Exactly my experience on a Coromandel when tacking. All good fun.!

Ash

This is a very interesting experiment and I hope you continue with it - maybe you could post some photos of your panels in the technical illustration section, your avatar photo isn't really big enough to see any detail.

It also seems to me that this post would be better in a thread of its own.

Slieve,
my intuition agrees with you, but recent practical experience does not. When I had a flat sail on Malena, back in 1990, I fairly often helped her tack by grabbing the boom and backing the sail a little.

With a cambered panel sail fitted, she (and my later boats) did not need this help. Anyway, I thought it would not help, so I didn’t try.

However, recently I have backed the sail a couple of times, and it works just fine: That indicates that the CP moves far forward at small angles of attack (even when the panels are not fully inflated). Funny, but that is what I observe. On «normal» sailing angles (of attack) it is clear that the CP moves forward at low angles and moves aft as the angle rises, and this helps my boats to steer themselves with a locked helm.

Cheers, Arne

Slieve McGalliard wrote:

David Tyler wrote:35% is rather too much balance, I feel. The sail will overbalance and snatch on, instead of feathering, when going to windward. 25% is safer, but still marginal.

Derek Van Loan specifies an angle of 125 to the luff, 35 above horizontal, and that would be the lowest angle I'd want to use for a  junk sail with maximum balance. 

(Except for special cases - split junk and my latest wingsail, for example)

It's possible that the above comments by David could be misleading.

Bill Belcher, in his book on Yacht Wind-Vane Steering, points out that for a flat plate the resultant pressure (or centre of pressure) acts at about 33% of the chord back from the leading edge, and tends to move further aft with increasing angle of the foil to the airflow. He also says that with a streamlined section the centre of pressure is further forward at 24 to 29% of the chord, depending on the streamline shape used.

The stiff battened flat junk sail acts very much like a flat plate, so should be stable up to near 33% balance. In his book on the Chinese Sailing Rig Derek van Loan draws his rig with 27% balance.

David Tyler wrote:35% is rather too much balance, I feel. The sail will overbalance and snatch on, instead of feathering, when going to windward. 25% is safer, but still marginal.

Derek Van Loan specifies an angle of 125 to the luff, 35 above horizontal, and that would be the lowest angle I'd want to use for a  junk sail with maximum balance. 

(Except for special cases - split junk and my latest wingsail, for example)

It's possible that the above comments by David could be misleading.

Bill Belcher, in his book on Yacht Wind-Vane Steering, points out that for a flat plate the resultant pressure (or centre of pressure) acts at about 33% of the chord back from the leading edge, and tends to move further aft with increasing angle of the foil to the airflow. He also says that with a streamlined section the centre of pressure is further forward at 24 to 29% of the chord, depending on the streamline shape used.

The stiff battened flat junk sail acts very much like a flat plate, so should be stable up to near 33% balance. In his book on the Chinese Sailing Rig Derek van Loan draws his rig with 27% balance.

When you bring asymmetric section rigs into the equation the picture gets even more cloudy. Although built symmetrical a cambered panel sail is aerodynamically asymmetric. In the model world it is standard practice to build a balanced rig with 33% of the total sail area in front of the mast/ pivot to obtain light but positive sheet loads. The Carbospar Aero Rig was a full-size copy of the model balanced rig. Edward Hooper's Amiina has a split rig with the mast aimed for 35% batten length balance and has not become unstable.

There is a tie up between yard angle and balance in many rigs, but I believe it is more important to try to concentrate the tip vortex to get the lowest possible induced drag, and get the best lift/ drag ratio. I believe a low yard angle helps to achieve this. Van Loan uses 125º, and on Amiina we used 120º on the basic rig. In this year's Island race the top panel was reefed out, leaving a yard angle of only 100º. This did not seem to effect the performance, but with the untapered rig it was not aesthetically pleasing.

There is a lot of room for experimenting. You could learn a lot at little expense by making a scale model of a rig. I learned a lot about rig stresses by making simple batten and boltrope outline models.

Cheers, Slieve.