Why Did 19th Century Sailors Dump Oil In The Sea While Sailing?

Sailors in the 19th century dumped oil in the sea to calm the waves around their ship, and it genuinely worked. A spoonful of oil spreads into an ultra-thin film that damps the tiny wind-driven ripples the wind uses to grip the water. With nothing to grab, the wind can no longer build the surface into steep, breaking waves, so the sea near the ship turns slick and smooth.

This might come as a shock, especially if you’re an environment enthusiast, but in the late 19th century, sailors used to dump oil in the sea on purpose! Sailors used to stockpile different kinds of oil (vegetable oil, mineral oil and even sperm whale oil) just so that they could dump it in water.

It may seem like an incredibly pointless exercise, but they did have a ‘good’ reason to spill those valuable oils in water. In fact, they knew that oil could help calm violent waves in the ocean.

Their trick worked, and in fact, it still does! There is an entire book on it, The Use of Oil to Lessen the Dangerous Effect of Heavy Seas, published in 1886 by George L. Dyer of the U.S. Hydrographic Office. It catalogs, captain by captain, which kinds of oil were poured where to tame which kind of storm.

Technology, back in the day, was not nearly as advanced as it is now, so ships back then were not as sturdy as the ones used these days to maneuver through choppy waters. This seemingly strange ‘oil-spill’ technique helped them to calm waves and somewhat “master” storms.

How Does Oil Help To Calm Ocean Waves? – Immiscible Liquids

The reason is one you’ve surely heard of before: the mutual hatred between water and oil. They are immiscible liquids; in other words, they don’t mix into each other under ordinary conditions. If you put these liquids in a container and shake them violently, they may seem to mix at first, but will separate all by themselves after some time.

This mutual hatred between oil and water results in this fantastic application for calming violent waves.

Under normal conditions, i.e., in the absence of spilled oil, wind first roughens a calm sea into a fine pattern of tiny ripples. Those little ripples give the wind something to push against, and once it has that grip, it keeps feeding energy into the surface, building the ripples up into the steep, breaking waves that make a storm so dangerous.

However, when oil is spilled on water, it spreads out into an astonishingly thin film, often just one molecule thick. That film stiffens the surface and smooths out exactly those tiny wind-driven ripples the wind relies on to get a grip. With the ripples damped, the wind slides over the water instead of grabbing it, so it can no longer pump energy in to build up large, breaking waves. In effect, the oil acts as a shield that keeps the wind from working on the sea. A small quantity of oil dropped overboard by sailors would calm the waves around their ship to a remarkable degree.

So why does such a vanishingly thin layer do so much? You see, as a ripple passes, it stretches and squeezes the oil film along the surface. The film resists being stretched, and that tug-of-war drags on the water just beneath it and quietly bleeds energy out of the ripple. Scientists call this Marangoni damping, and it is why a film you could barely measure can flatten the small waves that feed a storm.

The oil film helps in a second way too. In a gale, the sea throws up spray and breaking spume that roughen the surface even further and give the wind extra purchase. By holding the surface together and stopping waves from breaking, the film cuts down on that flying spray, taking away yet another way for the storm to whip up the sea.

Believe it or not, spilling even a small amount of oil, often less than a gallon (about 4 liters) per hour, could calm the broken water around the ship to a great extent. Not just any oil would do, though. Fish and vegetable oils worked far better than mineral oils, because their molecules spread out into an even film instead of clumping together. There was one important limit: the trick tamed the steep, breaking waves whipped up by the local wind, but it could not iron out the large swells rolling in from a distant storm. Sailors used it to keep their own patch of sea slick enough to ride out the worst of it.

Benjamin Franklin And The Pond Of Glass

If this all sounds like a sailor’s tall tale, consider that one of history’s sharpest minds put it to the test. Benjamin Franklin first grew curious about it in 1757, when, on a voyage to England, he watched the wakes of two ships stay oddly smooth and was told it was the greasy water the cooks had tipped overboard.

He could not let the idea go. Years later, in 1769, he tried it himself on a wind-ruffled pond at Clapham Common in London. He dribbled barely a teaspoon of oil onto the water, and, as he later wrote, it spread until perhaps half an acre of the pond was “as smooth as a looking-glass.” Franklin described the whole affair in a letter to his friend William Brownrigg on 7 November 1773, and the account was read to the Royal Society and printed in its Philosophical Transactions in 1774 under the wonderfully blunt title Of the Stilling of Waves by Means of Oil.

Franklin never worked out just how thin his film was, but the experiment quietly handed science a tool. More than a century later, Lord Rayleigh and Agnes Pockels used the very same teaspoon-on-water trick to measure the size of a single molecule, pegging the film at roughly a nanometer thick. A sailor’s storm remedy had become one of the first windows onto the molecular world.