Controllable Pitch Propeller (CPP) vs Fixed Pitch Propeller (FPP)

6/11/2015 ControllablePitchPropeller(CPP)VsFixedPitchPropeller(FPP)

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ControllablePitchPropeller(CPP)VsFixedPitchPropeller(FPP)

JANUARY28,2015BYSOUMYACHAKRABORTY

Mostofthepropellersthatareusedinshipsarefixedpitchpropellers(FPP).Whatdoesthispropellertermmeans?Letsunderstandthisfromabeginnerspointofview.

Ifyoulookatapropelleraxially,youwillnoticeatwistinthepropellerbladealongthelengthofthepropellerhub.Itisthistwistinthebladethatresultsintheforwardmotionofthepropellerwitheachrotation.

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Partsofscrewpropeller

Itisprettymuchwhatyouseeonascrew.Theangleofthethreadisactuallywhatiscalledthepitchangle.Themorethepitchangle,themoredistanceistravelledbythescrewinonerotation,thatis,themoreisthepitch.Similarly,propellersactontheprincipleofscrew.Hencethetermscrewpropeller.

ButwhatisafixedpitchpropellerortheFPP?

Thepropellerisdesignedinawaysuchthat,ifyoucutthepropellerradiallyatanyrandomradiusfromitscentre,thenthecutsectionofthepropellerbladeswillhaveacertainpitch(magnitudedependsuponhowtwisteditisalongthelengthofthehub).Nowifyourotatethiscutsectioninwater,theaxialdistancetravelledinonerotationisthepitchofthissection.Ifyouvarytheradiusatwhichyoucutasection,thetwistofthebladealsovariesfromtheroottothetip.Thismakesitevidentthateachsectionofthepropellerhasaparticularpitch.

Therootofthepropellerwillhaveadifferentpitchfromthetipofthepropeller.SothepitchvariationfromtheroottothetipisasshowninFigure1.Thequestionnowis,ifeachsectionhasadifferentpitch,thenwhatistheresultantpitchofthepropeller?Answeristheresultantpitchistheaverageofallthepitchesofeachsectionofthepropeller.



Bladesectionandvariationofpitchwithsection

Noteverycarefully,thatthepitchvariesalongwiththebladesection(orincreasingdistancefromthepropellercentre),butthenetaverageorthenetpitchofthepropellerremainsconstant.Suchapropellerisafixedpitchpropeller.

Now,letsgobacktotheconceptofascrew.Whatifyoucouldsomehowchangetheangleofinclinationofthethreadsalongthescrew?

Ifyoucould,whatwouldyouactuallybedoingischangingthepitchangleandsubsequently,thepitchofthescrew.Thisisexactlywhythecontrollablepitchpropellerwasdevelopedbytheindustry.Wewillbedealingwiththeapplicationsindetailalittlelater.Butfornow,itisvitaltoknowthefundamentalworkingofaControllablePitchPropeller(CPP).

Thetwist(tobereferredaspitchfromnowon)oftherootsectionofthebladeisalwayswithrespecttotheangleitmakeswiththepropellerhub.Whatifwecanchangetheangleoforientationoftheroot?Andsincealltheotherbladesectionshadtheirpreviouspitchanglealready,theyattainanewpitchangle.Inotherwords,youareabletocontrolthepitchoftheentirepropeller,bychangingtheorientationoftherootsectiononthehub.Thisisdonebythemeansofahydrauliccylinder.



Whentherequiredorientationorpitchofthepropellerisobtained,thebladesarelockedinthatposition.Sothebladesarenotanintegralpartof(ornotcastedalongwith)thepropellerhub.Theyaremountedonspindlesthathaveaxesperpendiculartotheshaftaxis.



ImageCredits:Stahlkocher/Wikipedia

ThefollowingpointswilldiscusstheapplicationsandtheirrelationswiththeadvantagesanddisadvantagesofCPPandFPPsystems:



Thecontrollablepitchpropellerscanbeusedtoruntheshipinforwardandasterndirectionboth,withouttherequirementtochangethedirectionofrotationoftheengine.How?Well,thepropellerworksontheprincipleofliftgeneratedbyeachaerofoilsectionoftheblade.Whatifwecanchangetheorientationoftheblade,suchthatthedirectionoflifeforceisoppositetowhatitwasduringforwardrunningoftheship?Thatsexactlywhatisachievedbychangingthepitchangleoftheblades,asshowninFigure2.

Bladesectionsshowninvariouspitchorientations

Anonreversibleenginecanbeusedforbothforwardandasternoperationoftheship.Hence,theweightoftheengineandpropulsionmachineryisreducedconsiderablywhencomparedtowhatisrequiredinpropulsionsystemsequippedwithfixedpitchpropellers.

Sincethepitchofthepropellerdeterminestheamountofthrustgeneratedbythepropeller,achangeinthepitchanglecanbringaboutachangeinthespeedoftheship.So,acontrollablepitchpropellerisusefulinchangingthespeedoftheshipwithoutchangingthespeedorrpmofthemainengine.Butwhyarewefocusingonthisadvantage?Whathappensifweneedtoreducetheenginerpmtoreducetheshipsspeed?Well,manyshipshaveashaftdrivendieselalternatorsystem.Sotherpmofthemainengineshaftcontrolstheelectricpowerproductiononboard.Inthatcase,ifthespeedwastobereducedusingafixedpitchpropeller,itwouldrequireareductionintheshaftrpm,whichwouldresultinreductionofpowergenerated.

Duetotheabovereason,thespeedoftheshipcanbehandledfromthenavigationbridgedirectly.In



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caseoffixedpitchpropellers,themarineengineerneedstobeintimidatedinordertochangetheshipsspeed.ThereforeCPPtechnologyisusedforobtainingfasterresponseofspeedchange.

Theefficiencyofasternconditionincaseoffixedpitchpropellersislessthanthatofcontrollablepitchpropellerinasterncondition.

However,therearemanydisadvantagesofusingacontrollablepitchpropellertoafixedpitchpropeller.Itisimportanttoknowthemtoo.

Thepitchcontrolmechanisminstalledinthehubiscomplicatedenoughtodemandproperinspectionatregularintervals.Theinstallationprocessisalsomorecomplicatedascomparedtothatoffixedpitchpropellersystem.

Controllablepitchpropellershaveaveryhighinitialcost.Andthiscostincreasesrapidlywithincreasingdiameterofthepropeller.

Duetopitchcontrolmechanismshousedinsidethehub,thelengthandthediameterofthehubisalsohighascomparedtothefixedpitchpropellersystems.

Toconclude,itisvitaltoknowtheapplicationsofcontrollablepitchpropellers.Theseareusedinshipsthatarerequiredtooperateatvariablespeedsevenwhenoperatingatconstantpowerrequirements.Suchshipsaretugs,trawlers,ferries,icebreakers,andsmallscalewarshipsthatareequippedwithgasturbines.

Overtoyou..

WhataccordingtoyouarethemostimportantfeaturesofCPPandFPP?

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AboutSoumyaChakrabortySoumyaispursuingNavalArchitectureandOceanEngineeringatIMU,Visakhapatnam,India.Passionateaboutmarinedesign,hebelievesintheimportanceofsharingmaritimetechnicalknowhowamongindustrypersonnelandstudents.HeisalsotheCoFounderandEditorinChiefofLearnShipDesignAStudentInitiative.

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COMMENTS

AbhiVermasaysJanuary31,2015at7:14pm

beenadeckcadetforabout18monthsnow..sailedonbothtypeofpropulsionsystem..CPP.itsthebestformanoeuvring.beingonbridgewhenenteringportorchannel.whenthepilotseesaCPP.Youcanseethegrinonhisfacereducestensiononthebridgealot

FergusGloversaysMarch4,2015at6:12am

1.Whataretheadvantagesanddisadvantagesoffixedpropellerinship?2.whataretheadvantagesanddisadvantagesofcontrollablepitchproperller?



POSTCOMMENT

TanvirBiswassaysApril4,2015at2:16pm

Canweusetheazipodinamerchantshipinsteadofcpporfpp?

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Controllable Pitch Propeller (CPP) vs Fixed Pitch Propeller (FPP)