Question: Why are some patches of the river’s surface smooth and other rippled during a drizzle?
This question arises from observation of the texture of the water’s surface. In a drizzle, rain droplets ripple the surface of the water. Although the rain is falling uniformly, smooth patches appear of the water’s surface amidst the ripples. These smooth patches are distinct and persistent. They are often long and narrow bands along the length of the river, usually where ships passed. Similarly, in light air or breezes, when the water is stirred with ripples and wavelets, smooth water is observed in patches, often in a ship’s wake.
A friend, an avid water-gazer, first asked me this question early in my tenure as lighthouse keeper. Still unschooled in the subtleties of river knowledge, I did not have a ready answer at the time. Even when I did some research, I could not find enough information to formulate a satisfying answer. Recently, while onboard the sloop Clearwater, an observant child asked me the same question. When I offered my tentative findings, the sloop captain overheard and stepped over to offer another explanation. I said that it was due to surfactants on the water. He said that it was from the turbulence of a ship’s wake. His answer seemed equally plausible, so I decided to revisit the question and do some more research. As it turns out, both explanations, the sloop captain’s and mine, are right. Both turbulence and surfactants play roles in the formation and persistence of these long, narrow, flat water bands in the wake of a ship. While the initial flattening of the smaller waves is due to turbulence, the band of flat water persists due to surfactants concentrated in the stern wake.
The turbulence created by the ship’s hull and propellor temporarily flattens out the smaller waves by dissipating the energy of the wave patterns into disorganized turbulent motion. Also, a large amount of foam and bubbles are created when waves break at the ship’s bow and by cavitation from the propellers. Dissipative forces of turbulence are at work immediately astern of a passing ship. Gradually, the turbulence settles down, yet a calm persists long far astern. In the far wake region, surfactants play a dominant role.
The flat water in a stern wake persists long after a ship passes. This persistent band of calm is due to naturally-occurring surfactants (surface-active agents) concentrated in the stern wake. Throughout the water column are many living and dead organisms, which spread natural surfactants such as fatty acids and proteins. The hull of a passing ship compacts surfactant substances at the surface into slick bands at either side of the stern wake. Surfactants also being deposited astern by rising bubbles which scour naturally-occurring surfactants from the water column. These bubbles are generated by the bow wave and propellors. The accumulated surfactants reduce the surface tension astern of the ship, damping ripples and wavelets.
The water’s surface forms a “skin” like a thin sheet of stretched rubber or the head of a snare drum. Any disturbing force such as a rain drop or breeze hitting this taut skin causes a tiny wave or ripple. A very small amount of any surfactant substance (oil, protein, soap) spreads on the surface to produce a film one molecule thick. This thin film reduces the water’s surface tension. Without surface tension, the water is like a loosened drum head. Without adequate surface tension, ripples and wavelets cannot form. The small ripples from rain droplets cannot form in areas with low surface tension, which is why the long, narrow bands of surfactants from ships are especially noticeable during a drizzle. These slicks of surfactants in a ship’s stern wake persist long after a ship has passed.
As it turns out, these bands of flat water trailing a ship are of particular interest to the military for detecting and tracking vessels on the open ocean. Most of the available information on this flat water phenomenon is from naval research articles related to ship detection.
WAKEX 86: A Ship Wake/Films Exploratory Experiment
SURFACTANTS IN THE WAKE OF A SURFACE SHIP AND THEIR INFLUENCE ON AMBIENT WAVES
THE TURBULENT SCAR REGION IN THE WAKE OF A SURFACE SHIP
As a side note on surfactants, the knowledge that oil smooths rough waters predates Pliny and Plutarch who remarked upon the phenomenon in the first century AD. In their time, this phenomenon was already widely known to fishermen, who poured oil on the water to increase their ability to see fish below the surface, and by sailors, who would attempt to calm troubled seas. Benjamin Franklin reported his own observations of smoothing waves with oil to the British Royal Society in 1774.