Reading the ripples

To the editor: What do a cast iron kettle, a handful of pebbles and a cat’s paw have in common? They have all been used at various times by seafarers becalmed at sea to discover the set and drift of the current that may be sweeping them silently from their intended course. Today, it is a simple matter of switching on the GPS and letting the satellites track the change in your position. In earlier times, however, sailors used their powers of observation and much ingenuity to tackle this problem, but many of their skills are no longer practiced or have been forgotten. Here are a few that you may like to investigate should the fat controllers turn off the satellites.

Sailing along the coast or in an estuary, the signs of a current are usually quite obvious. For example, a wake of bubbles will stream from buoys or other moored objects in the direction of the current and, unless the wind is very strong, anchored yachts will point up to it.
Sometimes, the current drags a line of debris along its edge and, in clear shallow waters, marine plants may be seen deflected along its course. On a windy day in an estuary or a river, when the tide is ebbing or flowing, the current will be seen working with or against the wind through the behavior of the waves. If the wind is against the current, the wave faces become short and steep, and the sea is choppy. When the wind is with the current, the waves are much smoother and longer. In a light wind and calm sea, a related but less dramatic effect may be observed. With the wind and current working together long, smooth patches on the surface will often indicate where the current is at its strongest and offering less resistance to the wind. A stronger current often indicates the deeper water.
When the tide changes direction and opposes the wind, the faster moving currents will offer more resistance to the wind and the effect will be reversed; rougher surfaces with sharper crests will replace the smooth ones.

But what are the indications of a current out at sea where there are no reference points, such as moored objects, structures or headlands? Unless you pass through a patch of rough and confused water where two currents collide, you may not be aware of its presence. Or, when sailing close-hauled, you may find your boat is sailing rather closer to, or farther from, the wind than you would normally anticipate, and suspect that a current is affecting the apparent wind.

At sea, many of the surface signs that work so well in an estuary or river are still there, and the ancient navigators who voyaged across the Pacific in sailing canoes made good use of them along with other indicators. They knew, for example, that when they entered a strong current there was often a change in the color of the sea, or in its temperature and that of the surrounding air. When the wind increased, they studied the whitecaps on the wave crests. If the caps tumbled over gently and seethed into a long streak, the current was running with the wind. If the caps peaked abruptly, fell and were drawn back to windward, the current was flowing against the wind. Joshua Slocum made much use of this during his pioneering circumnavigation of the world aboard Spray. In a moderate wind, currents of 0.5 knots and upward may be detected by comparing the amount of whitecaps visible upwind with those visible downwind. When the sea is viewed downwind, an opposing current can appear to produce as many whitecaps as when the sea is viewed upwind, whereas with the current and wind working together, the disturbance viewed downwind diminishes considerably.

In calmer seas or in a gentle swell with light winds, Polynesian navigators studied the ripples formed by the wind on the surface of the sea. It is a technique that has often been reported by their descendants but the descriptions, usually accompanied by piano playing gestures, were not always fully understood. As a light breeze blows across smooth water, it stretches the surface into small wavelets with rounded crests and v-shaped troughs, which are more pronounced to windward. These are known as capillary waves and, resisted by surface tension or the water’s “skin,” they soon disappear. If the wind’s speed increases, larger or gravity waves are formed but capillary waves may still be observed running ahead of, or passing through, the crests. When carried forward with the current, these small ripples are often difficult to follow and can disappear quite rapidly. With wind opposing the current, however, they contract and are more persistent; their paths are easier to follow and they may hold their shape for some time.

The secret of reading ripples lies in how well their shape and persistence can be judged against a backdrop of many other surface disturbances. The use of a pebble is an old trick and one that is still practiced occasionally by water engineers checking the velocity of water flowing in concrete channels and culverts. You can try it out ashore from a bridge by casting a pebble into a stream or river. When the water is still, ripples will spread evenly from the point of emanation. With the water flowing, however, the ripples will accelerate downstream but their passage upstream will be impeded depending on the speed of the current. In a powerful current, no ripples will travel upstream from the point of entry.

The same technique may be used to detect the set of a strong current in fairly calm water. With no supply of pebbles or stones at hand, cast a shackle pin attached to the end of a light line so you can retrieve it, but always keep an eye on its point of entry. Try to assess the direction in which the ripples escape freely and the direction in which their momentum is restrained. Whichever method you adopt to study the ripples, keep your observations well clear of the boat’s wake or wash.

Today’s oceanographers use acoustic Doppler equipment to establish the profiles of currents at different depths, but two hundred years ago becalmed ships’ captains had a very lo-tech solution. They borrowed an iron kettle from the galley, attached it to a long line and lowered it from a tender. It sunk into the depths where the current is generally weaker and the stronger surface current dragged the line in its direction. A simple procedure but a risky one, for it broke the first law of the sea: Never upset the ship’s cook.

Tony Crowley is a former Merchant Navy navigator who sails for pleasure on the East Coast of England. He is the author of the “Sailing Quiz Book” and “The Lo-Tech Navigator,” published by Sheridan House.

By Ocean Navigator