POWERCATAMARANSDON’TLIKETOGOUPHILL.IhavebeenaskedonanumberofoccasionswhatitisthatsetstheLeopard51and43Power-catsapart fromothersandwhy it is thatwebelieve theirefficiencyandride isbetterthanthosetheyareincompetitionwith.Foranymotoryacht,beitamono-ormulti-hull,therearemanyconsiderationsaffectingperformance. In this article I will just concentrate on some and make some simpleassumptions for the purpose of explaining the workings on the Leopard powercatamarans.If the yacht is a slowone (staying in displacementmode)weight,wetted surface andwaveresistanceremainthekeyplayers.Irrespectivehowmayhullsyouconsider;theirperformancewillbeprettyidentical.Howeverifitisafastcraft(notoperatingintheclassicdisplacementmode),thenotherfactorscomeintoplayanditistherethatyouwillseeadistinctdifferenceinbehaviorbetweenfastmono-hullsandfastmulti-hulls.A mono-hull will move over its bow wave and start planing, thereby significantlyreducing wetted surface and wave resistance. It can do this because it has sufficientplaning area (as a single hull, it is sufficiently wide). Power catamarans haven’t gotanywhereclosetosufficientplaningareaandremaininwhatyoucouldcalla“forced”displacementmode. Tohavea lowenoughresistancetheyhavetosolelyrelyonveryslenderhullssothatthewaveresistanceathigherspeedsremainssmall.An important design consideration to look at for catamarans is the largest crosssectional areayouare trying topush forward through thewater. Basically this is theareaoftheunderwaterbodyyouwouldseeifyoustandexactlyinfrontoftheyacht.Thebigger it is, themoreresistance itwillgenerate, themorepoweryouneedor thesloweryouryachtwillgoifyouonlyhaveasetamountofhorsepoweravailable.Thisbringsustothepointofthetrimofapoweryacht.Withazerotrimanglethisareaisthesmallestandassuchitstandstoreasonthatitwouldhavetheleastresistanceandgiveyou themost speed. However,allyachtsunderpower tend to trimby thestern(backwards).Thishappensbecausethebowwaveliftsthefrontandthepropellertendstopull thesterndown. Onamono-hull this isoftenonlymomentarywhentheyachtgetsontheplane(jumpsoverthebowwave)afterwhichitthenlevelsout.Amono-hullcanalsomakeuseoftrimtabsastheyhaveawidestern,theycanfitlargetrim-tabsorputthemfaraparttomakethemmoreeffective.Bymeansofusingthesetrim-tabstheskipper can make sure that the boat goes at the best running trim giving the leastresistancefordifferentloadconditions.Onapowercatamarannoneoftheseoptionsareavailable.Itdoesn’tplaneandithasn’tgotawideenoughtransomfortrim-tabstomakeanyworthwhiledifference.Tomakemattersworse,becausewearetryingtomakethehullsasslenderaspossibletoreducethewaveresistance,thedirectsideeffectofthatisthatthosesamehullsarenowalsobecomingfarmoresensitivetotrim.

So how do you solve this, especiallywhen there are conflicting requirements to dealwith?Youneedtocontrolthetrimoftheyacht,butyoucan’tusetrimtabsandyoucan’tmakethehullswiderasthatwillgiveyoutoomuchwaveresistance.The solutionwe looked to for this problemwas to design a hull shape,which is lesssensitive to trimover a rangeof loading conditions atdifferent speeds. Thewaywewanted tomake it lesssensitive to trimwasby findingaperfectbalancebetween thetrim generated by the bow wave and the downward force of the propellers to becounteractedbyadynamic lifting force in the sterngeneratedoutof a specific tunneldesigntopartlyhousethepropellerin.OntheLeopardcatamaransthetunneldesigninwayofthepropellerissuchthatthereissuchaforce,tryingtokeeptheboatleveloverarangeofspeedsandconditions.Theideaitselfsoundssimpleenough,andpropellersintunnelsarealsonothingnewasitisusedfordecadestoimprovepropellerefficiency.Buttheideatousethetunnelsnotonlytoimprovepropellerefficiency,butalsotoreducethetrimoftheyachtisfarlesscommonandmayevenbeunique.TogetthisrightwestartedanextensiveCFD(ComputationalFluidDynamics)programwithNUMECAinGermany.WewouldsupplytheinitialHullshapesandNUMECAwouldmodel and run the CFD study and report back to us for further evaluation. The CFDstudyallowedustocarefullylookattheresultingtrimangles(Figure1)andassociatedpowerrequirementstodrivetheyacht.

Figure1:EarlyresultsoutofaCFDTrimStudy

Theresultoftheprogramwhichranoveraperiodof6monthswastheLeopard51PCwhich proved that the concept worked. The 51 PC has a low running trim in allconditionsandexcelsinspeedandfuelefficiencycomparedtosomevesselswithsimilarPowertoWeightratio’sbyasmuchas22%,primarilybecausetherunningtrimiskeptundercontrolatalltimes.With the success of the 51, the demand grew for a smaller power cat with similarofferingsintermsofperformance,butinasmallerandmoreeconomicalpackage.

Sothechallengewasontoseeiftheresearchdoneonthe51couldbecontinuedforthe43andimprovedupon.Todothisonasmallervesselsuccessfullyweneededtoworktoanevenlowerrunningtrim. To do this at a lower speed (compared to that of the 51) the upward forcegenerated in the stern had to become bigger. Thiswas done bymoving the volumedistribution further aft than on the 51 andmaking it wider in the aft sections. Thedangerhereisthatifwewouldbecometoowide,thewaveresistancewouldgouptoomuchandtheresultwouldbeasignificantdropinspeed.

Figure2:WaterlinesprofilesofLeopardCatamarans.

Figure2showsacomparisonbetweenthewaterlineandtunnelprofilesoftheLeopard51and43.Onecanclearlyseethatthe43hasrelativelywiderhullsandmorevolumeattheback in order to create a bigger lift in the stern sections. That is hasn’t been toomuchcanprobablybestbeseeninFigure3.

Figure3:OverlayofWaterlineprofilesLeopardCatamarans.

Hereyoucanseethatthetunnelwidthandtransomwidtharemuchcloserifwelinethehullsuponthepointwherethepropellertunnelstarts.Theamountofarealostinthefrontexceedsthatwhichislostatthebackwhiletheoverallbeamhasn’tincreased.Assuch thispictureprobablybestdepicts the changes indesignbetween the twoyachtsandcanserveasanillustrationwhythe43managestooperateatanevenlowerrunningtrimthanthe51.Onapowercatamaranathighspeedeverythingisaboutthetrimangle,ifitistoomuch,all you will do is burn fuel as the resistance becomes excessive for the speed. Asexplained,power catamaransdon’tplaneanddon’t respond to trim tabs soyouronlyoptionistogetitrightfromthestartinthehulldesign.Whichbringsmetothetitleofthearticle:Powercatsdon’tliketrim.(Orinotherwords,goinguphill).AlexanderSimonis,June2016