Further Devels'nent Ofthe Tunny
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FURTHERDEVELS'NENT OF THETUNNY RIG E M H GIFFORDANO C PALNER Gi f ford and P art ners Carlton House Rlngwood Road Hoodl ands SouthamPton S04 2HT UK 360 1, lNTRODUCTION The idea of using a wing sail is not new, indeed the ancient junk rig is essentially a flat plate wing sail. The two essential characteristics are that the sail is stiffened so that ft does not flap in the wind and attached to the mast in an aerodynamically balanced way. These two features give several important advantages over so called 'soft sails' and have resulted in the junk rig being very successful on traditional craft. and modern short handed-cruising yachts. Unfortunately the standard junk rig is not every efficient in an aer odynamic sense, due to the presence of the mast beside the sai 1 and the flat shapewhich results from the numerousstiffening battens. The first of these problems can be overcomeby usi ng a double ski nned sail; effectively two junk sails, one on either side of the mast. This shields the mast from the airflow and improves efficiency, but it still leaves the problem of a flat sail. To obtain the maximumdrive from a sail it must be curved or cambered!, an effect which can produce over 5 more force than from a flat shape. Whilst the per'formanceadvantages of a cambered shape are obvious, the practical way of achieving it are far more elusive. One line of approach is to build the sail from ri gid componentswith articulated joints that allow the camberto be varied Ref 1!. This approach is claimed to give a very good performance, but suffers from the practi cal di sadvantage that the sai 1 area cannot be reduced by reefing. On large vessels, wherethe rig provides sail assistance contributing only a fraction of total power. this may not.be a limitation, but for smallvessels designed to exploit sail powerto the full, someform of area reduction appears to be vital!. The alternative of a sail which has both cambercontrol and provision for reefing has proved to be a very difficult specification to meet with a practical seagoing solution. Manyextremely complicated systems have been proposed, which whilst they may have worked for thei r enthusiastic inventors, have not been sufficiently practical to attract wider attention. The first practical solution to this problemhas comefr oman unexpected quarter. The Combewrightshad a simple objective, to build the cheapest boat that would take them safely across the Atlantic. They started with no preconceptions and little knowledgeof boats or the sea. They talked to people, listened and learnt. Their solution was a 32' catamaran with a wing sail - a wing sail with cambercontrol which they called the Tunny Rig. They realised their ambition, crossing the Atlantic last year without problem. Since then they have proceeded via the West indies to the Panamacanal and so to the West Coast of America. Their voyagehas provedthe practicality and seaworthiness of the ri g in its prototype form, and their basic concepts are embodied in new versions of the rig being developed by Gifford Technology. This developmenthas taken place over the past 18 months and has now reached a stage where a production prototype is being tested and corrmrercial applications are under serious consideration. 361 2. PR!NCIPLES OF THE TUNNYRIG TheTunny Rig is an aerofoil wingsail madefrom a set of shapedbattens connectedby panels of sail cloth Fig I!. Thebattens wrap aroundthe mast,thus formingthe aerofoil section, andcan be collapsedone on top of the other either for reefing or clearing awaythe sail. For almosttwo thirds of thefr lengththe battensare rigid frames,made fram thfn section timber. The final third of the length comprises detachable,flexible extensionsto the mainframe, interconnected only by a single tensioned line. The camber control comes from two lines, one from the end of each flexible section, whichare fastenedto a crossmember in the front part of the batten. By differential ly tensioning these lines the batten is warpedone way or another, the degree of waro being limited by the tension line joining the flexible extensions. ln the full rfg these control lines are tensfoned by a conwnonline runnfng downthrough the sail to over centre levers on the lower batten or boom!. 3. TUNNYRIG OEVELOPMENT Building on the Combewrfghts ' ori ginal concept the ri g has been developedin two phases, comprisingthe manufacturepf a 28m2sail for initial practi cal appraisal fol 1 owedby an 18.2m< sail for di rect comparativeevaluation against other rigs. Muchof the first stage was reported in Ref 2, notably the selectfon of the aerofoil section for the sail and the simplification of the internal mechani sm. The section chosenwas the NASAGA M, a recent product of research into aerofofls suftable for light aircraft. It provedfdeally suited to the Tunny Rig for several reasons: Relative insensitivity to surface roughness Thick section with thickness carried well forward Camberla'rgely concentratedin small proportion of the trailing edge Goodlift to drag ratio coupled with high maximumlift coefficient Goodaerodynamic properties retained in low wfnd speeds, i.e. low Reynolds numbers Figure 4 from Ref3! showsthe basic lift characteristics of the section at typical sailing boat Reynolds numbers. Ffgure Q shows its superiority in terms of maximumlift over other typical sections, in the practical condition of aerodynamically rough surfaces. The first developmentrig of 28m2area was built using this section and fitted to a catamaran trial s boat Fig 4!. Extensive tri als were carried out last suniaer in a wide range of wfnd strengths and a great deal was 1 earnt about the general handlf ng characteri st i cs and practfcality of the rig. The positive features demonstrated are: 362 Low sheet loads. A direct result of the sail area being y lanced on the mast as in the junk rig. Evenl distributed loadin . Since the sail does not rely for its shape on being set betweenattachment points where loads are highly concentrated, the sheet and'aerodynamicloads are evenly distributed so reducing the weight and quality of sail cloth required as well as the complexity of manufacture. Insensitivit to sheet osition. Because the sail is self supporting it does not rely upon the di rection of the sheet lead for its shape. Consequentlythe sheet can be taken to points which do not obstruct operation of the boat and which fit best with the overall deck layout, rather than dictating the layout as with many soft sail rigs. The sail cloth is supported by the Tunny og when being rai sed or as the sail is tacked. Practicali t of Reefin . Again in comnonwith the junk rig reefing is a quick and effective process. Easeof handlin . The light sheet loads meanthat the 28m2sail can be tended single handedwithout the need for a winch. Its docile behaviour when hoi sting or lowering also makes life much easi er for small crews. retained dri ve even when very close to the wind, tacking was a much more reliable pr ocess than is normally the case on a catamaran. Provided the sheet was freed off contrary to normal practice! as the boat was put i nto the wi nd the sai 1 would drive the boat round and through the wi nd. Close winded. The lower drag of the rig coupled with i ts stable Cambereffective. Trials conductedwith and without camber in the sail demonstratedan improvementi n dri ve result i ng from the a pp1i cat i on of camber . By pushing the sail out into a reverse mode the craft astern quite simply. Li fe is not al 1 gain howeverand there are of course somenegat ive points aboutthe rig. Comparedto, say, a Bermudianrig, the TunnyRig will alwaysbe heavier, thoughfurther developmentand the incorporation of more refined st ructural details could well reduce the margin cons i de rab1y. The other prob1 emswhi ch were foundon the fi rst ri g were 1 argely specific to that particular rig and were rectified for the next 18.6m2 vers i on. Excessive structural wei ht. In the absenceof rel i able 1 oading data the Tunniesand the sail cloth weremade greatly over strength and thus the rig wasmuch heavier than necessary. 363 NASA GA W! -1 Section ~f Srrr'incaOovynness ana ~ nolae Hvmbar If<seeet gecvian$4spe an Mastrrerm utr ~nrenr ~ et Attmhlaearaeal 1eynal4 Mrnaer Figure 1 Figure 2 TUNNEYkjc OH SAHGSYtPPER Figure 3 364 Creasin of sail cloth. Oueto deflection in the top batten gaff! t e top pane set bad y towards the trailing edge. Also it proved impossibleto complete'lyeliminate creasesaround the leading edge, even when extreme halliard tensions were applied. The sail was laced to the Tunnies far more necessary, making removal and adjustment very time consuming. Hi h camber control forces. This problem was linked to the overstr ong structure which resulted in the Tunnies bei ng stiff and difficult to deflect. Damae to control mechanism. The camberlines were tensioned by means of levers on each Tunny. These worked well but suffered damage when the sai 1 was lowered . Li it d ri rotati on Dueto the halliard arrangementand the down hau on t e oomthe rig could not swing out freely to large angles. Since it wasfinely balancedabout the mastthe aerodynamic forces were insufficient to force the sail out and thus it was difficult to reducethe dri ve by simplyfreei ng the sheet. Lack of visual clues to saiI setti n . Since the sail did not flap w en not set correct y it was di icult to determine visually the best sheeting angle. No reliable measureof erformance. Whilst the sail appeared subjectively to performwell it wasnot possible to obtain any vali d comparisonwith other rigs by which its true performancecould be judged- 4.