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M. W. Schacht



Crab Claw Tests

Nicholas Schneider sent me some results of his crab claw rig experiments, which are posted after the jump. Thanks Nic!

These are qualitative studies of slender foils and leading edge vortices for sailing craft.  The results of these experiments, suffering from improvised input and variable conditions were good enough to inspire further work but produced more questions than answers.

Description of rigs in drawings:

  • A. Original Sunfish rig, large camber, high weather helm and sheet tension.
  • B. Little camber due to spar support line, downhaul and highly tensioned leech line, lee helm, light sheet tension, taut flat sail does not luff, snaps through tack, less heel, nice sailer
  • .

  • C. Little to no camber due to heavier fabric, downhaul, compression strut and leech line, too much lee helm.
  • D. mast aft 2-’3”, almost neutral helm no sheet tension, reduced heeling but loss of easy tacking and some windward ability.
  • E. Fabric tension from apex to upper tip and leech line maintain flat, non-luffing leading edge for vortex formation, loose foot cambers lower area for conventional flow, too much lee helm.

  • F. Mast aft 1’-9” (total of 4’-0”), neutral helm in lulls, weather helm in gusts but weather cocking makes for difficult tacking.
  • G-1. Rigid wing pivoting in mast sleeve 2, lee helm, no sheet tension.
  • G-2.  Mast aft 1’-9”, neutral helm and light sheet tension.
  • G-3. aft another 1’-0” with mast in sleeve 1 for more feedback.  Best tack with camber to leeward but tough tacking, think conventional flow kicks in to fight crossover. Sharp leading edged clean wing disappointing beyond being heavy and clumsy.
  • H. Cut down from G as in drawing with camber to leeward performance close to G but flat to leeward not good, could not even form spiral in tell-tails which G could on both tacts, might be maximum camber being so far aft.
  • B-H all had characteristics beyond my sailing experience. D sailed without sheet, assume CP at mast and movement of aerodynamic center with changing AOA trimmed sail as I steered with dead helm. Sailed both F and G3 same day in higher wind and open water chop and gentler conditions in harbor where G3’s larger area had some advantage but generally similar performance, excepting asymmetry of G. 

    Tell-tails of video tape applied to D-H and generally formed as in drawing. 1 floats above all, 2 and 3 spiral aft, slight drift to leeward, once neatly twisted up and unwound on opposite tack, 4 rises up to join them at best trim, 5 steadily streaming aft, 6 sagging slightly. Rounding up puts all streaming aft, falling off, 1-6 float to leeward and most in lee field hunt forward. Tell-tail chafe greatest on leading edge between apex and mast, and in trailing vortex. Good gusts produce a tighter vortex, there is a delay in acceleration until it forms.  H, with camber to leeward has 3, 4 and 5 rise up along trailing edge to spiral. 

    Sailing F with flat foot in harbor 10-15KT wind, best tack was with mast and strut to leeward, opposite tack best in 15-20KT in bay pounding up wind with weather helm and tell-tail 5 rising up to join 2-4 in a more compact back flowing spiral. Assume high speed flow may extend vortex further aft.  In harbor weaker vortex may be assisted by disturbance of supports.  Flipped mast and strut to opposite side, performance followed suit. Varied sweep angle of D and F to improve windward ability.  D and F sailed flat footed and loose footed for lower edge camber.  Flat F sailed with leeward lower end plate which brought low line of leeward tell-tails parallel with boom. Rolled top spar to develop twist. All results, perhaps too nuanced for my variable conditions, were inconclusive. 

    Seems less camber is best, easing or loss of tensioning device has adverse effect yet wing favors camber side and good high wind performance may result from increased camber in loaded fabric.  Removal of lower aft area, sheet tension and helm indicate effective area of sail is forward yet slender wing’s center of pressure should be further aft. Suspect mast disturbance but clean wing has same characteristics.

    Telltails were quite useful during sailing,but better studied with boat beached, or better yet, indoors at 1/12 scale on tabletop with grated fan and cassette tape tell-tails on and around sheetmetal planforms. Formation and position of LEV and some surface flow direction and pressure can be visualized. In 1/12 model of H, using moveable telltail on wand in 6 MPH flow, cassette tape slides over top of LE and rolls into vortex on lee surface below with rapid rhythmic slapping; tape streams under and a detached vortex forms below and leeward of bottom edge, large drag inducing tip vortex? The small differences between sharp clean wing and sail rig with its high parasitical drag, and the small differences in performance between D and F with its reduced SA favors this area for improved L/D. Combined vortex and potential[conventional] lift puts CP aft of mid point in symmetrical delta wing studies. Half delta large tip condition compromises conventional flow causing forward CP as confirmed in sailing,but be skeptical.

    Adding tapered end plate, or pitching apex end up 20 degrees, lifts bottom vortex up onto lee surface. At 10 degrees AOA weak top and bottom vortices, conventional flow in mid aft section; 20-40 total vortex and best suction; 50 less suction and bottom detaching; 60 total separation. With flat to leeward 10-20 conventional flow; 30 transition; 40-50 middling vortex action; 60 detached. Results loosely concur with sailing experience. In models, LE with straight or curved sweepback, with sharp, round, blunt, even end plates all easily formed vortices but planform, streamwise camber and kant may help maintain attachment. In general at 20-40 AOA vortex detaches out toward streamwise flow at ½ LE, suspected drag of long telltail but short ones indicate the same. In B model LEV forms on lower half of LE and detaches out and bends down to horizontal streamwise flow at mid level, leaving tip area in somewhst confused conventional flow. Surprised it sailed so well; I might be missing something here. More sweepback, streamwise aft camber and kant twisted to leeward toward tip follows vortex trajectory delaying separation. Looks good but no idea of L/D consequences.

    Other than B, these rigs handled poorly but always got me home. Sunfish hull not ideal and focus of study was drag reduction but conclusions should apply to any LEV sail. The crab claw in low position, lower spar parallel to streamwise flow, produces large tip vortex and, as sweepback is reduced, spar increasing becomes a blunt trailing edge and LEV eventually fails to form on increasingly vertical LE. Yet sail performs with this theoretically poor design indicating much to learn and potential for improvement. Time for sailing. I have sail rig F and wing H in need of a low drag platform in measurable conditions. It is winter: ice, wind meter, GPS and an iceboat would give me a speed ratio; big work for a small number.

    Posted: 21/Jan/2008


    Comments are closed for this entry.
    BWD said:

    interesting you mention ice.
    I think maybe with a crab claw with the top spar at a low angle (<45 anyway), you may see effects a bit like a DN.

    I think effects parallel th DN in some ways, but expand past usual idea of twist control. It is more like some of what you see with a traction kite, and may encompass local and overall AOA and camber.

    This would seem compatible with the practice of pulling of mast to windwrd in higher wind (decr. AOA/decr. camber), and letting it sag leeward in light (incr. AOA especially in low to mid sections, incr. camber low)...

    ...And with a sheet from peak of yard (analogous to a junk’s sheets), which I think I have seen, you might tune the camber further….

    For more testing I would look to a system allowing the mast to move as on a universal joint; getting the most from the sails is likely dependent on more control over 3-d orientation of rig than for conventional rigs. I would also make sure the spars could be made to bend!

    Overall I think it probably works more like a kite -remember the people who developed them also made (& may still?) fantastic kites….

    The delay you report responding to a gust reminds me of how a kite responds to power, first with extra pressure, then with the lift moving more forward.

    Thinking of some crabclaw experiments myself, thanks!

    Posted: 24 Jan 2008 - 10:47
    nicholas schneider said:

    The DN,like Sunfish has aft rake of 20 degrees,found no LEV at this angle. I am working with 60 at 1/2 height and area. Never sailed a kite but find no camber best for LEV. Tip sheet could add twist and streamwise camber near tip to extend vortex attachment,but this and mast movement too refined at this stage,besides rig so low needs no stays,and stayed mast compression requires stiffer fuselage.I have to get on the ice before it melts.No overall weight yet but T planform is 13X8 feet with optional runner positions at6 and 4 for lower expected heeling force.Do not hesitate to experiment,any results,especially ones you do not expect.                                  Thanks for input

    Posted: 25 Jan 2008 - 7:05
    peter said:

    are you still trying with crabclaw sails?
    If yes i would like to have some contact to exchange experience.



    Posted: 23 Apr 2008 - 6:25
    nicholas schneider said:

    Feedback from Boat Design Forums offers an explanation of limited windward performance of crab claw, or any LEV driven sail. In conventional flow, on the cambered forward facing surface aft of leading edge, general area of upwash and peak pressure, the total force vector perpendicular to that area has a forward component; suction is on a forward facing surface reducing drag. LEV forms only on the backside of foil with aft component adding to drag.
      If above analysis is correct, a rounded or even flat leading edge would allow a forward facing surface that sharp would not. The SST Concorde has a drooping wing edge, and on Proafile there is a triangular section spar with base forward and apex trailing; both present some forward surface at peak pressure. Less sweepback should rotate this force forward, though this brings the lower spar up as a blunt, or even cupped trailing edge adding to drag.
      The crab claw rig was a good solution for proas avoiding the discomfort and stress of windward sailing in fragile craft with exposed crew on long passages, but contemporary masochistic sailors expect better windward performance. A single upper spar with compression strut or wishbone supporting the clew allows less sweepback with good trailing edges; similar to a sailboard rig, with its mast as the upper spar, rotated from verticle to horizontal for LEV formation. Cable tension in unsupported concave edges could control sail shape, flat for low sweep, cambered for high sweep upwind performance, but running rigging might be a challange.
      As water warms up will sail F rig with raised apex for second lower LEV working edge to maximize offwind performance and mull over windward options; non flapping flex gliding, http://jeb.biologists.org/cgi/reprint/58/1/225.pdf is quite intriguing in form and performance.
      So Peter, post your work here and generate more input.

    Posted: 09 May 2008 - 4:07
    nicholas schneider said:

    Gary Dierkin’s T2 rig manifests some of my proposals and he seems pleased with his prototype’s performance. His tensioning detail for sail shape is ingenious and critical for performance; flat for LEV off wind,stiffer yard and concave trailing edge would help here, and cambered for up wind presenting more forward facing surface, and it has a clean trailing edge. Control of sweepback with yard horizontal for LEV and vertical for upwind could automatically control tension with right halyard location. I think best potential for proa development is in rig improvement where inefficiencies are greatest but I’ve yet to sail one so read this with skepticism

    Posted: 26 May 2008 - 8:45
    Dave Kellogg said:

    I have really become interested in the crab claw rig for my sailing dinghy.  I like the over the top boom and have discovered a way to make that happen using a sailboard mast base.  Using the sailboard mast base at the top of the stubby mast and control lines to the outer edges of the crossboom and lines on the bow as well as stern I should have excellent control in all aspects of the crab rig.  I would be interested in hearing from anyone that may have tried this application.  I plan on making an airfoil on the boom and mast if things go well with testing the basic rig.  Dave Kellogg

    Posted: 06 Sep 2008 - 8:04
    nicholas said:

    sailboard mast base good ideal. Hard to understand complete rig without drawings but controls should produce flat horizontal sail for off wind, cambered vertical for on wind, both positions with clean trailing edges. Try using telltails, video tape works well, to see air flow. Good luck!

    Posted: 07 Sep 2008 - 8:12
    Dave Kellogg said:

    Since I’m building the dinghy to be a stowaway over the cabin of my 30 ft sailboat it’s taken a little more time to engineer this thing than I wanted.  When finished, at least enought to start testing I’ll post some sketches of the claw rig, one nice thing about the overhead neutral setting of the sail, it makes a wonderful sunshade at high noon….  chuckle   Dave

    Posted: 07 Sep 2008 - 2:05
    hans lammens said:

    here a short movie from a 62 feet catamaran with crab claw rig.

    Posted: 06 Dec 2008 - 12:16
    Pascal Lutz said:

    Jean Neyret, a French priest in the pre-war Pacific, carried out serious practical experiments while in Fidji. He insists very much on the yard being very close to vertical for maximum performance. See his 2 fantastic volumes “Pirogues Océaniennes”, in particular vol2 page 161 (Mariannes Isl.), where he pays tribute to Admiral Pâris’ keen eye.

    Posted: 05 Dec 2010 - 10:14