Ti Gitu also sails best when the sail is pressed against the mast. We first noticed this in the Med and had a chance to test this when behind an island in the Caribbean in perfectly flat water and a nice steady f 3 to 4 wind. The only explanation I could think of (as already mentioned) is that when the mast is buried in the sail the drag of the mast is lessened.

We are currently fitting new cambered panel sails made as individual panels between each batten. Not sailed yet but just hoisting them in the marina looks good. Will write it all up later in the year.

John wrote:

I have a cambered junkrig sail on my boat. My expectation was that the sail should sail better with wind from the mast side, i.e. with the sail in good foil shape instead of pressing against the mast. In actual performance, I was suprised to find the reverse result. The boat sail better with the sail pressing against the mast! I played with simulation software trying to find out the effect of the distorted sail when pressed against the mast. It showed that there isn't much damage on the laminate flow. (see image). My guess was that the transmission of propulsion force is more "solid" with the battens pressing against the mast, instead of transmission through the parrels on the reverse side.

It's a well known phenomena and it occurs with all sails that lean against a mast on one tack and away from the mast on the other. So junks, balanced and standing lug rigs all experience it but dipping lugs don't (they  "dip" the sail for each tack).

The best explanation for it that I've come across is that on the tack where the mast is exposed, the high parasitic drag of the mast is in full effect and you pay the price. On the other tack, the mast is buried in the sail, inside the lammar flow and the "air" does not "see" it, so it's drag is much reduced.

If one wants to reduce the effect the answer is to go for a skinny carbon fiber mast. There is still an effect but the smaller you can make the mast section, the less effect the mast will have. I have seen this when I have replaced wooden masts with steel ones or carbon ones. Both tacks always improve but for the one where the mast is clear of the sail, the improvement is most marked.

Bonjour

On the John's images we can see that the air flow remains stucked to the sail longer on the "bad tack". It is sometihing welle know in aérodinamic. The turbulent flow has less a tendency to leave the extrado surface.

It seems that the images are representative of "solid" sail with a very small incidence angle. If it's true it is not representative of a junk sail headwind. the incidence would be rather about 10°.

On some aircrafts, spoilers (small asperitiies on the surface) are added to the surface to prevent the flow to leave the surface.

So, in the exemple presented by John, the turbulences due to the mast in the airflow maintains the flow attached to the surface.

An other explanation, especialy whith flats sails, may be the turbulences around the mast create a virtual camber of the airflow. So with flat sails, the bad tack becomes the better one but with cambered sails the effect should be less evident.

Eric

Very informative experiments John! Keep it up !

So this is also confirmed (by AD) with the SJR rig?!

I have a cambered junkrig sail on my boat. My expectation was that the sail should sail better with wind from the mast side, i.e. with the sail in good foil shape instead of pressing against the mast. In actual performance, I was suprised to find the reverse result. The boat sail better with the sail pressing against the mast! I played with simulation software trying to find out the effect of the distorted sail when pressed against the mast. It showed that there isn't much damage on the laminate flow. (see image). My guess was that the transmission of propulsion force is more "solid" with the battens pressing against the mast, instead of transmission through the parrels on the reverse side.