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Rotating sail ships and why your curve ball doesn’t curve.

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The Flettner-sail ship, Barbara, 1926.

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.

Force diagram of Flettner rotor (Lele & Rao, 2017)

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. Multiply this by a drag coefficient, CD, that is about 1 for a non-spinning cylinder, and about 2 for a fast spinning cylinder. FD= CDDhρv2/2.

A spinning cylinder has lift force too. FL= CLDhρv2/2.

Numerical lift coefficients versus time, seconds for different ratios of surface speed to wind speed, a. (Mittal & Kumar 2003), Journal of Fluid Mechanics.

As graphed in the figure at right, CL is effectively zero with sustained vibrations at zero spin, α=0. Vibrations are useless for propulsion, and can be damaging to the sail, though they are helpful in baseball pitching, producing the erratic flight of knuckle balls. If you spin a cylindrical mast at more than α=2.1 the vibrations disappear, and you get significant lift, CL= 6. At this rotation speed the fast surface moves with the wind at 2.1 times the wind speed. That is it moves significantly faster than the wind. The other side of the rotor moves opposite the wind, 1.1 times as fast as the wind. The coefficient of lift lift, CL= 6, is more than twice that found with a typical, triangular, non-rotating sail. Rotation increases the drag too, but not as much. The lift is about 4 times the drag, far better than in a typical sail. Another plus is that the ship can be propelled forward or backward -just reverse the spin direction. This is very good for close-in sailing.

The sail lift, and lift to drag ratio, increases with rotation speed reaching very values of 10 to 18 at α values of 3 to 4. Flettner considered α=3.5. optimal. At this α you get far more thrust than with a normal sail, and you can go faster than the wind, and far closer to the wind than with any normal sail. You don’t want α values above 4.2 because you start seeing vibrations again. Also more rotation power is needed (rotation power goes as ω2); unless the wind is strong, you might as well use a normal propeller.

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. Controlling the rotation speed is somewhat difficult but important. Flettner sails were no longer used by the 1930s because fuel became cheaper and control was difficult. Normal sails weren’t being used either for the same reasons.

In the early 1980s, there was a return to the romantic. Famous underwater explorer, Jacques Cousteau, revived a version of the Flettner sail for his exploratory ship, the Alcyone. He used aluminum sails, and an electric motor for rotation. 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 claim was likely bragging, but he bragged a lot. Even with today’s computer systems, people are needed to steer and manage things in case something goes wrong. The energy savings were impressive, though, enough so that some have begun to put Flettner sails on cargo ships, as a right. This is an ideal use since cargo ships go about as fast as a typical wind, 10- 20 knots. It’s reported that, Flettner- powered cargo 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: it’s likely you’re not spinning it fast enough. To get a good curve, you want the ball to spin at α =3, or about 1.5 times the rate you’d get by rolling the ball off your fingers. You have to snap your wrist hard to get it to spin this fast. As another approach, you can aim for α=0, a knuckle ball, achieved with zero rotation. At α=0, the ball will oscillate. It’s hard to do, but your pitch will be nearly impossible to hit or catch. Good luck.

Robert Buxbaum, March 22, 2023. There are also Flettner airplane designs where horizontal, cylindrical “wings” rotate to provide high lift with short wings and a relatively low power draw. So-far, these planes are less efficient and slower than a normal helicopter. The idea could bear more development work, IMHO. Einstein had an eye for good ideas.

Three identical strangers, and the genetics of personality

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Inheritability of traits is one of the greatest of insights; it’s so significant and apparent, that one who does not accept it may safely be called a dullard. Personal variation exists, but most everyone accepts that if your parents are tall, you are likely to be tall; If they are dark, you too will likely be dark, etc., but when it comes to intelligence, or proclivities, or psychological leanings, it is more than a little impolite to acknowledge that genetics holds sway. This unwillingness is glaringly apparent in the voice-over narration of a popular movie about three identical triplets who were raised separately without knowing of one another. The movie is “Three identical strangers”, and it recounts their meeting, and their life afterwards.

Triplets, raised separately, came out near identical.

As one might expect, given my introduction, though raised separately, the three showed near identical intelligence, and near identical proclivities: two of them picked the same out-of-the way college. All of them liked the same sort of clothes and had the same taste in women. There were differences as well: one was a more outgoing, one was depressed, but in many ways, they were identical. Meanwhile, the voice-over kept saying things like, “isn’t it a shame that we never saw any results on nature/nurture from this study.” Let me clear this us: genetics applies to psychology too. It’s not all genetics, but it is at least as influential as upbringing/ nurture.

This movie also included pairs of identical twins, raised separately, they also showed strong personality similarities. It’s a finding that is well replicated in broader studies involving siblings raised separately, and unrelated adoptees raised together. Blood, it seems, is stronger than nurture. See for example the research survey paper, “Genetic Influence on Human Psychological Traits” Journal of the American Psychological Society 13-4, pp 148-151 (2004). A table from that paper appears below. Genetics plays a fairly strong role in all personal traits including intelligence, personality, self-control, mental illness, criminality, political views (even mobile phone use). The role is age-dependent, though so that intelligence (test determined) is strongly environment-dependent in 5 year olds, almost entirely genetic in 25-50 year olds. One area that is not strongly genetic, it seems, is religion.

In a sense, the only thing surprising about this result is that anyone is surprised. Genetics is accepted as crucial for all things physical, so why not mental and social. As an example of the genetic influence on sports, consider Jewish chess genius, Lazlo Polgar: he decided to prove that anyone could be great at chess, and decided to train his three daughters: he got two grand masters and an international master. By comparison, there are only 2 chess grand masters in all of Finland. Then consider that there are five all-star, baseball players named Alou, all from the same household, including the three brothers below. The household has seven pro baseball players in all.

Most people are uncomfortable with such evidence of genetic proclivity. The movie has been called “deeply disturbing” as any evidence of proclivity contradicts the promise of education: that all men are equal, blank slates at birth that can be fashioned into whatever you want through education. What we claim we want is leaders — lots of them, and we expect that education will produce equal ratios of woman and men, black and white and Hispanic, etc. and we expect to be able to get there without testing for skills, — especially without blind testing. I notice that the great universities have moved to have testing optional, instead relying on interviews and related measures of leadership. I think this is nonsense, but then I don’t run Harvard. As a professor, I’ve found that some kids have an aptitude and a burning interest, and others do not. You can tell a lot by testing, but the folks who run the universities disagree.

The All star Alou brothers share an outfield.

University heads claim that blind testing is racist. They find that some races score poorly on spacial sense, for example, or vocabulary suggesting that the tests are to blame. There is some truth to these concerns, but I find that the lack of blind testing is more racist. Once the test is eliminated, academia finds a way to elevate their friends, and the progeny of the powerful.

The variety of proclivities plays into an observation that you can be super intelligent in one area, and super stupid in others. That was the humor of some TV shows: “Big Bang Theory” and “Fraser”. That was also the tragedy of Bobby Fischer. He was brilliant in chess (and the child of brilliant parents), but was a blithering idiot in all other areas of life. Finland should not feel bad about their lack of great chess players. The country has produced two phone companies, two strong operating systems, and the all time top sniper.

Robert Buxbaum, May 15, 2022