What’s A Whirlpool?

It’s bath night for my son, and we’re nearly finished. The last step is our ritual of unstopping the drain and letting him play until all of the water runs out. This is generally a time of frantic activity as he tries to pack in as much play as possible. This night, he really pays attention to the way that his toys are caught by the current and begin spinning above the drain. He grabs one, pulls it away, and watches as it drifts back and starts spinning again.

“It’s like a whirlpool,” I observe.

“Yeah!” he agrees. And then, without missing a beat, adds “What’s a whirlpool?”

Uhm… uh… You know, I don’t actually know what a whirlpool is. Not really. Oh, sure I know what they look like – I have, ever since I watched 20,000 Leagues Under the Sea. But I don’t actually know what they are. All I can say is that I’m pretty sure they’re not places where water is draining out of the ocean through cracks in the sea floor.

Pretty sure.

Wikipedia defines a whirlpool as “a body of swirling water produced by the meeting of opposing currents”. The article goes on to note that powerful whirlpools are often known as maelstroms, and that any whirlpool with a downdraft is properly known as a vortex. Also, “the most powerful whirlpools are created in narrow, shallow straits with fast flowing water.”

The Times of India provides a paragraph-long explanation that, while generally agreeing, goes into more detail:

A whiirlpool is a large, swirling body of water produced by ocean tides. When flowing water hits any kind of barrier, it twists away and spins around rapidly with great force. This creates a whirlpool. Whirlpools can occur in a small area where a piece of land juts out into a river, causing the water to swirl around. They can also occur in the middle of the ocean when one current meets an opposing current, as when an incoming tide hits the ebb current of the last tide. Strong winds can also whip up the water into whirlpools.

Basically – for whatever value ‘basically’ has when dealing with a complex problem in physics – a vortex is a mass of fluid rotating on an axis and flowing towards the center. The speed of rotation is fastest at the center, and slows down as you move away thanks to the conservation of angular momentum. Whirlpools are an expression of vortexes in water, just like hurricanes and tornadoes are expressions of vortexes in the air. But what actually causes them? Well, here’s my layman’s understanding of the process.

You start with a moving mass of fluid (which can be water or air, because both are considered fluids for this purpose), which is better known as a current (in the water) or wind (in the air). The moving mass is not a single solid object, and so it will deflect if it hits an obstacle – whether solid object or another moving mass. If the deflecting obstacle is moving as well, or if the deflecting object is curved, it will cause the deflected mass to pick up a little spin. This spin is then exaggerated by the fact that the rest of the moving mass is still coming, forcing it to spin more. Thanks to the conservation of angular momentum, this forces the spinning section to pick up speed as the curve becomes tighter. A vortex is formed.

So, are whirlpools dangerous? The answer is an unequivocal it depends. Like all currents, their danger depends on how powerful they are and how experienced you are. You should always respect them and take them seriously, but you don’t need to fear them. Most of them.

Well, go ahead and fear that one. It’s the Saltstraumen Maelstrom, off the coast of Norway, and has the strongest tidal currents in the world – up 20 knots (about 23 mph, or 37 kph).  It can kill you.