"A wing coefficient of lift without a flap can be 1.5. A sail has a hard time exceeding .8 typically around .6.  A Wing with a flap can have a lift coefficient around 3 (but the drag goes up.)  Lift/drag ratios of sails are around 30 to 1, where a wing can be around 140 - 1.   With a flap the wing drops down to the 30-1 range similar to the sail.

A wing can be around 4 -5 times more efficient than a wing sail depending on how perfectly the sail is trimmed. If the sail is poorly trimmed who knows? ^^"

I find it hard to believe that the difference is so huge. Mind now that I don't have a formal degree in aerodynamics. All I know is what I've learned from reading on the subject in recent years, and playing with software. I also don't have much experience in sailing. I own a small keel boat. I bought it two years ago, and have been sailing it for two summers. That is all the sailing experience I have. So take what I write with that in mind.

I've learned to use softwares like xflr5. When playing around with it, I have not been able to find anything special with a single element wing compared to a multi element wing / wing with flap. I have found that a double element wing with about 50:50 chord ratio has a very similar lift/drag curve to a single element wing of similar shape. The difference in drag (I assume) comes from the roughness around the gap between the elements, and higher wetted area. But it's minor.

The figure of L/D=140 sounds like the performance of a wing section in infinite span. A peak L/D of 30 with infinite span sounds very low? a sail is that bad? why?
I have not found much difference in peak L/D between a single element wing, double element wing, or "sail" (very thin wing). Something tells me you are using different Reynolds numbers when comparing. Which would maybe make sense, since a craft like the Greenbird would sail fast, and a keel boat slow, in the same wind. But I don't think that is a fair comparison.

Now that's what playing with the software tells me. But since I don't have any real experience with this, and never have spoken/written to anyone about this, I don't know if I'm just looking at numbers in a computer, with no connection to the real world, or if the results are in fact usable.

Are you sure that a sail would have a hard time exceeding Cl=.8?

When I look at the jib on my boat, I think it makes a shape very similar to the lee side of a wing. I don't see why the sail would be so bad at "bending" the air stream, compared to a wing?

Are you sure these figures are not for low aspect ratio sails?

At AR=1-2 a wing would not reach Cl=.8 either.

Talador wrote:
"If the boat is heavy and draggy, a lot of inefficient sail area is better than a wing."

I agree with that. I guess it would be possible to sum everything which is dragging, and calculate the "total L/D". Then the optimum would probably be a very big/low aspect sail on such a boat.

Bjorn