Wing tip vortex

I was watching a doco about the design of the Airbus A380 and it was talking about how they minimise the problem with vertical ends to prevent the wind circulating. This begged the question, why aren't sails designed with some sort of horizontal wing on top to minimise the equivalent problem. Of course it would have to be measured in as sail area but I would have thought the gain would be more substantial than the minimal loss in sail area. On the doco they said it increased efficiency by up to 7%.

I would have thought it would be easy to do with a bit of normal sail and maybe a couple of small stiff battens.

 

The "search" tool on this forum returned this:
http://www.boatdesign.net/forums/sailboats/winglets-sails-22521.html

 

Way back in the day, certain big budget boats tried shelves attached to the boom. They were called "Park Avenue" booms. The shelf constituted a winglet on the bottom end of the sail. The foot of the sail traveled on numerous sail tracks across the horizontal plank. Such a boom was clumsy and whether effective or not they were ultimatly abandoned.

 

I have seen this a few times before. Quite often there is a misunderstanding of the physics which leads to thinking that all winglets must be good. They're not.

Consider the flow situation at an airliner wing tip. Firstly, it's at the end of a high-aspect ratio, tapered wing with nice clean incident airflow. Secondly, airliners fly for long periods at (near) constant speed and lift. This means that you optimise the wing for (pretty much) one condition. The airliner wing has a nice, clean tip vortex, that can be controlled with a wingtip device.

Now consider the yacht sail. Firstly, there's the mast-head, which is a structural requirement, but rather screws up the flow. Secondly, you're sweeping the "wing" forward, so you lose the nicely controlled vortex rollup. Thirdly, even if you did change the planform, you're never operating at constant conditions, so you're looking to achieve an improvement over a significant range of conditions, which will lead you to a poor solution.

In short, if you want to improve sail efficiency, increase the aspect ratio and use a constant chord or slightly tapered (straight trailing edge) "wing". Unfortunately these are not easy to use. Hence why they are still quite specialist.

All the best,

Tim B.