Just when you think you've seen all the gorgeous helices in the world, here comes another one. This one trails behind propellers underwater. What causes this helical structure to form? And why is it a problem?
This beautiful structure is probably unfamiliar to you unless you're a manatee or a mermaid. These helices trail behind the propellers of boats as they move through the water. Similar structures of bubbles crowd around underwater pumps - although those bubbles aren't arranged as prettily as these. What causes the bubbles to appear in the first place? Cavitation.
As water slips around the propellers of a boat, there are places where it experiences sudden, extreme loss of pressure. The propeller is making a "hole" in the water, and the more powerful that propeller is the more water it clears away. Water appears - for the most part - as a liquid on Earth because the atmosphere on Earth keeps it under pressure. Drop the pressure and the individual molecules of water come apart. The water boils, even at very low temperatures. When propellers lower the pressure on certain areas of the water, the water boils, and we see these bubbles.
Cavitation has proved to be something of a curse to engineers. The bubbles have a short life, and when they snap closed, they can create powerful shockwaves. These can disrupt the flow of the water near the propeller, and can even damage the metal itself. Power has a price.