526WEATHEROBSERVATIONSSolution:First startingfrom the centerofa maneuver-If a vessel is proceeding at 12 knots, 6 knots constitutesingboard,plottheship'svectorer,at240,length18knotsone-half(0.5)unit,12knotsone unit, 18knots1.5 units,24(using the 3-I scale).Next plot the relative wind's vectorknotstwounits,etc.fromr,inadirectionof100(thereciprocalof280°)length30knots.Thetruewind is fromthecenter to theendofthisExample2:Ashipisproceedingon course270°atavector or line ew.speed of 10 knots.The apparent wind is from 10o off theAlternatively,you can plot the ship's vectorfrom theport bow, speed 30 knots.center,then plottherelative wind's vector toward the cen-Required: The relative direction, true direction, andter, and see the true wind's vector from the end of this linespeed of the true wind by table.totheend of theship's vector.Useparallel rulerstotrans-fer the wind vector to the center for an accurate reading.Solution: The apparent wind speed isAnswer:Truewind isfrom315°at20knots.30 - 3.0 ships speed units10On a moving ship, the direction of the true wind is al-ways on the same sideand aft of thedirection of theEnterTable30with3.0and10°and find therelativedirec-apparent wind.Thefaster the ship moves,the more the ap-parent wind draws ahead of the true windtionofthetruewindtobe15°offtheportbow(345°relative)Solution can also bemadewithout plotting,inthe fol-andthespeedtobe2.02timestheship'sspeed,or20knots,approximately.Thetruedirectionis345°+2700=2550lowingmanner:On a maneuveringboard,label thecircles5,10,15,20,etc.,from the center,and drawvertical lines tan-Answers:Truewindfrom345°relative=255°true,at20knots.gent to these circles. Cut out the 5:1 scale and discard thatpart having graduations greater than the maximum speed ofthe vessel.Keep this sheetfor all solutions. (For durabilityByvariations ofthisproblem,onecan findtheappar-thetwopartscanbemountedoncardboardorothersuitableent wind from thetrue wind, the course or speed requiredmaterial.)Tofind true wind, spot in point 1 byeye.Placetheto produce an apparent wind from a given direction orzero of the 5:1 scale on this point and align the scale (invert-speed, or the course and speed to produce an apparented) using the vertical lines. Locate point 2 at the speed of thewind ofagiven speed fromagiven direction.Suchprob-vesselasindicatedonthe5:1scale.Itisalwavsverticallybe-lems often arise in aircraft carrier operationsand in somerescue situations. See “Pub. 217, Maneuvering Boardlowpoint1.Readtherelativedirectionandthespeedofthetrue wind, using eye interpolation if needed.Manual"formoredetailed information.Atabular solution canbe made using Table30,Direc-When wind speed and direction are determined by thetion and Speed of True Wind in Units of Ship's Speed.Theappearanceofthesea,theresultistruespeedanddirec-entering valuesfor this table are the apparent wind speed intion. Waves move in the same direction as the generatingunits of ship's speed,and thedifferencebetween thehead-wind, and are not deflected by earth's rotation.If a winding and theapparent wind direction.Thevalues takenfromvane is used,the direction of the apparent wind thus deter-the table are the relative direction (right or left) of the truemined can be used with the speed of the true wind towind, and the speed of the true wind in units of ship's speed.determinethedirection of thetruewind byvectordiagram.WINDANDWAVES3711.EffectsOfWindOnTheSeaThese pictures (courtesy of Environment Canada)present the results ofa project carried out on board the Ca-There is a direct relationshipbetween the speed of thenadianOceanWeatherShipsVANCOUVERandQUADRA at Ocean Weather Station PAPA (50°N.,wind and the state of the sea.This is useful in predicting the145°W),betweenApril1976andMay1981.Theaimoftheseaconditionsto beanticipated whenfuturewindspeedforecastsareavailable.Itcanalsobeusedtoestimatetheproject was to collect colorphotographs of the sea surfacespeed of the wind, which may be necessary when an ane-as it appears under the influence of the various ranges ofmometer is notavailable.wind speed, as defined byTheBeaufort Scaleof WindWind speeds are usually grouped in accordance with theForce.The photographs represent as closely as possibleBeaufort scale,named afterAdmiral Sir FrancisBeaufortsteady-state sea conditionsovermanyhours for each Beau-(1774-1857),who devised it in 1806.As adopted in 1838fort windforce,exceptForce12,forwhichnophotographsBeaufort numbers ranged from 0 (calm)to 12 (hurricane).Theare available.They weretaken from heights ranging from12-17metersabovethe sea surface;anemometer heightwasBeaufort wind scaleand sea statephotographswhichare atthe28meters.end ofthischapter canbeused toestimatewind speed
526 WEATHER OBSERVATIONS Solution: First starting from the center of a maneuvering board, plot the ship’s vector er, at 240°, length 18 knots (using the 3–1 scale). Next plot the relative wind’s vector from r, in a direction of 100° (the reciprocal of 280°) length 30 knots. The true wind is from the center to the end of this vector or line ew. Alternatively, you can plot the ship’s vector from the center, then plot the relative wind’s vector toward the center, and see the true wind’s vector from the end of this line to the end of the ship’s vector. Use parallel rulers to transfer the wind vector to the center for an accurate reading. Answer: True wind is from 315° at 20 knots. On a moving ship, the direction of the true wind is always on the same side and aft of the direction of the apparent wind. The faster the ship moves, the more the apparent wind draws ahead of the true wind. Solution can also be made without plotting, in the following manner: On a maneuvering board, label the circles 5, 10, 15, 20, etc., from the center, and draw vertical lines tangent to these circles. Cut out the 5:1 scale and discard that part having graduations greater than the maximum speed of the vessel. Keep this sheet for all solutions. (For durability, the two parts can be mounted on cardboard or other suitable material.) To find true wind, spot in point 1 by eye. Place the zero of the 5:1 scale on this point and align the scale (inverted) using the vertical lines. Locate point 2 at the speed of the vessel as indicated on the 5:1 scale. It is always vertically below point 1. Read the relative direction and the speed of the true wind, using eye interpolation if needed. A tabular solution can be made using Table 30, Direction and Speed of True Wind in Units of Ship’s Speed. The entering values for this table are the apparent wind speed in units of ship’s speed, and the difference between the heading and the apparent wind direction. The values taken from the table are the relative direction (right or left) of the true wind, and the speed of the true wind in units of ship’s speed. If a vessel is proceeding at 12 knots, 6 knots constitutes one-half (0.5) unit, 12 knots one unit, 18 knots 1.5 units, 24 knots two units, etc. Example 2: A ship is proceeding on course 270° at a speed of 10 knots. The apparent wind is from 10° off the port bow, speed 30 knots. Required: The relative direction, true direction, and speed of the true wind by table. Solution: The apparent wind speed is Enter Table 30 with 3.0 and 10° and find the relative direction of the true wind to be 15° off the port bow (345° relative), and the speed to be 2.02 times the ship’s speed, or 20 knots, approximately. The true direction is 345° + 270° = 255°. Answers: True wind from 345° relative = 255° true, at 20 knots. By variations of this problem, one can find the apparent wind from the true wind, the course or speed required to produce an apparent wind from a given direction or speed, or the course and speed to produce an apparent wind of a given speed from a given direction. Such problems often arise in aircraft carrier operations and in some rescue situations. See “Pub. 217, Maneuvering Board Manual”, for more detailed information. When wind speed and direction are determined by the appearance of the sea, the result is true speed and direction. Waves move in the same direction as the generating wind, and are not deflected by earth’s rotation. If a wind vane is used, the direction of the apparent wind thus determined can be used with the speed of the true wind to determine the direction of the true wind by vector diagram. WIND AND WAVES 3711. Effects Of Wind On The Sea There is a direct relationship between the speed of the wind and the state of the sea. This is useful in predicting the sea conditions to be anticipated when future wind speed forecasts are available. It can also be used to estimate the speed of the wind, which may be necessary when an anemometer is not available. Wind speeds are usually grouped in accordance with the Beaufort scale, named after Admiral Sir Francis Beaufort (1774-1857), who devised it in 1806. As adopted in 1838, Beaufort numbers ranged from 0 (calm) to 12 (hurricane). The Beaufort wind scale and sea state photographs which are at the end of this chapter can be used to estimate wind speed. These pictures (courtesy of Environment Canada) present the results of a project carried out on board the Canadian Ocean Weather Ships VANCOUVER and QUADRA at Ocean Weather Station PAPA (50°N., 145°W), between April 1976 and May 1981. The aim of the project was to collect color photographs of the sea surface as it appears under the influence of the various ranges of wind speed, as defined by The Beaufort Scale of Wind Force. The photographs represent as closely as possible steady-state sea conditions over many hours for each Beaufort wind force, except Force 12, for which no photographs are available. They were taken from heights ranging from 12-17 meters above the sea surface; anemometer height was 28 meters. 30 10- 3.0 = ships speed units
527WEATHEROBSERVATIONS3712.EstimatingTheWindAt Seastrongcurrents,shallowwater,swell,precipitation,ice,andwind shifts.Their effects will be described laterObservers on board ships at sea usually determinetheA wind of a given Beaufort Force will, therefore, pro-speed of thewind by estimatingBeaufortForce,asmer-duceacharacteristicappearanceofthe seasurfaceprovidedchant ships may not be equipped with wind measuringthat it has been blowing for a sufficient length of time,andinstruments.Through experience,ships'officers havede-over a sufficiently long fetch.velopedvarious methods ofestimatingthisforce.TheIn practice,themariner observes the sea surface,not-effect of the wind on the observer himself, the ship's rig-ingthesizeofthewaves,the white caps,spindrift,etc.,andging,flags, etc.,is used as a guide, but estimatesbased onthen finds the criterion which best describes the sea surfacethese indications give therelative wind whichmust be cor-as he saw it. This criterion is associated with a Beaufortrected forthe motionof theshipbeforean estimate of thenumber,forwhich a correspondingmeanwind speed andtruewindspeedcanbeobtainedrange in knots are given.Since meteorological reports re-The most common method involves the appearance ofquirethat wind speeds be reported inknots,the mean speedthe sea surface. The state of the sea disturbance, i.e. the di-forthe Beaufort number maybe reported,or anexperiencedmensionsofthewaves,thepresenceofwhitecaps,foam,orobserver may judge that the sea disturbance is such that aspray,depends principally on three factors:higherorlowerspeed withintherangefor theforce is moreaccurate.1.Thewind speed.Thehigherthespeed of thewind,This method should beused with caution.The sea con-the greater is the sea disturbance.ditions described for each BeaufortForce are"steady-state"2.The wind's duration. At any point on the sea, theconditions; ie.the conditions which result when the winddisturbance will increase the longer the wind blowshasbeenblowingfora relativelylong time,and over agreatat a given speed, until a maximum state of distur-stretch of water. At any particular time at sea, though, thebance is reached.duration of the wind or the fetch, or both, may not have3.Thefetch.This is thelength ofthe stretchof waterbeen great enough to produce these"steady-state"condi-over whichthe windactsonthe sea surfacefromtions.When a high wind springs up suddenly afterthe same direction.previously calm or near calm conditions, it will requiresome hours, depending on the strength ofthe wind, to gen-For a given wind speed and duration, the longer theerate waves of maximum height.The height of the wavesfetch, thegreater is the sea disturbance.Ifthefetch is short,increasesrapidlyinthefirstfewhoursafterthecommence-suchas a fewmiles, the disturbancewill be relatively smallmentof theblow,but increases at a much slowerratelaterno matter how great the wind speed is or how long it hason.beenblowing.There are other factors which can modify the appear-At the beginning of thefetch (such as at a coastlineanceofthesea surfacecausedbywindalone.Thesearewhen the wind is offshore)after the wind has been blowingTheoreticalFetch (nautical miles), withBeaufortDuration ofwinds,(hours),maximumwaveunlimited duration offorceofwith unlimited fetch, toheight (ft)blow, to produce percentwind.producepercentof maxi-unlimiteddurationof maximum wave heightmum wave height indicated.indicated.and fetch.50%75%90%50%75%90%535281.58313258123.510306071222205.52175150975540162515028093285117019200450Table3712.Durationofwindsand length of fetchesrequiredforvariouswindforces
WEATHER OBSERVATIONS 527 3712. Estimating The Wind At Sea Observers on board ships at sea usually determine the speed of the wind by estimating Beaufort Force, as merchant ships may not be equipped with wind measuring instruments. Through experience, ships’ officers have developed various methods of estimating this force. The effect of the wind on the observer himself, the ship’s rigging, flags, etc., is used as a guide, but estimates based on these indications give the relative wind which must be corrected for the motion of the ship before an estimate of the true wind speed can be obtained. The most common method involves the appearance of the sea surface. The state of the sea disturbance, i.e. the dimensions of the waves, the presence of white caps, foam, or spray, depends principally on three factors: 1. The wind speed. The higher the speed of the wind, the greater is the sea disturbance. 2. The wind’s duration. At any point on the sea, the disturbance will increase the longer the wind blows at a given speed, until a maximum state of disturbance is reached. 3. The fetch. This is the length of the stretch of water over which the wind acts on the sea surface from the same direction. For a given wind speed and duration, the longer the fetch, the greater is the sea disturbance. If the fetch is short, such as a few miles, the disturbance will be relatively small no matter how great the wind speed is or how long it has been blowing. There are other factors which can modify the appearance of the sea surface caused by wind alone. These are strong currents, shallow water, swell, precipitation, ice, and wind shifts. Their effects will be described later. A wind of a given Beaufort Force will, therefore, produce a characteristic appearance of the sea surface provided that it has been blowing for a sufficient length of time, and over a sufficiently long fetch. In practice, the mariner observes the sea surface, noting the size of the waves, the white caps, spindrift, etc., and then finds the criterion which best describes the sea surface as he saw it. This criterion is associated with a Beaufort number, for which a corresponding mean wind speed and range in knots are given. Since meteorological reports require that wind speeds be reported in knots, the mean speed for the Beaufort number may be reported, or an experienced observer may judge that the sea disturbance is such that a higher or lower speed within the range for the force is more accurate. This method should be used with caution. The sea conditions described for each Beaufort Force are “steady-state” conditions; i.e. the conditions which result when the wind has been blowing for a relatively long time, and over a great stretch of water. At any particular time at sea, though, the duration of the wind or the fetch, or both, may not have been great enough to produce these “steady-state” conditions. When a high wind springs up suddenly after previously calm or near calm conditions, it will require some hours, depending on the strength of the wind, to generate waves of maximum height. The height of the waves increases rapidly in the first few hours after the commencement of the blow, but increases at a much slower rate later on. At the beginning of the fetch (such as at a coastline when the wind is offshore) after the wind has been blowing Beaufort force of wind. Theoretical maximum wave height (ft) unlimited duration and fetch. Duration of winds, (hours), with unlimited fetch, to produce percent of maximum wave height indicated. Fetch (nautical miles), with unlimited duration of blow, to produce percent of maximum wave height indicated. 50% 75% 90% 50% 75% 90% 3 2 1.5 5 8 3 13 25 5 8 3.5 8 12 10 30 60 7 20 5.5 12 21 22 75 150 9 40 7 16 25 55 150 280 11 70 9 19 32 85 200 450 Table 3712. Duration of winds and length of fetches required for various wind forces