Into the Maelstrom (or Not)
Today we're going to go in search of a real live sea monster, one that has graced the pages of fiction and nonfiction alike. The legendary Maelstrom is a whirlpool that eats men and ships alike; anything that gets caught up in its great funnel is sucked to the depths and crushed forever, never to return to the surface or the light. Does such a mighty beast actually exist somewhere in the world?
The word maelstrom comes from the Dutch for "grinding current", and has come to refer to any current-induced whirlpool. There are a lot of them throughout the world, including famous ones that reappear with each tide. Maelstroms first became infamous among the public at large when Edgar Allen Poe wrote his fictional 1841 short story A Descent into the Maelström, in which an ancient mariner recounts his tale of being aboard a ship that was sucked down to its doom:
The story referenced an actual maelstrom in Norway called the Moskstraumen, an area between some islands off the coast where the incoming and outgoing tides collide. Herman Melville referenced the same maelstrom ten years later in Moby-Dick. At its height, the Moskstraumen comprises an area 8 kilometers wide that's peppered with maelstroms up to 50 meters across. Today, tourist boats and airplanes go out there daily to show off the spectacle.
Tourists? In the Maelstrom? How could that be? Why don't they get sucked down?
As you can probably surmise, Poe grossly exaggerated the Moskstraumen. It is not, as the old mariner described, a single near-vertical funnel "hundreds of feet" across. A maelstrom that large couldn't have enough energy to hold back enough water to form much of a funnel. Let's take a quick look at the science of maelstroms. First of all, we're not going to discuss eddies. An eddy is a current flowing around in a circle. It has no vortex in the center, and it's really just a conventional current where all the water is flowing around at the same speed. Many of the maelstroms you might think you see are just eddies. For example, there was a famous eddy about a half a kilometer across in a Japanese harbor following the 2011 tsunami — it was NOT a maelstrom. For our purposes today, eddies are not the type of whirlpool we're going to discuss.
There are two very different types of whirlpools that are commonly seen in water, and it's important to differentiate between the two. The most common is a suction vortex, seen when you pull the plug in a bathtub or sink. All the water moves toward the drain, and when it collides in the middle, its momentum is converted to angular momentum and a vortex forms. A vortex is nature's way for an agitated fluid to seek its lowest energy state.
We've discussed the physics of vortices before on Skeptoid, but we didn't discuss whether they actually suck you down. They don't, really. This is a huge misconception. The hole in the center of a whirlpool is not the result of water moving downward, but of water moving outward. As it rushes around the vortex going faster at the center, its momentum tries to carry it in a straight line, away from the center. The centrifugal effect pushes the water away from the center of the whirlpool. At the surface, where water pressure is least, its possible for a hole to form. But deeper, as increasing water pressure imposes a higher centripetal force to close that central hole, the hole gets smaller, so we end up with a funnel shape. There is no downward current.
This is better illustrated in the second type of whirlpool, which is the type of the Maelstrom of legend. Rather than the agitating force being an outward flow through a drain at the bottom, the agitating force is two opposing currents colliding. This is a constant input of energy, which the water must disspel. A vortex is the most efficient way for this to happen. Most maelstroms around the world are of this type, caused by river currents or tidal currents. Depending on a number of factors, such as the size of the body of water and the speed of the differential, the vortices might be small or large, many or few. A ship pulling away from a dock might create a lot of little fast vortices, while a tide churning through a large inlet might create one huge slow one.
But in none of these whirlpools is there a downward current at the center; the physics of where the water goes just don't work that way. If you were swimming on the surface near one, you would go around and around, but you wouldn't be drawn toward the center. If you swam into the center, you would descend along with the surface to the depth of the funnel, but once at the bottom you would just be spinning like a top. There is nothing pulling you down.
There are two effects that you've probably observed that don't seem to square with this explanation. First is when you stir a bucket of water that has some sand in it. Notice that all the sand at the bottom collects in the center. You might ask how could that be, if water does not flow toward the center of a vortex? When you stir a bucket, you're not creating a vortex. In a vortex, water moves slowest at the outside and fastest at the center; when you stir a bucket, the opposite is true. You're creating a uniformly rotating mass, like a record on a turntable. Fast moving water at the outside, where you're stirring, creates turbulence where it moves against the bottom of the bucket, kicking up sand. The sand bounces around in the turbulence and settles at the center, where current and turbulence are the least.
The second effect is seen when you stir a cup of tea, all the leaves go toward the center, which suggests that a current is pulling them there. Wrong; there is no current toward the center, which is why the swimmer doesn't get sucked in. Pressure is lowest in the center of the vortex — the "eye of the storm", as it were. It's also lowest at the center of a uniformly rotating mass, like the stirred bucket. Anything lighter than water floats toward the center; squeezed out of the high-pressure areas toward the low-pressure center. A lucky few divers have seen threadlike vortices caused by currents stretching all the way from the sea floor to the surface, their visible filaments made of tiny bubbles and other lightweight debris from the water.
Poe got another technical detail wrong, and that's the way the ship traveled bow-first all the way around the funnel, as if it were sailing forward at high speed. Because fluid in a vortex moves faster at the middle, a boat pointing north would point north all the way around. The physics of a vortex are such that any rigid object placed in it would be held in its same orientation at every point, regardless of how far it is from the center. If Poe had known that, he could have sent his ancient mariner on an even more terrifying and sickening tumble into the abyss.
Maelstroms are indeed dangerous to boats, however, but not because of the threat of being sucked underwater. In the most violent maelstroms, currents are strong enough and rotate at a sharp enough angle that small boats can be capsized. Water is pretty dense and heavy, and exerting that much pressure from different angles on a small boat can easily manhandle the hull in just about any direction. Larger boats can be knocked aside by the sideways current, but maelstroms wide enough to encompass a larger boat are lower energy than the small, sharp ones; so in practice, a maelstrom is not likely to give a large boat much more than a thrill ride. Water has so much mass that the gravity easily overcomes any centrifugal effect in a large maelstrom. The square-cube law tells us that as the dimensions of the maelstrom double, the mass cubes; thus, funnels are biggest in smaller whirlpools, and small or nonexistent in the largest.
Famous maelstroms are found all over the world. The most violent is the Saltstraumen, also in Norway, where the tide rushes through a narrow inlet. However the real danger here is from turbulence caused by the rocks, creating boiling currents that flow up, down, and all around; but the maelstroms themselves are scarely more than a few meters across at their best. Scotland, Canada, Japan, and New Zealand all have their share of famous tidal maelstroms, and none approach the type of monstrosity described by Poe, nor should we expect them to.
Let the myth that whirlpools suck things underwater be put to rest. It seems to make sense, and the shape of the funnel makes the suction appear to be obvious; but the science behind the Maelstrom shows that it just ain't so. But please don't email me that the Mythbusters tried swimming in one and found the claim to be plausible — they stirred a giant bucket, creating a uniformly rotating mass, not a vortex; and Adam never tried letting go of the tether. If he had, he would have been stuck spinning in the middle at the lowest point and would have struggled, but there simply wasn't any downward current that could have sucked him to the bottom. So swim with a bit more confidence.
Correction: An earlier version of this incorrectly used the terms centrifugal and centripetal. –BD
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