Posted by Janne Käpylehto on July 5th, 2011
I’m currently working with radar measurement data from a friendly Finnish institute and I’m teaching our software to cope with all the oddities that comes from real-life measurements. Also, the sailing season has begun and I’m reading sailing blogs and equipment tests. How are these related? Well, this is about a pet peeve of mine: Bad sailing science.
A lot of people write about aerodynamics on the world wide web, and a surprising number of people write things that are wrong. Very wrong indeed. The situation is so bad that NASA itself, the respectable space agency that should know a bit about the subject, has set up a website not only to educate about the subject, but to openly mock the people who get it wrong. This is not sailing but flying, but a sail and a wing are very much the same thing. You can have a look here.
Not that this helps much. Some sites manage to link to the NASA site, quote from it, and then get it wrong. http://blog.everydayscientist.com/wp-content/uploads/physics-sailing.pdf says ‘hi’ to NASA and claims that high wind speeds cause low pressure.
An example. Here is an explanation taken from a textbook about sailing, about how the wind propels a sailboat. I happen to have it on my shelf (name of author withheld to protect the guilty) and there are several web sites who claim the same.
“A sail, when viewed from above, is curved. The wind has to travel a longer path around the ‘outer’ side of the sail and a shorter path past the ‘inner’ side of the sail. Because the air has to meet again behind the sail, the wind on the ‘outer’ side has to travel faster than the ‘inner’ side. Due to the Bernoulli principle the higher speed causes a lower pressure on the ‘outer’ side compared to the ‘inner’ side and this pressure difference pushes the boat forward.”
This is the first sentence of the explanation, and the general discussion builds on this. Trouble is, this is nonsense. Even more trouble is, a bunch of schoolchildren can prove that this is wrong. How?
Well, this is basic scientific procedure. The sentence about the air having to meet behind the sail is just a claim, a theory. But the theory can be verified easily. All you need is a sail. A sail of an optimist or a 420 children’s boat will do, you raise the sail on a sailing boat while it is on land. You turn the boat so you have some pressure on the sail. Now you take something that can float in air and let it float past the sail, on both the inner and outer side at the same time. A china cracker will do, as it makes smoke. Similarly confetti, saw dust, cherry blossom. Just hold them before the mast and let go.
And what do we learn? Well, the two halves of dust or smoke don’t meet behind the sail at all. The dust that travels around the ‘outer’ path wins the race easily, even though it travels along the longer path.
The claim that the wind passed the sail on the left and on the right synchronously and then meets up behind the sail is wrong. It doesn’t happen. Unfortunately this brings down the entire explanation of the textbook like a house of cards. And I have to wonder, if it is so easy to make this experiment, how could people who write textbooks on the theory of sailing avoid testing the claim they use as a foundation for an entire chapter?
This is why we measure during development. Because deep inside we know that our best theory about how mathematics can solve a problem could be very wrong. And no, I will not attempt to explain to you why the outside lane dust wins the race. I’ve seen university professors making themselves look like donkeys on this matter. I have a theory, but I’m not going to tell you. Maybe you figure it out with a pencil and paper? Please be careful when consulting web sites.
Chief Software Architect, Radar technology
PS: Before the internet mocks me for not getting it, I am fully aware that the textbook explanation quoted above has another glaring fault: Claiming that Bernoulli says that high speed ‘causes’ low pressure. Bernoulli’s principle doesn’t mention cause and effect, wisely so.