High Aspect Jibs -- Theory and Practice

[Dockhead]

Last year I had a carbon laminate blade jib made for my boat, about 95%, so quite a bit smaller than the standard overlapping yankee jib (120%).

It has taken me more than a year to get all the kinks out of the sheet lead system, and also I had to have the sail shortened slightly to get enough luff tension on it, something I only just now managed to get around to.

But I knew it was a cracking sail from the very first time out with it last year, a sparkling overnight run across the Channel to Guernsey from Lymington, on a beam reach, with a couple of hours with more than 11 miles run each.


I expected it to be mainly useful for sailing upwind in true wind of more than 20 knots -- that's what it was designed for. The previous year I bashed 1500 miles from Finland to the UK with great frustration at how hard it was to make miles towards the true wind, in strong conditions. Two days and nights tacking across the North Sea, dodging oil platforms, in ugly weather, hardly getting anywhere, was the final straw.


I only now -- after a season and a half and a couple thousand miles -- feel like I can draw conclusions about this sail. And I realize that I don't really understand why it works the way it does, which is why I'm starting this thread.

Compared to the 120% yankee, the blade produces vastly less heeling moment, and the difference is much greater than I expected. I have never (!!) reefed it, not once, with the wind ahead of the beam. It works well up to well over 35 knots apparent, which is well into the usable wind range for my staysail, so instead of reefing, I just furl it and change down to the staysail (this begs the question of whether it could be a hank-on, and perform even much better). Over 30 knots apparent it needs to be flattened to reduce drag, but that is easily done.

It does not backwind the main at all, and the main seems to work much better without interference from the jib. I don't understand why that is true -- I always thought that the main needs circulation of air from around the jib, or vice versa. Main and jib don't really seem to work together when I have the blade up, yet the performance is excellent. In fact I even sail hard on the wind, with no main at all, sometimes. I can point almost as high without the main, than I can with it. This goes against everything I was always taught.


The most surprising thing about this sail, is the LOWER end of the wind range. It seems to work as well or better than the 120% yankee, right down to no wind at all, so long as the wind is well ahead of the beam.

I had a delightful sail today, hard on the wind in only 5 to 8 knots of wind, something I wouldn't have even bothered to put the sails up in with my old Dacron sails. I have a rough target speed of 2/3 true wind speed, with these new sails, when sailing upwind. I would have expected the blade to be poor in these light winds, but I was exceeding my target speeds during the entire passage. At one point I was making 5.5 knots of boat speed with 6.5 knots of true wind. Astonishing. But why? How? This is far below the designed wind range for this sail, yet I am sure that I could not achieve these speeds with the larger yankee. There must be some aerodynamic explanation, but I sure don't understand it. It doesn't make any sense to me -- my first instinct is that I must be using the 120% yankee wrong in these conditions, or the yankee is not as well cut as the blade, but more speed than this is simply not possible, in conditions like this in a cruising boat like mine (with the dinghy not even folded, but sitting fatly in the davits and making windage).

I feel a loss of power with the blade only with the wind behind the beam, and so it's really only with less than 15 knots of wind and wind behind the beam, that I want the yankee up. The blade has become my primary headsail.


If someone can explain any of these, I would be interested to hear it. I know the principle of high aspect wings and drag, but lift is still some kind of function of area, and not of leech length, as far as I know. It doesn't make any sense to me.