7 February 2017

Analysis of 1883 at-sea rescue leads to new understanding of wave energy

Posted by nbompey

By Robert Monroe

Charles “Chip” Cox.
Credit: Scripps Institution of Oceanography

A team of oceanographers has developed a new model for ocean wave energy, using an 1883 account of how a ship’s crew dumped oil into stormy seas and calmed the waves enough to save the crew of a sinking ship.

Scientists at Scripps Institution of Oceanography at the University of California San Diego and Woods Hole Oceanographic Institution describe how a key detail recorded by a ship’s captain led to the new model, which includes a novel concept about how short surface waves affect wind stress at the sea surface.  The study is described in a paper published online today Geophysical Research Letters, a journal of the American Geophysical Union. It was the last in the career of famed Scripps oceanographer Charles “Chip” Cox, who passed away on Nov. 30, 2015, just eight days after completing the paper’s initial draft.

In a series of research papers in recent years, Cox had considered the physics behind damping waves by pouring oil onto the surface, a technique used since antiquity. This topic had been studied by scientists for centuries (including Benjamin Franklin in 1774) without finding a satisfactory answer to the question of how it works. Cox himself had been interested in the topic since his days as a graduate student and fisherman, before he joined the Scripps staff as an assistant research oceanographer in 1954.

Most records of real-life oil-dumping events did not include enough observational data to help oceanographers understand how it calms waves. But a detailed account of the rescue of a sinking ship’s crew by Thomas Greenbank, master of the sailing ship Martha Cobb, allowed Cox and his colleagues to create a wave dynamics model from it. Cox used a magazine account of Greenbank’s story following his rescue of the crew of another vessel, the Grecian, on Nov. 15, 1883 in the Atlantic Ocean. The Grecian, overloaded with grain bound for Portugal, had been damaged in a storm five days out of port, and began taking on water.

By the time the ships met, the stormy ocean had destroyed life boats on both of their decks, leaving only a dinghy for the Martha Cobb to use in a rescue attempt. Greenbank judged that the dinghy would be destroyed if it were cast into the breaking seas and decided to wait for conditions to improve. But after several hours with no change in ocean conditions, he decided to pump out the vessel’s bilges into the ocean, since the bilge water contained a petroleum product (the Martha Cobb’s cargo). Unfortunately, this was not very effective in calming the ocean waves.  So the Martha Cobb crewmembers tried something else: they dumped five gallons of fish oil into the water and waited.

There were several details in the account that Cox and colleagues found important. One concerned the use of fish oil.  As opposed to motor oil, for example, it is known as a “polar oil”: its molecules have two “poles,” one of which clings to water, while the other end is repelled by water.  That characteristic keeps films of fish or vegetable oil more coherent on the surface of a raging sea, while mineral oils typically break up into clumps on the water surface. The coherence of the fish oil gave it a greater ability to damp down waves, like a large blanket thrown over the water surface.

One other detail Greenbank mentioned was the crucial information that Cox’s team needed to make their wave dynamics model possible: how long it took for the oil to take effect. He described a “magical” transformation of the oil-covered waters 20 minutes after the oil was dumped overboard. The breakers crashing around the two vessels then abated long enough for the Martha Cobb to launch the dinghy. Over several trips, all 10 members of the sinking Grecian were ferried to safety.

Cox and co-authors Xin Zhang of Scripps and Timothy Duda of Woods Hole Oceanographic Institution used the timing information to estimate how far the fish oil slick spread. Both ships were being pushed by the wind during the storm, drifting one to three kilometers (.6-1.8 miles) or more while the slick grew. The slick drifted as well but, hugging the ocean surface, did not do so as quickly. The slick acted like a wake, with its aerodynamic efficiency improved in comparison to the choppy surface around it. The reduced wind stress provided a shield from high seas that lasted about an hour, according to Greenbank’s account, enough time to complete the rescue.

In contrast to previous theories, the model results suggest that surface slicks attenuate waves not by reducing the energy of waves themselves but by changing the structure of the wind so that the energy it contributes to wave formation is greatly reduced.  The team’s study raises further questions about how long such oil films can last and about how the structure of wind changes in response to their presence. Other questions could consider how such films could influence weather, including whether they could be used to attenuate hurricanes.

“This could be a very significant advance in our understanding of the complicated coupling processes at the air-sea interface,” said Zhang.

Cox wrote an article on the Grecian rescue and its social and political aftermath for the International Journal of Maritime History in 2015. He had already been working on the model before that article was published, said Zhang; one month before his death he had been excited to see the results of an experiment in which canola oil was dropped into the ocean off the California coast with a similar effect on waves. Zhang said that inspired him to accelerate completion of the study manuscript.

“My dad never lost his curiosity about what makes the oceans work,” said Caroline Cox, who had helped her father research his 2015 article. “To me, it is truly inspirational that he could return in his 90’s to a subject he had researched as a graduate student, pull together historical information, and develop a model.”

Cox had funded the collection of related field data himself and supplemented it with donations to the Hydraulics Laboratory at Scripps.

— Robert Monroe is a media specialist and editor at Scripps Institution of Oceanography at the University of California San Diego. This post originally appeared on the Scripps website