It might seem like one of those legends that have been handed down from one seaside inn to another, but it is a true story: for centuries the sailors poured oil into the stormy seas to calm the fury of the waves.
And the surprising thing is that this method (called “storm oil“) really worked, thanks to the physics of fluids: poured onto the surface of the stormy sea, the oil creates a very thin film that prevents the wind from gripping the water, reducing the formation of breaking waves, the most dangerous for ships. From Benjamin Franklin’s eighteenth-century experiments with a teaspoon of oil in a pond to modern hydrodynamic laboratories, science has then understood how this veil of molecules alters the surface tension and “defuses” the tiny ripples from which the waves they gain strength.
Pouring oil on stormy seas: a very ancient method
The practice of throwing oil into rough seas is very old.
The Roman naturalist Pliny the Elder mentioned it in his writings in the 1st century AD, telling how divers released a little oil from their mouths to smooth the surface of the water and let more light reach the bottom. The reason is simple: a rippled surface scatters and refracts light in multiple directions, while a smooth surface makes it easier to see what’s underneath.
The practice has continued into more modern times, however, as the episode documented by the Grecian ship attests. On November 15, 1883, the Grecian ended up in the clutches of the stormy sea, and the waves were so frightening that the ten men of the crew had already given up for dead. Not even the nearby ship, the Martha Cobb, could help her due to the very high and rough waves.
That’s when the captain of the Martha Cobb, Thomas Greenbank, spilled about 19 gallons of fish oil overboard. Within about twenty minutes, although the waves were still gigantic, the crests seemed to have disappeared in the area around the two boats. This was how the Martha Cobb managed to launch a lifeboat into the sea and save the Grecian’s entire crew.
Just over a century earlier, however, a very famous scientist and politician had become aware of this method, and had spoken about it for the first time in the Philosophical Transactions (published in 1774): Benjamin Franklin.
In fact, in 1757, while crossing the Atlantic with a fleet of sailing ships, the scientist noticed that the water behind some ships appeared strangely calm, even if the surrounding sea was choppy by the wind. When he asked for explanations, he was told that fatty waste thrown into the sea could have that effect.
Initially skeptical, he later decided to conduct an experiment. So, one day, he went to a pond whose surface was agitated by the wind, and poured a little olive oil on the water.
Initially the wind pushed the oil towards the shore, and nothing else happened. Franklin didn’t give up: he moved to the other side of the pond, where the wind was blowing from, and tried again.
Then, it worked: a small amount of oil leveled an area of about 2,000 square meters, and he was sure that the method was not a sailor’s legend.
How does a small amount of oil “calm” the waves
It may seem like a paradox that even just a small amount of oily substance can have an effect on waves that violently hit a ship during a storm.
But the fact is that the oil does not fight the brute force of the sea, but rather “hits” the mechanism that creates and grows the waves: the way in which the wind transfers its energy to the water.
Before a giant wave forms, the wind begins to blow across the flat surface of the water, creating tiny wrinkles called capillary waves.
These ripples, a few millimeters high, are very important, because they transform the smooth surface of the sea into a sort of sandpaper. And it is precisely by clinging to these small roughnesses that the wind manages to push efficiently, transferring its energy to the water and swelling increasingly higher waves.
If we remove the wrinkles, however, we remove the wind’s grip, and that is precisely what the oil does (fish oil was used a lot at the time, but the same goes for vegetable oils).
Being surfactant and insoluble substances, the drops of oil do not sink or mix, but spread out visibly, creating an elastic and invisible film on the water.
When the motion of the waves compresses and expands this molecular layer, it creates local differences in the cohesive force of the water (surface tension). In physics, this phenomenon is called Marangoni effect: put simply, microscopic forces are generated that continuously counteract the movement, dissipating the energy and “stretching” the small ripples (as seen in the video below).
In this way, three results are obtained:
- The surface of the sea becomes smooth again
- The wind loses its grip: no longer finding roughness to push against, the air slides over the surface, transferring much less energy.
- The waves stop breaking: the enormous mass of water continues to move, but loses the violent white crest which overturns and sinks the boats
In essence, therefore, the oil does not have the strength to physically block a wall of water, but it changes the way in which the wind and the sea exchange energy.
Modern experiments that have confirmed this method
In 2019, a group of researchers from ISMAR-CNR and Ca’ Foscari University of Venice, together with Chinese colleagues from the First Institute of Oceanography in Qingdao, published in Ocean Science the results of an experiment that demonstrated how a thin film of fish oil can influence the waves generated by the wind.
To do this, the researchers filled a large experimental tank and simulated a wind of around 22 km/h (6 meters per second).
On theclean waterthe surface rippled and well-defined waves developed. But by adding a thin layer of fish oil to the water, the normal formation of waves generated by the wind was almost eliminated (the residual surface fluctuations were minimal, on the order of a millimeter). But the most surprising fact, perhaps, is another: the aerodynamic stress exerted by the wind on the oily surface has been reduced by as much as 95%.
It must be said, however, that there is a limit: when the researchers also added longer waves to the tank, similar to the long sea found in the open sea, the oil continued to reduce the small waves generated by the wind, but in a much less effective way.
Four years later, in 2023, a study that appeared in the Journal of Fluid Mechanics instead focused on the true terror of sailors: breaking waves. To be clear, let’s talk about those steep waves whose crest curls forward and collapses violently.
Also in this case the researchers worked in a tank, reproducing the breaking waves in controlled conditions. Instead of oil, however, they used a water-soluble surfactant (Triton X-100) added in small quantities.
In a second series of tests they left the water exposed to the air for up to 21 hours, so that it spontaneously collected the substances present in the environment.
In both cases the result was similar: even a few traces of surfactant were enough to profoundly change the behavior of the wave. The jet of water curving forward lost its typical shape, and the way the wave trapped air upon impact also changed.
From this experiment, the researchers discovered that it is not so much lowering the surface tension uniformly that makes the difference, but the continuous voltage differences which are created along the profile of the wave as it moves, because they are capable of dampening its violence in the bud.
How technology overcame storm oil
Storm oil was a mandatory lifesaver for decades on steamers and lifeboats in many Western countries.
The obligation came from the United Kingdom in 1894, with the Merchant Shipping Act which required ships to always carry oil on board “for use in stormy weather”.
In 1952, federal regulations in the United States required certain lifeboats to carry 3.8 liters of storm oil, and this requirement was only abolished in 1983, supplanted by enclosed, self-righting, motorized lifeboats, which being designed to resist capsizing and flooding without needing to still the surrounding water made the practice useless.
Furthermore, modern navigation relies on radar, satellite routes and steel hulls with geometries specifically designed to cut through rough seas, drastically reducing the situations in which a ship finds itself at the mercy of the fury of the sea.
Storm oil has been abandoned, however, also due to its environmental impact: as is now well known to all of us, deliberately spreading oily substances in the open sea (even if of organic origin) is an act incompatible with the protection of the marine environment and with international regulations against water pollution.
The latter, however, was certainly not a thought for the sailors of the past! They only cared about one thing, surviving the storm and returning home safely, and they relied on instinct to do so. Without knowing physics, they were nevertheless exploiting one of the most incredible laws of fluid physics.
