Ship Wakes are Made of Mini-Implosions

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If you've ever been on a boat or watched a James Bond movie, you'll be familiar with the sight of a trail of white bubbles and froth trailing along behind a boat. These trails of bubbles are part of the ship's wake. I always assumed that these bubbles were caused by the propeller entering the water from the air. After all, when cannonballs or divers hit the water, a cavity of air forms behind them. This cavity generally doesn't last long enough for our eyes to detect it, although it's easy to see the bubbles that are left behind as the cavity collapses. The following super-awesome slow motion video shows the cavities forming behind spheres as they hit water.

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Of course, most of my readers are living in the 21^st century, where we use submerged propellers instead of paddlewheels.

.You've probably already figured out the flaw in my logic; propellers are completely under the water, so the bubbles can't be coming from the surface.

So where do the bubbles come from?

When a propeller spins quickly enough, the pressure on the water near the trailing edge of the propeller blade gets so low that the water vaporizes, forming a bubble of air. If you remember, lower pressure leads to lower boiling points, which is why water boils on a lower temperature on everest than it does at sea level. As the propeller blade moves away from the bubble, the pressure of the water takes back over, and the bubble implodes back into itself. This implosion is called cavitation.

In this video, bubbles form in a jar as atmospheric pressure is decreased, and then implode as the pressure is increased again.

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Here, we can see cavitation in action. This video uses a strobe light to slow the motion down, and you can see the cavitation bubbles forming on the back of the propeller blades. The faster the propeller goes, the more bubbles are formed.

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These cavitation bubbles have effects in addition to forming a bubble trail behind the ship. First of all, each mini implosion makes a loud noise. The noise of a cavitation bubble collapsing is so loud that underwater animals like the snapping shrimp and the mantis shrimp use them to communicate and to stun prey. The cavitation implosions are also the main source of underwater noise from ships. With increased number of ships in the ocean every year, noise from ships is becoming more and more of a concern for marine organisms that depend on noise to communicate and find food. These imploding bubbles also aren't very good for ships, since each tiny implosion causes a little bit of structural damage.

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Added together, tiny bubbles can eat holes in steel. In the end, it turns out that the bubble-obsessed fish from Finding Nemo wasn't crazy after all. He was just planning on using bubbles to destroy his enemies.