CHAPTER 38 WEATHER ROUTING PRINCIPLES OF WEATHER ROUTING
3800. Introduction
Ship weather routing develops an optimum track for ocean voyages bad on forecasts of weather,a condi-tions,and a ship’s individual characteristics for a particular transit.Within specified limits of weather and a condi-tions,the term optimum is ud to mean maximum safety and crew comfort,minimum fuel consumption,minimum time underway,or any desired combination of the factors. The purpo of this chapter is to acquaint the mariner with the basic philosophy and procedures of ship weather rout-ing as an aid to understanding the routing agency’s recommendations.
The mariner’s first resources for route planning in rela-tion to weather are the Pilot Chart Atlas and the Sailing Directions(Planning Guides).The publications give cli-matic data,such as wave height frequencies and ice limits, for the major ocean basins of the world.They recommend specific routes bad on probabilities,but not on specific conditions.
The ship routing agency,acting as an advisory rvice, attempts to avoid or reduce the effects of specific adver weather and a conditions by issuing initial route recom-mendations prior to sailing,recommendations for track changes while underway(diversions),and weather adviso-ries to alert
the commanding officer or master about approaching unfavorable weather and a conditions which cannot be effectively avoided by a diversion.Adver weather and a conditions are defined as tho conditions which will cau damage,significant speed reduction,or time loss.
The initial route recommendation is bad on a survey of weather and a forecasts between the point of departure and the destination.It takes into account the hull type,speed capability,cargo,and loading conditions.The ship’s progress is continually monitored,and,if adver weather and a conditions are forecast along the ship’s current track,a recommendation for a diversion or weather adviso-ry is transmitted to the ship.By this process of initial route lection and continued monitoring of the ship’s progress for possible changes in the forecast weather and a condi-tions along a route,it is possible to maximize the ship’s speed and safety.
In providing optimum sailing conditions,the advisory rvice also attempts to reduce transit time by avoiding the adver conditions which may be encountered on a shorter route,or if the forecasts permit,diverting to a shorter track to take advantage of favorable weather and a conditions. The greatest potential advantage for this ship weather rout-ing exists when:(1)the passage is relatively long,about 1,500miles or more;(2)the waters are navigationally unre-stricted,so that there is a choice of routes;and(3)weather is a factor in determining the route to be followed.
U of this advisory rvice in no way relieves the commanding officer or master of responsibility for prudent amanship and safe navigation.There is no intent by the routing agency to inhibit the exerci of professional judg-ment and prerogatives of commanding officers and masters.
3801. Historical Perspective
The advent of extended range forecasting and the develop-ment of lective climatology,along with powerful computer modeling techniques,have made ship routing systems possible. The ability to effectively advi ships to take advantage of favor-able weather was hampered previously by forecast limitations and the lack of an effective communications system.
Development work in the area of data accumulation and climatology has a long history.Benjamin Franklin,as deputy postmaster general of the British Colonies in North America, produced a chart of the Gulf Stream from information supplied by masters of New England whaling ships.This first mapping of the Gulf Stream helped improve the mail packet rvice be-tween the British Colonies and England.In some passages the sailing time was reduced by as much as14days over routes pre-viously sailed.In the mid-19th century,Matthew Fontaine Maury compiled large amounts of atmospheric and oceano-graphic data from ships’log books.For the first time,a climatology of ocean
weather and currents of the world was available to the mariner.This information was ud by Maury to develop asonally recommended routes for sailing ships and early steam powered vesls in the latter half of the19th century. In many cas,Maury’s charts were proved correct by the sav-ings in transit time.Average transit time on the New York to California via Cape Horn route was reduced from183days to 139 days with the u of his recommended asonal routes.
In the1950’s the concept of ship weather routing was put into operation by veral private meteorological groups and by the U.S.Navy.By applying the available surface and upper air forecasts to transoceanic shipping,it was possible to effectively avoid much heavy weather while generally
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sailing shorter routes than previously.
Optimum Track Ship Routing(OTSR),the ship rout-ing rvice of the U.S.Navy,utilizes short range and extended range forecasting techniques in route lection and surveillance procedures.The short
range dynamic fore-casts of3to5days are derived from meteorological equations.The forecasts are computed twice daily from a data ba of northern hemisphere surface and upper air ob-rvations,and include surface pressure,upper air constant pressure heights,and the spectral wave values.A signifi-cant increa in data input,particularly from satellite information over ocean areas,can extend the time period for which the forecasts are uful.
For extended range forecasting,generally3to14days, a computer arches a library of historical northern hemi-sphere surface pressure and500millibar analys for an analogous weather pattern.This is an attempt at lective climatology by matching the current weather pattern with past weather patterns and providing a logical quence-of-events forecast for the10to14day period following the dy-namic forecast.It is performed for both the Atlantic and Pacific Oceans using climatological data for the entire peri-od of data stored in the computer.For longer ocean transits, monthly values of wind,as,fog,and ocean currents are ud 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 the problem of finding an MTT for a specific flight is much simpler than for a transoceanic ship passage becau an air-craft’s transit time is much shorter than a ship’s.Thus, marine minimum time tracks require significantly longer range forecasts to develop an optimum route.
Automation has enabled ship routing agencies to de-velop realistic minimum time tracks.Computation of minimum time tracks makes u of:
spire欧盟缩写1.A navigation system to compute route distance,
time enroute,estimated times of arrival(ETA’s),
and to provide6hourly DR synoptic positions for
the range of the dynamic forecasts for the ship’s
current track.
2.A surveillance system to survey wind,as,fog,
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and ocean currents obtained from the dynamic and
climatological fields.
3.An environmental constraint system impod as
part of the route lection and surveillance process.
Constraints are the upper limits of wind and as
desired for the transit.They are determined by the
猜测ship’s loading,speed capability,and vulnerability.
The constraint system is an important part of the
route lection process and acts as a warning sys-
tem when the weather and a forecast along the
prent track exceeds predetermined limits.
4.Ship speed characteristics ud to approximate
ship’s speed of advance(SOA)while transiting the
forecast a states.
Ship weather routing rvices are being offered by many nations.The include Japan,United Kingd
om,Rus-sia,Netherlands,Germany,and the United States.Also, veral private firms provide routing rvices to shipping industry clients.
There are two general types of commercial ship routing rvices.The first us techniques similar to the Navy’s OTSR system to forecast conditions and compute routing recommendations.The cond asmbles and process weather and a condition data and transmits this to ships at a for on-board processing and generation of route recom-mendations.The former system allows for greater computer power to be applied to the routing task becau powerful computers are available ashore.The latter system allows greater flexibility to the ship’s master in changing parameters, lecting routes, and displaying data.
3802. Ship And Cargo Considerations
ocmcontractingShip 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 lection process prior to sailing and the surveillance procedure while underway.A ship’s character-istics identify its vulnerability to adver conditions and its ability to avoid them.
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Generally,ships with higher speed capability and less cargo encumbrances will have shorter routes a
nd be better able to maintain near normal SOA’s than ships with lower speed capability or cargoes.Some routes are unique be-cau of the type of ship or cargo.Avoiding one element of weather to reduce pounding or rolling may be of prime im-portance.For example,a20knot ship with a heavy deck cargo may be verely hampered in its ability to maintain a 20knot SOA in any as exceeding moderate head or beam as becau of the possibility of damage resulting from the deck load’s characteristics.A similar ship with a stable car-go under the deck is not as vulnerable and may be able to maintain the20knot SOA in conditions which would dras-tically slow the deck-loaded vesl.In towing operations,a tug is more vulnerable to adver weather and a condi-tions,not only in consideration of the tow,but also becau of its already limited speed capability.Its slow speed adds to the difficulty of avoiding adver weather and a conditions.
Ship performance curves(speed curves)are ud to es-timate the ship’s SOA while transiting the forecast a states.The curves indicate the effect of head,beam,and fol-lowing as of various significant wave heights on the ship’s speed.Figure3802is a performance curve prepared for an 18 knot vesl.
With the speed curves it is possible to determine just how costly a diversion will be in terms of the required dis-tance and time.A diversion may not be necessary where the duration of the adver con
ditions is limited.In this ca,it may be better to ride out the weather and as knowing that
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a diversion,even if able to maintain the normal SOA,will not overcome the incread distance and time required.
At other times,the diversion track is less costly be-cau it avoids an area of adver weather and a conditions,while being able to maintain normal SOA even though the distance to destination is incread.Bad on in-put data for environmental conditions and ship’s behavior,route lection and surveillance techniques ek to achieve the optimum balance between time,distance,and accept-able environmental and akeeping conditions.Although speed performance curves are an aid to the ship routing agency,the respon by mariners to deteriorating weather and a conditions is not uniform.Some reduce speed vol-untarily or change heading sooner than others when unfavorable conditions are encountered.Certain waves with characteristics such that the ship’s bow and stern are in successive crests and troughs prent special problems for the mariner.Being nearly equal to the ship’s length,such wavelengths may induce very dangerous stress.The de-gree of hogging and sa
gging and the associated danger may be more apparent to the mariner than to the ship routing agency.Therefore,adjustment in cour and speed for a more favorable ride may be initiated by the commanding officer or master when this situation is encountered.3803. Environmental Factors
Environmental factors of importance to ship weather routing are tho elements of the atmosphere and ocean that may produce a change in the status of a ship transit.In ship routing,consideration is given to wind,as,fog,ice,and ocean currents.
While all of the environmental factors are
buy的过去分词important for route lection and surveillance,optimum routing is normally considered attained if the effects of wind and as can be optimized.
Wind:The effect of wind speed on ship performance is difficult to determine.In light winds (less than 20-knots),ships lo speed in headwinds and gain speed slightly in fol-lowing winds.For higher wind speeds,ship speed is reduced in both head and following winds.This is due to the incread wave action,which even in following as results in incread drag from steering corrections,and indicates the importance of a conditions in determining ship per-formance.In dealing with wind,it is also necessary to know the ship’s sail area.High winds will have a greater adver effect on a large,fully l
oaded container ship or car carrier than a fully loaded tanker of similar length.This effect is most noticeable when docking,but the effect of beam winds over veral days at a can also be considerable.
Wave Height:Wave height is the major factor affect-ing ship performance.Wave action is responsible for ship motions which reduce propeller thrust and cau incread drag from steering corrections.The relationship of ship speed to wave direction and height is similar to that of wind.Head as reduce ship speed,while following as increa ship speed slightly to a certain point,beyond which they re-tard it.In heavy as,exact performance may be difficult to predict becau of the adjustments to cour and speed for shiphandling and comfort.Although the effect of a and swell is much greater than wind,it is difficult to parate the two in ship routing.
In an effort to provide a more detailed description of the actual and forecast a state,the U.S.Navy Fleet Nu-merical Meteorology and Oceanography Center,
Figure 3802. Performance curves for head, beam, and following as.
542WEATHER ROUTING
Monterey,California,produces the Global Spectral Ocean Wave Model(GSOWM)for u by the U.S.Navy’s Opti-mum Track Ship Routing(OTSR)rvice.This model provides energy values from12
different directions(30°ctors)and15frequency bands for wave periods from6 to26conds with the total wave energy propagated throughout the grid system as a function of direction and frequency.It is bad on the analyzed and forecast plane-tary boundary layer model wind fields,and is produced for both the Northern and Southern Hemispheres out to72 hours.For OTSR purpos,primary and condary waves are derived from the spectral wave program,where the primary wave train has the principal energy(direction and frequency),and the condary has to be20percent of the primary.
Fog:Fog,while not directly affecting ship perfor-mance,should be avoided as much as feasible,in order to maintain normal speed in safe conditions.Extensive areas of fog during summertime can be avoided by lecting a lower latitude route than one bad solely upon wind and as.Although the route may be longer,transit time may be less due to not having to reduce speed in reduced visi-bility.In addition,crew fatigue due to incread watchkeeping vigilance can be reduced.
North Wall Effect:During the Northern Hemisphere fall and winter,the waters to the north of the Gulf Stream in the North Atlantic are at their coldest,while the Gulf Stream itlf remains at a constant relatively warm tem-perature.After passage of a strong cold front or behind a developing coastal low pressure system,Arctic air is sometimes drawn off the Mid-Atlantic coast of the United States and out over the warm waters of the Gulf Stream by northerly winds.This cold air is warmed as it pass ove
r the Gulf Stream,resulting in rapid and inten deepening of the low pressure system and higher than normal surface winds.Higher waves and confud as result from the winds.When the winds oppo the northeast t of the current,the result is incread wave heights and a shorten-ing of the wave period.If the opposing current is sufficiently strong,the waves will break.The phenome-na are collectively called the“North Wall Effect,”referring to the region of most dramatic temperature change between the cold water to the north and the warm Gulf Stream water to the south.The most dangerous as-pect of this phenomenon is that the strong winds and extremely high,steep waves occur in a limited area and may develop without warning.Thus,a ship that is labor-ing in near-gale force northerly winds and rough as, proceeding on a northerly cour,can suddenly encounter storm force winds and dangerously high breaking as. Numerous ships have foundered off the North American coast in the approximate position of the Gulf Stream’s North Wall.A similar phenomenon occurs in the North Pacific near the Kuroshio Current and off the Southeast African coast near the Agulhas Current.
Ocean Currents:Ocean currents do not prent a significant routing problem,but they can be a determining factor in route lection and diversion.This is especially true when the points of departure and destination are at relatively low latitudes.The important considerations to be evaluated
are the difference in distance between a great-circle route and a route lected for optimum cur-rent,with the expected increa in SOA from the following current,and the decread probability of a di-version for weather and as at the lower latitude.For example,it has proven beneficial to remain equatorward of approximately22°N for westbound passages between the Canal Zone and southwest Pacific ports.For east-bound passages,if the maximum latitude on a great-circle track from the southwest Pacific to the Canal Zone is be-low24°N,a route passing near the axis of the Equatorial Countercurrent is practical becau the incread distance is offt by favorable current.Direction and speed of ocean currents are more predictable than wind and as, but some variability can be expected.Major ocean cur-rents can be disrupted for veral days by very inten weather 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 ice should be avoided becau of the difficulty of detection and the potential for collision.Deck ice may be more difficult to contend with from a ship routing point of view becau it is caud by freezing weather associated with a large weather system.While mostly a nuisance factor on large ships,it caus 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 problem
s with the environment.Certain operations should benefit from a-sonal planning as well as optimum routing.For example, towing operations north of about40°latitude should be avoided in non-summer months if possible.
老罗英语培训机构3804. Synoptic Weather Considerations
A ship routing agency should direct its forecasting skills to avoiding or limiting the effect of weather and as associated with extratropical low pressure systems in the mid and higher latitudes and the tropical systems in low lat-itude.Seasonal or monsoon weather is also a factor in route lection and diversion in certain areas.
Despite the amount of attention and publicity given to tropical cyclones,mid-latitude low pressure systems gener-ally prent more difficult problems to a ship routing agency.This is primarily due to the fact that major ship traf-fic is sailing in the latitudes of the migrating low pressure systems,and the amount of potential exposure to inten weather systems, especially in winter, is much greater.
Low pressure systems weaker than gale intensity (winds less than34knots)are not a vere problem for most ships.However,a relatively weak system may generate pro-longed periods of rough as which may hamper normal
