

Wave Height: Wave height is the major factor affect-ing ship performance. Wave action is responsible for shipmotions which reduce propeller thrust and cause increaseddrag from steering corrections. The relationship of shipspeed to wave direction and height is similar to that of wind.Head seas reduce ship speed, while following seas increaseship speed slightly to a certain point, beyond which they re-tard it. In heavy seas, exact performance may be difficult topredict because of the adjustments to course and speed forshiphandling and comfort. Although the effect of sea andswell is much greater than wind, it is difficult to separate thetwo 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,

Figure 3802. Performance curves for head, beam, and following seas.

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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 maybe less due to not having to reduce speed in reduced visi-bility. 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 Streamin the North Atlantic are at their coldest, while the GulfStream itself remains at a constant relatively warm tem-perature. After passage of a strong cold front or behind adeveloping coastal low pressure system, Arctic air issometimes drawn off the Mid-Atlantic coast of the UnitedStates and out over the warm waters of the Gulf Stream bynortherly winds. This cold air is warmed as it passes overthe Gulf Stream, resulting in rapid and intense deepeningof the low pressure system and higher than normal surfacewinds. Higher waves and confused seas result from thesewinds. When these winds oppose the northeast set of thecurrent, the result is increased wave heights and a shorten-ing of the wave period. If the opposing current issufficiently strong, the waves will break. These phenome-na are collectively called the “North Wall Effect,”referring to the region of most dramatic temperaturechange between the cold water to the north and the warmGulf Stream water to the south. The most dangerous as-pect of this phenomenon is that the strong winds andextremely high, steep waves occur in a limited area andmay develop without warning. Thus, a ship that is labor-ing in near-gale force northerly winds and rough seas,proceeding on a northerly course, can suddenly encounterstorm force winds and dangerously high breaking seas.Numerous ships have foundered off the North Americancoast in the approximate position of the Gulf Stream’sNorth Wall. A similar phenomenon occurs in the NorthPacific near the Kuroshio Current and off the SoutheastAfrican coast near the Agulhas Current.

Ocean Currents: Ocean currents do not present a

significant routing problem, but they can be a determiningfactor in route selection and diversion. This is especiallytrue when the points of departure and destination are atrelatively low latitudes. The important considerations tobe evaluated are the difference in distance between agreat-circle route and a route selected for optimum cur-rent, with the expected increase in SOA from thefollowing current, and the decreased probability of a di-version for weather and seas at the lower latitude. Forexample, it has proven beneficial to remain equatorwardof approximately 22°N for westbound passages betweenthe Canal Zone and southwest Pacific ports. For east-bound passages, if the maximum latitude on a great-circletrack from the southwest Pacific to the Canal Zone is be-low 24°N, a route passing near the axis of the EquatorialCountercurrent is practical because the increased distanceis offset by favorable current. Direction and speed ofocean currents are more predictable than wind and seas,but some variability can be expected. Major ocean cur-rents can be disrupted for several days by very intenseweather systems such as hurricanes and by global phe-nomena 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 from sea-sonal planning as well as optimum routing. For example,towing operations north of about 40° latitude should beavoided in non-summer months if possible.

3804. 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 systems gener-ally present more difficult problems to a ship routingagency. This is primarily due to the fact that major ship traf-fic is sailing in the latitudes of the migrating low pressuresystems, and the amount of potential exposure to intenseweather systems, especially in winter, is much greater.

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 normal

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Figure 3804a. Generalized 10% frequency isolines of gale force winds for October through January.

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Figure 3804b. Generalized 10% frequency isolines of gale force winds for January through April..

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Figure 3804c. Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or typhoon intensity, 1957-1974.

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Figure 3804d.. Area of initial detection of high percentage of tropical cyclones which later developed to tropical storm or hurricaine intensity, 1946-1973

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work aboard ship. Ship weather routing can frequently limitrough conditions to short periods of time and provide morefavorable conditions for most of the transit. Relatively smallships, tugs with tows, low powered ships, and ships withsensitive cargoes can be significantly affected by weathersystems weaker than gale intensity. Using a routing agencycan be beneficial.

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.A recommendation for a diversion by a routing agency wellin advance of the intense weather and associated seas willlimit the duration of exposure of the ship. If effective, shipspeed will not be reduced and satisfactory progress will bemaintained even though the remaining distance to destina-tion is increased. Overall transit time is usually shorter thanif no track change had been made and the ship had remainedin heavy weather. In some cases diversions are made to avoidadverse weather conditions and shorten the track at the sametime. Significant savings in time and costs can result.

In very intense low pressure systems, with high windsand long duration over a long fetch, seas will be generatedand propagated as swell over considerable distances. Evenon a diversion, it is difficult to effectively avoid all unfavor-able conditions. Generally, original routes for transoceanicpassages, issued by the U.S. Navy’s ship routing service, areequatorward of the 10% frequency isoline for gale forcewinds for the month of transit, as interpreted from the U.S.Navy’s Marine Climatic Atlas of the World. These are shownin Figure 3804a and Figure 3804b for the Pacific. To avoidthe area of significant gale activity in the Atlantic from Octo-ber to April, the latitude of transit is generally in the lowerthirties.

The areas, seasons, and the probability of developmentof tropical cyclones are fairly well defined in climatologicalpublications. In long range planning, considerable benefitcan be gained by limiting the exposure to the potential haz-ards of tropical systems.

In the North Pacific, avoid areas with the greatest prob-ability of tropical cyclone formation. Avoiding existingtropical cyclones with a history of 24 hours or more of 6-hourly warnings is in most cases relatively straightforward.However, when transiting the tropical cyclone generating ar-ea, the ship under routing may provide the first report ofenvironmental conditions indicating that a new disturbanceis developing. In the eastern North Pacific the generatingarea for a high percentage of tropical cyclones is relativelycompact (Figure 3804c). Remain south of a line from lat.9°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 no

tropical systems are known to exist. See Figure 3804d.In the Atlantic , sail near the axis of the Bermuda high or

northward to avoid the area of formation of tropical cyclones.Of course, avoiding an existing tropical cyclone takes prece-dence over avoiding a general area of potential development.

It has proven equally beneficial to employ similar con-siderations for routing in the monsoon areas of the IndianOcean and the South China Sea. This is accomplished byproviding routes and diversions that generally avoid the ar-eas of high frequency of gale force winds and associatedheavy seas, as much as feasible. Ships can then remain insatisfactory conditions with limited increases in routedistance.

Depending upon the points of departure and destination,there are many combinations of routes that can be used whentransiting the northern Indian Ocean (Arabian Sea, Bay ofBengal) and the South China Sea. For example, in the Arabi-an Sea during the summer monsoon, routes to and from theRed Sea, the western Pacific, and the eastern Indian Oceanshould hold equatorward. Ships proceeding to the PersianGulf during this period are held farther south and west to putthe heaviest seas on the quarter or stern when transiting theArabian Sea. Eastbound ships departing the Persian Gulfmay proceed generally east southeast toward the Indian sub-continent, then south, to pass north and east of the highestsouthwesterly seas in the Arabian Sea. Westbound ships outof the Persian Gulf for the Cape of Good Hope appear to havelittle choice in routes unless considerable distance is added tothe transit by passing east of the highest seas. In the wintermonsoon, routes to or from the Red Sea for the western Pa-cific and the Indian Ocean are held farther north in theArabian Sea to avoid the highest seas. Ships proceeding tothe Persian Gulf from the western Pacific and eastern IndianOcean may hold more eastward when proceeding north in theArabian Sea. Ships departing the Persian Gulf area will haveconsiderably less difficulty than during the summer mon-soon. Similar considerations can be given when routing shipsproceeding to and from the Bay of Bengal.

In the South China Sea, transits via the Palawan Pas-sage 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.

3805. Special Weather And EnvironmentalConsiderations

In addition to the synoptic weather considerations inship weather routing, there are special environmental prob-lems that can be avoided by following recommendationsand advisories of ship routing agencies. These problemsgenerally cover a smaller geographic area and are seasonalin nature, but are still important to ship routing.

In the North Atlantic, because of heavy shipping traf-fic, frequent poor visibility in rain or fog, and restrictednavigation, particularly east of Dover Strait, some mariners

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prefer 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 the sum-mer frequently transit via Cabot Strait and the Strait of BelleIsle, where navigation aids are available and icebergs aregenerally grounded.

Icebergs are a definite hazard in the North Atlantic fromlate February through June, and occasionally later. The haz-ard of floating ice is frequently combined with restrictedvisibility in fog. International Ice Patrol reports and warningsare incorporated into the planning of routes to safely avoiddangerous iceberg areas. It is usually necessary to hold southof at least 45°N until well southeast of Newfoundland. TheU.S. Navy ship routing office at the Naval Atlantic Meteorol-ogy and Oceanography Center in Norfolk maintains a safetymargin of at least 100 miles from icebergs reported by the In-ternational Ice Patrol. Also, in a severe winter, the DenmarkStrait may be closed by ice.

In the northern hemisphere winter, a strong high pres-sure system moving southeast out of the Rocky Mountainsbrings cold air down across Central America and the westernGulf of Mexico producing gale force winds in the Gulf of Te-huantepec. This fall wind is similar to the pampero, mistral,and bora of other areas of the world. An adjustment to ship’strack can successfully avoid the highest seas associated withthe “Tehuantepecer.” For transits between the Canal Zoneand northwest Pacific ports, little additional distance is re-quired to avoid this area (in winter) by remaining south of atleast 12°N when crossing 97°W. While avoiding the highestseas, some unfavorable swell conditions may be encounteredsouth of this line. Ships transiting between the Panama Canaland North American west coast ports can stay close along thecoast of the Gulf of Tehuantepec to avoid heavy seas duringgale conditions, but may still encounter high offshore winds.

In the summer, the semi-permanent high pressure sys-tems over the world’s oceans produce strong equatorwardflow along the west coasts of continents. This feature is mostpronounced off the coast of California and Portugal in theNorthern Hemisphere and along Chile, western Australia,and southwest Africa in the Southern Hemisphere. Veryrough seas are generated and are considered a definite factorin route selection or diversion when transiting these areas.

3806. Types Of Recommendations And Advisories

An initial route recommendation is issued to a ship orrouting authority normally 48 to 72 hours prior to sailing, andthe process of surveillance begins. Surveillance is a continu-ous process, maintained until the ship arrives at its

destination. Initial route recommendations are a compositerepresentation of experience, climatology, weather and seastate forecasts, operational concerns, and the ship’s seagoingcharacteristics. A planning route provides a best estimate ofa realistic route for a specific transit period. Such routes areprovided when estimated dates of departure (EDD’s) are giv-en to the routing agency well in advance of departure, usuallya week to several months. Long range planning routes arebased more on seasonal and climatological expectations thanthe current weather situation. While planning routes are anattempt to make extended range (more than a week) or longrange (more than a month) forecasts, these recommendationsare likely to be revised near the time of departure to reflectthe current weather pattern. An initial route recommendationis more closely related to the current weather patterns by us-ing the latest 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 is in-tended 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 currently un-favorable weather conditions. Adjusting the departure time isan effective method of avoiding a potentially hazardous situ-ation where there is no optimum route for sailing at theoriginally scheduled time.

A diversion is an underway adjustment in track and isintended to avoid or limit the effect of adverse weather con-ditions forecast to be encountered along the ship’s currenttrack. Ship’s speed is expected to be reduced by the encoun-ter with the heavy weather. In most cases the distance todestination is increased in attempting to avoid the adverseweather, but this is partially overcome by being able to main-tain near normal SOA. Diversions are also recommendedwhere satisfactory weather and sea conditions are forecast ona shorter track.

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.

Evasionis a recommendation to the commanding offic-er or master to take independent action to avoid, as much aspossible, a potentially dangerous weather system. The shiprouting meteorologist may recommend a general directionfor safe evasion but does not specify an exact track. The rec-ommendation for evasion is an indication that the weather

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and sea conditions have deteriorated to a point where shiph-andling and safety are the primary considerations andprogress toward destination has been temporarily suspended,or is at least of secondary consideration.

A weather advisory is a transmission sent to the shipadvising the commanding officer or master of expected ad-verse conditions, their duration, and geographic extent. It isinitiated by the ship routing agency as a service and an aid tothe ship. The best example of a situation for which a forecastis helpful is when the ship is currently in good weather butadverse weather is expected within 24 hours for which a di-version has not been recommended, or a diversion whereadverse weather conditions are still expected. This type ofadvisory may include a synoptic weather discussion, and awind, seas, or fog forecast.

The ability of the routing agency to achieve optimum condi-tions for the ship is aided by the commanding officer or masteradjusting course and speed where necessary for an efficient andsafe ride. At times, the local sea conditions may dictate that thecommanding officer or master take independent action.

3807. Southern Hemisphere Routing

Available data on which to base analyses and forecastsis generally very limited in the Southern Hemisphere.Weather and other environmental information obtained fromsatellites offers the possibility of improvement in southernhemisphere forecast products.

Passages south of the Cape of Good Hope and CapeHorn should be timed to avoid heavy weather as much aspossible, since intense and frequent low pressure systems arecommon in these areas. In particular, near the southeastcoasts of Africa and South America, intense low pressuresystems form in the lee of relatively high terrain near thecoasts of both continents. Winter transits south of Cape Hornare difficult, since the time required for transit is longer thanthe typical interval between storms. Remaining equatorwardof about 35°S as much as practicable will limit exposure toadverse conditions. If the frequency of lows passing these ar-eas is once every three or four days, the probability ofencountering heavy weather is high.

Tropical cyclones in the Southern Hemisphere present asignificant problem because of the sparse surface and upperair observations from which forecasts can be made. Satellitesprovide the most reliable means by which to obtain accuratepositions of tropical systems, and also give the first indica-tion of tropical cyclone formation.

In the Southern Hemisphere, OTSR and other shipweather routing services are available, but are limited in ap-plication because of sparse data reports, from whichreliable short and extended range forecasts can be pro-duced. Strong climatological consideration is usually givento any proposed southern hemisphere transit. OTSR proce-dures for the Northern Hemisphere can be instituted in theSouthern Hemisphere whenever justified by basic data in-put and available forecast models.

3808. Communications

A vital part of a ship routing service is communicationbetween the ship and the routing agency. Reports from theship show the progress and ability to proceed in existing con-ditions. Weather reports from the ship enrich the basic dataon which analyses are based and forecasts derived, assistingboth 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 OTSR re-ceive some operational or weather-dependent change whileunderway.

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. Informa-tion on seakeeping changes initiated by the ship is desirablein a coordinated effort to provide optimum transit conditions.New satellite communications services are making possiblethe transmission of larger amounts of data than possiblethrough traditional radio messages, a development whichsupports systems using on-board analysis to generate routes.

3809. Benefits

The benefits of ship weather routing services are prima-rily in cost reduction and safety. The savings in operatingcosts are derived from reductions in transit time, heavyweather encounters, fuel consumption, cargo and hull dam-age, and more efficient scheduling of dockside activities. Thesavings are further increased by fewer emergency repairs,more efficient use of personnel, improved topside workingconditions, lower insurance rates as preferred risks underweather 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 heavy weath-er. This is especially important on modern, automated shipswith reduced crews.

3810. Conclusion

The success of ship weather routing is dependent uponthe validity of the forecasts and the routing agency’s abilityto make appropriate route recommendations and diver-sions. Anticipated improvements in a routing agency’srecommendations will come from advancements in meteo-rology, technology, and the application of ocean waveforecast models.

Advancements in mathematical meteorology, coupledwith the continued application of computers, will extendthe time range and accuracy of the dynamic and statisticalforecasts.

550 WEATHER ROUTING

Technological advancements in the areas of satelliteand automated communications and onboard ship responsesystems will increase the amount and type of information toand from the ship with fewer delays. Ship response and per-formance data included with the ship’s weather report willprovide the routing agency with real-time information withwhich to ascertain the actual state of the ship. Being able topredict a ship’s response in most weather and sea condi-tions will result in improved routing procedures.

Shipboard and anchored wave measuring devices contrib-ute to the development of ocean wave analysis and forecastmodels. Shipboard seakeeping instrumentation, with input ofmeasured wave conditions and predetermined ship responsedata for the particular hull, enables a master or commanding

officer to adjust course and speed for actual conditions.Modern ship designs, exotic cargoes, and sophisticated

transport methods require individual attention to eachship’s areas of vulnerability. Any improvement in the de-scription of sea conditions by ocean wave models willimprove the output from ship routing and seakeepingsystems.

Advanced planning of a proposed transit, combinedwith the study of expected weather conditions, both beforeand during the voyage, as is done by ship routing agencies,and careful on board attention to seakeeping (with instru-mentation if available) provide the greatest opportunity toachieve the goal of optimum environmental conditions forocean transit.