MAX EBB — S
outhern California waters are a welcome change from the rainy season in San Francisco Bay: The seabreeze is warm, the palm trees seem to belong, and best of all, there's a steady parade of millennials on stand-up paddleboards showing off their physiques. I could not help noticing two particularly shapely young women on an outrigger canoe gliding up the marina fairway, when I suddenly realized that I knew one of them. She must have recognized my boat at about the same time.
Asymmetrical collapse of a cavitation bubble near a solid boundary, showing the formation of the reentrant water jet. Solid lines are theory, dotted lines are measured. From "Cavitation and Bubble Dynamics," C. Brennen, Oxford University Press 1995.
"Lee!" I hailed as the outrigger executed a three-point course reversal so they could pull up to my boat starboardside-to without the ama getting in the way. It was Lee Helm, naval architecture grad student from the Bay Area, and her friend. I hardly ever see her when she's not in foulies or a wetsuit. "Never thought I'd run into you in this neck of the woods," I said. "We're like, here for the big outrigger canoe race," she explained. "Tomorrow we'll be racing an OC-6 to Catalina, and the paddling club was okay with us taking out one of their small boats after our team practice." Lee introduced the other woman on the boat, a grad student in marine biology. "Please come aboard and watch the sunset with me over a drink," I said as I tossed them the mainsheet tail to use as a temporary mooring line. But the outrigger canoe had no cleats. "You can just pass this around one of the crossbeams Page 84 •
Latitude 38
• March, 2019
and then back to me," I suggested. "It's called the Iako," Lee's friend corrected me. "It's a common mistake." Once the boat was secured, my two guests carefully stood up on their boat and climbed aboard mine. They followed me into the cabin, but had other ideas for the sunset beverage. "Bananas and mangos," observed Lee after inspecting the galley shelves. "We'll make smoothies, if that's cool with you." I broke out the 12-volt blender while Lee and her colleague sliced up fruit. "Listen to those little crabs," I said. "The clicking of those crab claws on my hull always reminds me that I'm tied up in Sothern California waters. I hope they're keeping my hull clean." "Actually they're snapping shrimp," said the marine biologist. "It's a common mistake. They're probably Cragnon synalpheus, one species of pistol shrimp. And it's not the sound of the claws snapping shut that makes the noise; that's a common misconception. It's the collapse of the cavitation bubble they produce. The claw locks open, big muscles stretch, and when the cocked claw is released it rams a piston-like structure into a cylinder-like cavity that sends a thin jet of water out at speeds on the order of 30 meters per second. This forms an asymmetrical cavitation bubble, and when the bubble collapses it makes a pop that's just about the loudest sound in the ocean — it's been measured at 218 decibels." "218 decibels! Good thing it's underwater," I said. "But what makes it so loud?" "A leaf blower is only 100 decibels," added Lee. "A rock concert is 120, a jet engine at 25 meters is 150. Even a shotgun blast is quieter than a pistol shrimp, at 170 decibels. And it's a log scale. Every time decibels go up another ten points, the loudness, like, doubles." "Anything above 85 is considered harmful to humans," said the marine biologist. 185 is impressive for a crustacean that's only five centimeters long." Lee tried to explain why cavitation is so noisy. "Cavitation 101," she began. "You know how they say 'you can't push on a rope?' This is like, you can't pull on water." The biologist held up a drinking straw that she'd noticed on a galley shelf and waved it at Lee. "Okay," Lee responded, "but it only seems like you can suck water. Remember the water is being pushed on all sides by air pressure. You can reduce the pressure on one side by sucking, so the pressure on the other side pushes it toward the sucking. The water is, like,
still being pushed, not pulled. Doesn't work in a vacuum. But then, if you suck hard enough to bring the pressure down to zero, or if, like, water is flowing around the back of a curved propeller blade so fast that the centrifugal force pulling the water away from the blade would make the pressure go below zero, then a vacuum cavity forms." "Actually it's not really a total vacuum," the biologist pointed out. "That's a common mistake." "For sure," said Lee, "because there's some water vapor and maybe some dissolved gas released, but the vapor pressure of sea water is very small — about one fiftieth of an atmosphere — so we'll ignore it for now. Point is, when a pump sucks too hard, or when you try to make a siphon go above 32 feet, or when fastmoving water goes around a sharp curve and centrifugal force tries to pull it apart — then you get a vacuum cavity." "Vapor pressure of sea water at 20 degrees C," said the biologist after a quick consult with her cellphone, "is 0.0226 atmospheres." "And it's the cavitation that makes all that noise?" I asked. "The noise is when the cavitation bubble collapses," Lee continued. "It's a singularity. Consider a vacuum bubble caused by cavitation. Assume it's, like, spherical, for now. When the pressure of the water around the bubble is very low, the bubble is stable, held in the spherical shape by surface tension. When the water pressure comes back up, the vacuum bubble starts to collapse. But as it collapses, the surface area of the bubble decreases by the square of the radius, because surface area is proportional to size squared. So the water has to move faster by the inverse square of the bubble size, because the same volume of water now has that much less surface area to move through. There are two physical rules for all this: Momentum is conserved, and, what goes in equals what comes out. When there's a vanishingly smaller area for the water collapsing into the bubble to move through, and when the size of the bubble approaches zero, then the speed of the inrushing water approaches infinity and the dynamic pressure when the bubble stops collapsing approaches infinity. Like, not really because there's viscosity and a little bit of gas in the bubble, but you get the idea. The bubble collapses with a bang." I thought for a moment about the implications of infinite velocity and infinite pressure on underwater objects subjected to cavitation. "And that explains how mere water can cause so much cavitation
