Sailing ships are wonderfully economic and non-polluting. They have unlimited range because they use virtually no fuel, but they tend to be slow, about 5-12 knots, about half as fast as Diesel-powered ships, and they can be stranded for weeks if the wind dies. Classic sailing ships also require a lot of manpower: many skilled sailors to adjust the sails. What’s wanted is an easily manned, economical, hybrid ship: one that’s powered by Diesel when the wind is light, and by a simple sail system when the wind blows. Anton Flettner invented an easily manned sail and built two ships with it. The Barbara above used a 530 hp Diesel and got additional thrust, about an additional 500 hp worth, from three, rotating, cylindrical sails. The rotating sales produced thrust via the same, Magnus force that makes a curve ball curve. Barbara went at 9 knots without the wind, or about 12.5 knots when the wind blew. Einstein thought it one of the most brilliant ideas he’d seen.
The source of the force can be understood with help of the figure at left and the graph below. When a simple cylinder sits in the wind, with no spin, α=0, the wind force is essentially drag, and is 1/2 the wind speed squared, times the cross-sectional area of the cylinder, Dxh, and the density of air. Add to this a drag coefficient, CD, that is about 1 for a non-spinning cylinder. More explicitly, FD= CDDhρv2/2. As the figure at right shows, there is a sort-of lift in the form of sustained vibrations at zero spin, α=0. Vibrations like this are useless for propulsion, and can be damaging to the sail. In baseball, such vibrations are the reason knuckle balls fly erratically. If you spin the cylindrical mast at α=2.1, that is at a speed where the fast surface moves with the wind, at 2.1 times the wind speed, and the other side side moves to the wind, there is more force on the side moving to the wind (see figure above) and the ship can be propelled forward (or backward if you reverse the spin direction). Significantly, at α=2.1, you get 6 times as much force as the expected drag, and you no longer get vibrations. FL= CLDhρv2/2, and CL=6 at this rotation speed
At this rotation speed, α=2.1, this force will be enough to drive a ship so long as the wind is reasonably strong, 15-30 knots, and ship does not move faster than the wind. The driving force is always at right angles to the perceived wind, called the “fair wind”, and the fair wind moves towards the front as the ship speed increases. If you spin the cylinder at 3 to 4 times the wind speed, the lift coefficient increases to between 10 and 18. This drives a ship with yet force. You need somewhat more power to turn the sails, but you are also further from vibrations. Flettner considered α=3.5. optimal. Higher rotation speeds are possible, but they require more rotation power (rotation power goes as ω2, and if you go beyond α=4.3, the vibrations return. Controlling the speed is somewhat difficult but important. Flettner sails were no longer used by the 1930s when fuel became cheaper.
In the early 1980s, the famous underwater explorer, Jacques Cousteau revived the Flettner sail for his exploratory ship, the Alcyone. He used light-weight aluminum sails, and an electric motor for rotation instead of Diesel as on the Barbara. He claimed that the ship drew more than half of its power from the wind, and claimed that, because of computer control, it could sail with no crew. This latter claim was likely bragging. Even with today’s computer systems, people are needed as soon as something goes wrong. Still the energy savings were impressive enough that other ship owners took notice. In recent years, several ship-owners have put Flettner sails on cargo ships, as a right. This is not an ideal use since cargo ships tend to go fast. Still, it’s reported that, these ships get about 20% of their propulsion from wind power, not an insignificant amount.
And this gets us to the reason your curve ball does not curve: you’re not spinning it fast enough. You want the ball to spin at a higher rate than you get just by rolling the ball off your fingers. If you do this, α = 1 and you get relatively little sideways force. To get the ball to really curve, you have to snap your wrist hard aiming for α=1.5 or so. As another approach you can aim for a knuckle ball, achieved with zero rotation. At α=0, the ball will oscillate and your pitch nearly impossible to hit, or catch. Good luck.
Robert Buxbaum, March 22, 2023. There are also various Flettner airplane designs where horizontal, cylindrical “wings” rotate to provide lift, power too in some versions. The aim is high lift with short wings and a relatively low power draw. So-far, these planes are less efficient and slower than a normal helicopter.
