《航海英语二三副》-Unit 7-2 ship stability and Strength

stableequilibrium 稳定平衡neutralequilibrium 随遇平衡unstableequilibrium不稳定平衡intactStability 完整稳性damagestability 破舱稳性initialStability 初稳性（小倾角稳性）stabilityatlargeangles大倾角稳性FreeSurfaceEffect 自由液面效应slacktank 部分装载舱rollingperiod/timeperiod（船舶）横摇周期angleofvanishingstability稳性消失角1.Words&Expressionscentroid 质心；矩心；面心criteria 标准，准则(criterion的名词复数)disturbance 干扰，扰乱equilibrium 平衡resultant 合力resultantmoment 合力矩rightinglever 复原力臂timeperiod 周期trimmingmoment 纵倾力距waterplane 水线面metacenter 稳心点metacentricheight 稳性高度metacentricradius 稳心半径TPC 每厘米吃水吨MCT 每厘米纵倾力距KG重心垂向距离（坐标）KB浮心的垂向距离（坐标）CenterofBuoyancy 浮心CenterofFloatation 漂心CenterofGravity 重心tensionstress 拉力hogging 中拱变形sagging 中垂变形heaving 垂荡surging 纵荡swaying 横荡rolling 横摇pitching 纵摇yawing 摇首2.Terms&DefinitionsHeel.Ashipissaidtobeheeledwhensheisinclinedbyanexternalforce.Forexample,whentheshipisinclinedbytheactionofthewavesorwind.List.Ashipissaidtobelistedwhensheisinclinedbyforceswithintheship.Forexample,whentheshipisinclinedbyshiftingaweighttransverselywithintheship.Thisisafixedangleofheel.StableequilibriumAshipissaidtobeinstableequilibriumif,wheninclined,shetendstoreturntotheinitialposition.Forthistooccurthecentreofgravitymustbebelowthemetacentre,thatis,theshipmusthavepositiveinitialmetacentricheight.UnstableequilibriumWhenashipwhichisinclinedtoasmallangletendstoheeloverstillfurther,sheissaidtobeinunstableequilibrium.ForthistooccurtheshipmusthaveanegativeGM.neutralequilibriumWhenGcoincideswithM,theshipissaidtobeinneutralequilibrium,andifinclinedtoasmallangleshewilltendtoremainatthatangleofheeluntilanotherexternalforceisapplied.TheshiphaszeroGM.NotethatKG=KM.rollnaturalperiodTherollperiodofashipisthetimetakenbytheshiptorollfromonesidetotheotherandbackagaintotheinitialposition.stiffshipsWhenashiphasacomparativelylargeGM,forexample2mto3m,therightingmomentsatsmallanglesofheelwillalsobecomparativelylarge.Itwillthusrequirelargermomentstoinclinetheship.Wheninclinedshewilltendtoreturnmorequicklytotheinitialposition.Theresultisthattheshipwillhaveacomparativelyshorttimeperiod,andwillrollquicklyandperhapsviolently–fromsidetoside.Ashipinthisconditionissaidtobe‘stiff’,andsuchaconditionisnotdesirable.tendershipsWhentheGMiscomparativelysmall,forexample0.16mto0.20mtherightingmomentsatsmallanglesofheelwillalsobesmall.Theshipwillthusbemucheasiertoinclineandwillnottendtoreturnsoquicklytotheinitialposition.Thetimeperiodwillbecomparativelylongandaship,forexample25to35seconds,inthisconditionissaidtobe‘tender’.Asbefore,thisconditionisnotdesirableandstepsshouldbetakentoincreasetheGMbyloweringtheeffectivecentreofgravityoftheship

FreeSurfaceEffect

Whenatankispartiallyfilled,theliquid’scentreofgravitypositionwillchangeastheshipisinclined.LiquidinpartiallyfilledtankalwaysdecreasestheinitialmetacentricheightGM,rightingleverGZ,andangleofvanishingstability.Apartiallyfilledtankisknownasa“slacktank”.Thereductionofstabilitycausedbytheliquidsinslacktanksisknownasfree-surfaceeffect.Thisadverseeffectonthestabilityisreferredtoasa“lossinGM”orasa“virtualriseinverticalcentreofgravityKG”.FreeSurfaceEffectThefree-surfaceeffectcanendangertheshiporevenleadtoanegativemetacentricheight.Thereforethenumberofpartiallyfilledtanksshouldbekepttoaminimum.Whenballastingthevessel,onlyonetransversepairorasinglecenterlineballasttankshouldbefilledup.Atsea,asfaraspossible,ballasttanksshallbe100%fullorempty.Whenballasted,widedoublebottomtanksmustbealways100%full.NegativeGMAshiphavinganegativeinitialmetacentricheightwillbeunstablewheninclinedtoasmallangle.Angleoflollmaybedefinedastheangletowhichashipwithnegativeinitialmetacentricheightwilllieatrestinstillwater.TheGMvalueGMiscrucialtoshipstability.ThetablebelowshowstypicalworkingvaluesforGMforseveralship-typesallatfully-loadeddrafts.Fromthisitcanbeseenthattheshipwilloscillateabouttheangleoflollinsteadofaboutthevertical.IfthecentreofbuoyancydoesnotmoveoutfarenoughtogetverticallyunderG,theshipwillcapsize.Theangleoflollwillbetoportorstarboardandbacktoportdependingonexternalforcessuchaswindandwaves.Oneminuteitmayflopoverto3°Pandthensuddenlyflopoverto3°S.ThereisalwaysthedangerthatGwillriseaboveMandcreateasituationofunstableequilibrium.Thiswillcausecapsizingoftheship.Atdraftsbelowthefully-loadeddraft,duetoKMtendingtobelargerinvalueitwillbefoundthatcorrespondingGMvalueswillbehigherthanthoselistedinthetableabove.ForallconditionsofloadingitstipulatethattheGMmustneverbelessthan0.15m.3.IMOCriteriaTheareaundertherightinglevercurve(GZcurve)shallnotbelessthan0.055metre-radiansupto=30°angleofheelandnotlessthan0.090metre-radiansupto=40°ortheangleofdown-flooding.ifthisangleislessthan40°.Additionally,theareaundertherightinglevercurve(GZcurve)betweentheanglesofheelof30°and40°orbetween30°and,ifthisangleislessthan40°,shallnotbelessthan0.030metre-radians.TherightingleverGZshallbeatleast0.2matanangleofheelequaltoorgreaterthan30°.Themaximumrightinglevershalloccuratanangleofheelnotlessthan25°.Ifthisisnotpracticable,alternativecriteria,basedonanequivalentlevelofsafety,maybeappliedsubjecttotheapprovaloftheAdministration.TheinitialmetacentricheightGM0shallnotbelessthan0.15m.

IMO对普通货船的稳性要求1.

船舶在各种装载情况下的初稳性高度GM≥0.15m

2.

横倾角在0~30°之间静稳性曲线下的面积≥0.055m

3.

在0~40°(或小于40°的进水角θf)之间静稳性曲线下的面积不小于0.09m•rad.

4.

30°~40°(或小于40°的θf)之间静稳性曲线下的面积≥0.03m•rad.

5.

θ≥30°处的复原力臂不小于0.2m.

6.

最大复原力臂对应的角度最好大于30°，至少不少于25°

7.

满足天气蘅准数的要求

我国“海船稳性规范”对普通货船的稳性衡准数要求

1.

经自由液面修正后的GM≥0.15m

2.θ＝30°或θf处的GZ≥0.20m

3.Gzmax对应的角度θmax≥30°或当静稳性曲线有两个峰值时，第一个峰值对应的角度不小于25°

4.

稳性消失角θv不小于55°，即θv≥55°

5.

船舶在各种装载状态下的稳性衡准数不小于1，即Mhmin/Mw≥1；Mhmin的求取要经过横摇角θi和进水角θf的修正；Mw为风压力矩Mw=ρw•Aw•Zw，ρw－风压，Aw－横风受风面积，Zw－吃水一半到Aw中心的垂直距离TheendShipStrengthIntroductionAFormalSafetyAssessment(FSA)conductedbyJapanfortheIMOMaritimeSafetyCommitteeexaminedcasualtydatafrom1975to1996.Therewere2916reportedbulkcarriercasualties,resultinginthelossof1890lives.Between1978andAugust2000,datafromthesamesourcereported1,126liveslostinbulkercasualtiesattributabletostructurefailureorflooding.Figure1showsabreakdownofthesefatalities.The

strengthofships

isatopicofkeyinterestto

navalarchitectsandshipbuilders.

Ships

whicharebuilttoostrongareheavy,slow,andcostextramoneytobuildandoperatesincetheyweighmore,whilstshipswhicharebuilttooweaklysufferfromminorhulldamageandinsomeextremecasescatastrophicfailureandsinking.TheStrengthofShipLongitudinalStrength-总纵强度LocalStrength-局部强度TorsionalStrength-扭转强度

SheeringForce&BendingMomentTheshearingforceistheforcethatwantstoshiftthe(athwart-ship)planefromonepartoftheshiptoanother.Thelocaldifferencesbetweenupwardbuoyancyforcesandthelocalweightgiverisetosheari