Hinged, cambered or flat - or a hybrid?

  • 14 Nov 2010 21:49
    Reply # 462951 on 461931
    A couple of weeks ago, I visited Pete Hill, and saw his Kohler catamaran in build. Pete wanted to know all about my wingsails, with a view to going down that route. After I'd showed him all the drawings and photos, he decided, quite rightly, that it wasn't for him. A wingsail gives the very best performance in a junk-like sail, but it does so at a high cost in terms of materials, time and skill. In cost/benefit terms, a junk sail with either camber built into the panels, or with hinged battens has been proven to give good performance with less investment.
    Cambered panels have their disadvantages  - loose cloth to flog in a breeze, or to hang limp in a calm; uncertainties over batten stagger; diagonal creasing having to be addressed; and these disadvantages show up more, with more camber being built in. Hinged battens have their disadvantages - loss of integrity; the need for good engineering of the hinge; inability to put in a smoothly curved shape; S-bending if a second hinge is added forward to obtain an acceptable amount of camber with its maximum depth in an acceptable place.
    All these disadvantages show up more, the more camber that is added, to the point where I feel unhappy with either method when a camber of up to 10% is aimed for, an amount that I have now achieved without penalty in my wingsails.
    There is another way. I used it to a very minor extent on "Ivory Gull", way back in the '90s. A combination of both a single hinge and some camber in the panel can be very effective, without too many penalties.
    I made a sketch for Pete, which I have filed in "Members' online file storage/drawings/David's doodles", entitled "sketch for Pete Hill". It shows 1) a single hinge of ±12 degrees articulation, achievable with sufficient strength and integrity 2) the hinge at 50 % of chord, where it won't misbehave by trying to angle the wrong way 3) the sail aft of the hinge dead flat, behaving in a fully junk-like way - no flogging, always well behaved (a feature that I have retained in my wingsails), and able to resist diagonal creasing 4) the sail forward of the hinge with added shelf-foot camber, but to a much lesser degree than would be necessary without the hinge.
    Pete's boat will be very light and easily driven, so I drew the sketch at 6% camber (I haven't tried to alter it to 10 % yet).
    I know Pete can make good hinges - China Moon sailed from Brazil to Tasmania with them, so I have no worries about integrity. There is more work in the sailmaking than with a flat sail, but less cloth is needed than with cambered panels alone.
    Any comments or questions?
    Merged topic from TECHNICAL FORUM: 15 Aug 2018 20:49
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