World champion designer and skipper Graham Bantock guides ...

P44-47(DESIGN) 16/12/08 6:09 pm Page 45The generally light-air 2008 Europeans inCroatia were dominated by less extremedesigns. Foreground left to right, Karaoke,modified Topiko and Disco designs.Second row: Extreme, Taktic and Topiko.Left: the heavily flared German Test-5sfared less well at 56th and 61st overalldesigns were a lowered mast position anda raised or heavily cambered foredeckwhich permit each rig to be carried furtherup-range, thanks to the lower heeling armand the ability to shed water when thebow is depressed on a run.Since 1994 the TS2 had used a bulbwith a length:diameter ratio close to 7. Incontrast, almost all other contemporarydesigns featured a comparable ratio closerto 5. Bantock soon started to investigatethis area in more detail: ‘I went back tobasics and took a long look at the data inHoerner’s Fluid Dynamic Drag for lowspeeddrag of bodies of revolution.‘After fitting logical curves to the experimentaldata his calculations clearlyshowed a steadily decreasing drag forballast bulbs as the length:diameter ratioincreased. Obviously this also increasesstability – a real win/win situation.‘The only restriction appears to bepractical problems of long, thin ballaststrength and the increased moment ofinertia in yaw and pitch. In practice themoment of inertia issue is not as critical ason a comparable bigger boat, such as a V5ACC design, as One Metre models alreadyhave incredibly high inertia in pitch due tothe deep draught and tall rig. So the effectof the longer ballast on this still unquantifiedeffect is minimal.’During 1998 Bantock began to useballasts with L/D ratios around 10 and thishas since become the norm. That refinement,in combination with the Ikon hulldesign, developed with help from a VPP,proved successful enough to win the 1998European and 1999 world championships,both in a wide range of conditions.The new millennium: towards a sweetspotBy winning the 2000 Europeans in Franceand the worlds the following year inCroatia, Martin Roberts’ new Gadget byChris Dicks began to turn the tide, regainingthe high ground for narrower designs.The Gadget featured 10mm more beamthan a Widget and less tumblehome, givingit slightly more stability for windwardperformance. However, but for an unfortunateincident in the last race, the Europeansmay as easily have gone to Guillermo Beltrisailing an Ikon. Bantock placed third withthe new Italiko at 215mm beam.Like Ikon the Italiko features a heavilyflared hull and is aimed at best performancein No1 rig conditions. In fact, sofast was this design in the Adriatic driftersthat it timed out most of the A-fleet at bothmajor events in the lightest airs.As we moved further into the new decadefocus switched briefly from hull shape to sailhandling. Going into 2003 few would havepredicted Trevor Binks as a world championbut, prior to the 2003 worlds in Canada, heand his brother Ken had been busy testingBarry Chisam’s 215mm-wide Isis against theTS2 and found it really strong, especiallydownwind. But as well as finding themselvesa nice hull set-up, the brothers had also beenSEAHORSE 45

P44-47(DESIGN) 16/12/08 6:10 pm Page 46pioneering the latest Hitec 5745 servosystem for sail control and discovered significantadvantage in being able to gybe, sheeton or sheet out in an instant. Binks went onto dominate that year’s worlds with goodspeed and immaculate, crisp boat handling.The 2004 UK championship was sailedin very light airs and Michael Scharmerprovided brief discontinuity in the developmentcurve with a strong performancesailing his amazing 135mm wide, woodensparred design, eventually placing sixth.However, Bantock successfully defendedthe status quo, retaining his title with hisItaliko (now also using Hitec servos)But Scharmer had demonstrated that theargument for wider hulls was far from over.Slimming down again: replacingthe TS2Following Scharmer’s performance in theUK, and perhaps accepting that a shiftback towards a narrower, or rather lesswide, hull was essential, Australian CraigSmith was now replacing the long-servingTS2 skiff.Smith’s early developments did notprove entirely successful but by the time ofthe worlds in Mooloolaba in 2005 his newObsession was thoroughly tested andready for battle. At 230mm beam it sat justabove most other previous designs in termsof stability but featured a waterline beamand wetted surface area close enough tothe best light-airs boats to be in contentionall the time. Craig won the 2005 event by ahandful of points from Bantock’s Topikoand ahead of Jeff Byerley’s 240mm-widedesign Cockatoo sailed by Paul Jones.Last year’s world championship inFrance proved a truly competitive event,with the outcome decided between thethree top helms only in the last race. Aweek of widely varying conditions, fromlight air to Mistral, showed no particularoverall pointers for design except thatextreme boats are to be avoided! The eventwas narrowly won by Australian skiffsailor Brad Gibson using a Widget, aheadof Guillermo Beltri on a borrowed Topiko.Craig Smith used his Obsession to placethird, followed by a Widget, a Tonic, andby three more Topikos, one sailed byFrench sailor Guillaume Florent – whosubsequently won a bronze medal in theFinns in Quingdao.Over last winter the Topiko was furtherdeveloped to address one perceived weakness.Its bow is very clean right up to thedeck, allowing easy penetration of wavesand better airflow onto the headsailupwind. Downwind in steady conditions,with or without waves, the boat is very fasttoo, but in the gusty conditions moreprevalent on smaller ponds the Topiko isprone to bow burying. To address this thebow was given more flare and a transitionalchine was added along the aft 20 percent of the hull.The 2008 European championship inDubrovnik was dominated by light andfickle airs, lots of current and sloppy waves.Designs featuring in graphs and illustrations but not detailed in the main text: Oscar is a Peter Wilesdesign that has shown great downwind speed. Kite and Trinity are two earlier Bantock designs thatare still in current production in the USA by Mike Hughes. Vektor and Zig Zag are two more recentGraham Bantock designs and are optimised for reaching and windward/leeward courses respectivelyHowever, the best sailors were not undulytroubled by the challenging conditions.Guillermo Beltri showed excellent speed tohead the top four, which was an even mix ofPikantos and Topikos.The most obvious area of IOM developmentin 2008 was above the waterline,with a much wider than usual variety ofsailmakers and sail designs in the top halfof the latest world championship fleet.The current paradigmMoving into 2009, the ‘safe’ design spacelies within the 190-240mm range of maximumbeam with a waterline beam of170mm to 190mm – characteristic of awell-flared hull form.However, this is not the only successfulhull form, with slab-sided and eventumblehomed midsections still capable oftop performance. The 2006 UK nationalshad been won by Dave Potter sailing theLintel design of David Creed, a tumblehomed,high-prismatic hull with a developingchine from midships to the transomwhich went particularly well in No2 andNo3 rig conditions and big waves. A maximumbeam of 210mm just above thewaterline gives the Lintel high stability butalso high wetted surface area.Michael Scharmer continues to develophis ultra-narrow hull forms and earned agood win in the 2006 German championship.It seems that the downwindperformance of his earlier boats hasimproved with attention to the detail ofthe hull design, while the rig’s performanceis being improved by small increases inmaximum beam, enough to reduce mastcompression to a manageable level.In conclusion, the best performance inthe IOM fleet is no longer determined bythe simple ratio of stability per unit of hullwetted area alone. Other factors, such as ahull form suitable for low wave drag athigh speed, the quality of appendages, rigperformance, balance and so on, are nowimportant enough for the obvious featuresto be less of a determinant than in the past.Designing foils for low Reynoldsnumbers– David HollomThe problems of designing foil sections formodel racing yachts, which operate at verylow Reynolds numbers (Re), are verydifferent from the problems of designing foilsections for full-sized boats that operate athigh Re, though the same physical lawsapply to both. (If the reader is unfamiliarwith some or all of the technical terms suchas Re transition and separation bubble theyare referred to the series of articles ‘Go withthe Flow’, Seahorse issues 288, 289, 290and 292).Re measures the relationship betweenviscous and inertia forces in a fluid. At lowRe viscous forces, which tend to hold themolecules together, are relatively moreimportant than inertia forces and laminarflow over large chord lengths is relativelyeasier to achieve than at high Re when theinertia forces, which tend to throw themolecules asunder and thus destroy laminarflow, predominate. (A laminar boundarylayer can be important because its drag isconsiderably less than that for a turbulentboundary layer).The process is gradual but the crossoverRe where inertia forces begin to dominate isaround 1,000,000. Below this Re laminarflow is almost automatically achieved (flowon a flat plate is laminar to an Re of about1,000,000), but as Re rises above1,000,000 laminar flow becomesincreasingly more difficult to maintain andshaping of the foil, to maintain anaccelerating flow, is necessary to maintainlaminar flow up until the desired chord-wiseposition of transition to turbulent flow.Accelerating flow is important inmaintaining laminar flow because increasingvelocity means lower pressure so thataccelerating the flow means that the point of46 SEAHORSE