See James Brett and some discussion under Rozinante.  It seems to work very well on a small proa, though I do wonder how it will scale up.

I can see the advantages of a quick change when shunting, and also reduced lengths of sheets.  Also reasonably simple to make.

There is a (percieved?) disadvantage of putting the sail through the wind rather than feathing.  I understand that the wind pressure on the sail is greater when drawing aerodynamically, than when stall and flat to the wind,  so in most conditions not that greater worry.  That said in a nasty cross sea you would probably need to drop a panel or two during shunting.  Anyone who has sailed a junk will know this is as easy as sheeting in a bermudan mainsail. (which is why a junk can gain from always having the correct amount of sail).

My question is how efficient is this type of sail?  Being symetrical the centre of pressure is at 50% and in practice probably slightly further back.  Fantastic when the sail is free, but not considered the best close hauled.

(My choice of rig would be a junk rig wing sail,  as Bernard Fercot has on his cat)

Cheers
Mark Thomasson

Mark - 01 December 2011 05:45 AM

My question is how efficient is this type of sail?  Being symetrical the centre of pressure is at 50% and in practice probably slightly further back.  Fantastic when the sail is free, but not considered the best close hauled.

Quoting from ‘Sailing Theory and Practice’ by C. A. Marchaj, pg. 137-138:

“There is a general idea among sailing men that the position of the maximum camber should be 1/3 of the local sail chord, aft of the mast. This shape is supposed to produce the most effective aerodynamic performance. It originated by taking an analogy between sails and bird’s wings and with the usual position of maximum thickness of aeroplane wings. Nevertheless, experiments on sails have shown that this traditional view cannot be accepted without reservation.

“Figure 88 (not shown) gives the results of measurements made on three sail models, having the same camber of 1/13.5, but three different positions of the maximum. Sail 1 has maximum camber position at 1/3 of the chord length from the mast, sail 2 in the middle, and sail 3 at 2/3 of the chord. The differences between their resultant total aerodynamic force coefficients at similar incidences are very small, and do not exceed 4%, yet the sails are not alike and their merits will depend on the yacht’s course.

“On windward courses sail 2 is the best, since it has an aerodynamic drag angle typically about 2 degrees less that its rivals. When reaching, sail 3 shows some superiority of driving force. Sail 1 is everywhere inferior to either sail 2 or 3 or both. The best all-round compromise is sail 2, with its maximum bow at the mid-point of the chord and flat after-part of the sail.”