[Theo Jansen] has come up with an intriguing wind-powered strandbeest which races along the beach with surprising speed and grace. According to [Jansen], it “doesn’t have hinging joints like the classical strandbeests, so they don’t get sand in their joints and you don’t have to lubricate them.” It’s called UMINAMI, which appropriately means “ocean wave” in Japanese.
There are only videos of it in action to go on so far, but a lot can be gleaned from them. To make it easier to keep track of just a single leg, we’ve slowed things down and reddened one of them in the banner animation. Those legs seem to be providing a push but the forward motion is more likely supplied by the sails. The second video below shows it being pulled along by the type of strandbeest we’re all more used to seeing.
What follows is an analysis and best guess about how it works. Or you can just enjoy its graceful undulations in the videos below.
How is it all connected together? There are two sets of horizontal beams which span the length of the strandbeest around halfway down the height. These beams are fixed in length and seem to be for constraining the overall length. There are two sets of them, dividing the wave in the middle and possibly done that way to allow the two sections to tilt sideways independently of each other
From the views shown here, it’s clear that the legs are free to slide along those horizontal beams. Meanwhile, the beam along the top is longer but is also flexible, as are smaller segments in between the legs at the bottom. The top beam, however, keeps the tops of the legs evenly spaced apart whereas the smaller bottom segments only set a maximum distance between adjacent legs, while folding up to allow adjacent legs to touch. That’s why the legs come together only at the bottom of the wave.
What makes it undulate like a wave? Here’s our best guess from the videos and without experimenting. As the bottom of a leg catches in the sandy beach, it’s pulled along at the top by the sails, causing its top to pivot forward. While the leg is caught in the sand and is pulled along, it also pulls apart the legs in front of it while compressing the legs behind it. The compression has to exist because the first and last legs appear to be constrained by the middle horizontal beams, preventing all of the legs from being spaced apart at the same time. But that’s all just based on video analysis and could be wrong. What do you think? We’d love to hear your suggestions and even to see your own undulating strandbeests.
Thanks to [Jack Buffington] for suggesting this one through our tips line.