How does batten angle of the junk rig influence its (upwind) performance?
Batten angle in a junk rig is typically set to something between 0-15°. For example, Arne K. used 10° boomrise angle for his Johanna, which is the same angle as the batten angle for the lower parallelograms.
Now, in another thread we lively discussed the influence of the batten pockets of sails with sewn-in camber on aerodynamic phaenomena such as induced drag, tip vortex and the like. A story of a sailmaker was shared, calling those battens “aerodynamic fences”
We discussed the fencing effect of the sewn-in camber on induced drag – together with ideas about end plates – and speculated about how those “fences” aka batten pockets reduced that special kind of drag. Here again is one of the according sketches:
Now, about this 2D-sketch I missed something: the 3D. If the angle of the batten pockets does not align with the airflow, it creates separation, vortexes and as such: drag! Have a look at those sketches:
Parallel airflow at Arne's beautiful Broreman:
Not so parallel airflow on this one:
That batten angle/ boomrise angle is determined during sail design, and cannot be changed on the water (maybe only slightly by shifting the boom or the yard abow/abaft). However, not every boat sails with the same stiffness, i.e. a folkeboat heels more due to its hull shape than a for example a Maxi 77.
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Now, if we want a sail to better suit a given hull: Wouldn’t it be reasonable to have a lower boomrise/batten angle for the stiffer boat (more form stability) and a higher angle for a less stiff boat (less form stability)?
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Taking that thought further: How much sense does the upper triangle part of the HM sail do – aerodynamically? Those 2 high-angle battens at the top surely won’t do any good to the smooth airflow… Recent findings of Colin C. point towards the same direction: wouldn’t a all parallel sail be more effective - aerodynamically? (surely, there would be always pros and cons)
Taking that thought even further (and playing the devil’s advocat here): Might that also be one of the reasons, if not THE reason, why Slieve’s split rig is so effective? Please prove me wrong, but as far as I could research his SJR mostly has low, if not very low yard angle and almost parallel upper panels…
I’m very interested and open for any critical thoughts!
Paul
PS: of course this whole topic only relates to sewn-in camber panels (barrel cut, shelf foot, etc...). Flat cut sails or sails with camber created by batten-bending or similar are naturally not (or less) affected, as they lack those fencing batten pockets.