Nicely explained courtesy of ****yeahfluiddynamics.tumblr.com
"If you watch some of the sailing in Rio, you may hear commentators mention sailors being penalized for breaking Rule
42. Broadly speaking, Rule
42 says that sailors canít use their body to propel the boat. While it seems like a little rocking couldnít make much difference, it turns out events
have these rules for good reason.
One way to break Rule 42 is to perform sail flicking, demonstrated in the animation above. The sailor uses his or her body weight to roll the boat slightly, which causes the sail to flick. Aerodynamically speaking, weíd call this motion heaving. On the flexible sail, this unsteady motion decreases drag, allowing the boat to go faster. Done with the right frequency and amplitude, sail flicking actually makes the sailís drag become negative, thereby creating thrust!
The bottom image shows a visualization of the wake of a normal sail (left) and a sail being flicked (right). Both sails
shed vortices in the downstream direction, but the flicked sail has much stronger vortices, indicated by the darker colors. In addition to giving a sailor an illegal boost, sail flicking creates more difficult, turbulent conditions for any competitors downstream, so itís restricted in many (but not all) sailing events
. (Image credits: AP Photos; Reuters; National Solo, source; research
and flow diagram credit: R. Schutt and C. Williamson, pdf)"
Tagged "sail flicking" | **** Yeah Fluid Dynamics