Round Sails

Although I am not an engineer by trade, my interest in sailing has lead me to be fascinated with fluid mechanics (without the math).  The problems of cavitation cavitation  and super-cavitation were new to me until I saw some interesting articles about developing high-speed sailboats.

We all are told that sails act like airplane wings (or AC45/72 wings) that rely on the Bernoulli effect to create lift in the direction of lower pressure produced by the air moving more quickly over the curve in the sail.  Sometimes I am able to use that effect to move my Laser.

I recently ran across an interesting technique to do the same thing in a very different way - the use of the Magnus effect which, as my erudite readers will know, is the effect "in which a spinning cylinder past which an external stream is flowing receives a lateral thrust due to the lower pressure on the side of the cylinder where the cylinder’s motion is in the same direction as the streaming flow."  In other words, a spinning cylinder has lower pressure on one side which will create a force in that direction.  

So, to harvest this phenomenon on a boat, one simply makes a vertical cylinder, sets it to spinning and it acts like a wing or sail.  These cylinders are called Flettner rotors after Anton Flettner who was the first, along with a colleague, Prandtl, to have put such (patented) cylinders on 2 ships in the 1920s. 

A German wind-turbine company, Enercon, built a ship in 2010 using Flettner rotors, along with conventional propulsion. You can check it out (in German) in this video which has a good animation of the Magnus effect.

In the 1930s Thom reported significant increase in lift by adding discs to the cylinder. However, before any of you decide to add Thom discs to the Flettner rotors on your boat, you may wish to consult the findings reported by the Turbulence Mechanics Group of the University of Manchester which "failed to confirm the much enhanced lift coefficients that he reported."