THE MIGHTY SEA

Evening Star, Issue 21994, 2 April 1935, Page 11

THE MIGHTY SEA

; POWER UNLEASHED ACTWH OF WAVES STUDY FOR SCIENTISTS Every few years there is a winter seasbn on the Atlantic when storms follow one another in quick succession, when waves, batter small ships until their superstructure and steering gear give way, when the full unleashed -power of a gr6y gale brings disaster. The storm that whips up seas to heights difficult to measure, and that slices their tops off and hurls them in long lines of stinging spray, is a ma- , jestic and fearsome sight, says a writer in the 'New York Times.’ As the waves gnaw at the land, tearing sandspits apart, growling in thunder against' the rocks, they inspire awe. But to know their actual power one should be aboard a ship that is being forced into their f.eeth, and feel the drunken quiver of the hull, hear the crashing blows which boom below like the crack of doom. It is in such storms as these that small freighters are beaten apart and founder. They sit low in the water and the seas pile over them and strike their upper works with blows that crush and rend. They smash the bridge and dismantle the steering gear, np off hatch covers and fill the holds with water, tear boats and davits and stanchions from their places, and leave the ship a helpless hulk, her crew dependent on good fortune and the skill of rescuers; Then waves dance the Dance of Death, ruthless destroyers of mat) and his works. It is often the liners that come to port with tales of lifeboats smashed or carried away, s.teel booms bent and twisted, hatches stove in, and seamen injured. Big ships are seldom lost, but at times, paradoxically, they take more punishment than the smaller vessels. The reason is that a small vessel rises and falls with big waves, pitching as their slope changes, while a long one, tending to’ remain on an even keel aha span the waves, suffers from their breaking force as does a fixed structure on shore when assailed by the surf.

Some years ago a large cruiser off the Philippines was badly damaged by a typhoon, although a small gunboat passed through it unscathed. But when the small ship is heavily loaded and seas come aboard it is much more quickly vanquished than the liner. Man has been attempting to overcome the fury of the sea since the first day ft of recorded navigation, and he has done so to a tremendous extent. When one looks at pictures of stately galleons, with their high bows and poops, one wonders how they made _ their way around the world. The design of ships has been modified according to experience. but very slowly. The marine architect still struggles with the problem of the proper null and balance, so that a ship may ride the billows safely, smoothly, and with a minimum expenditure of power. But when the ship is finally sent to sea the task remains of guiding her through storms. It is part of her master’s strategy to lay a comfortable course through seas that he describes (with no thought of aesthetic appraisal) as “ ugly,” and in an emergency to smooth the wrinkled brow of tile deep with the magic of oil? And sometimes he finds himself called upon to go to the aid of another ship and to send out boats which by some miracle of courage and seamanship find their way through the turbulence. The things which brave men can do with a boat In a raging sea are beyond comprehension.

HQW WAVES ARE PRODUCED. While to the •••aiior tlie attack of the gea is » vit.aj problem of life and death,

to the physicist who seeks to explain the phenomena of waves it is a mathematical equation. It is remarkable that until a few decades ago science did hot know how waves were produced. To-day the process is partly explained by the knowledge of “ wind structure ” that came to Tight through the researches of Langley and his successors in the field of aeronautics.

The wind is never a perfectly smooth flow of air. It is always full of eddies, which involve more or less vertical motion. Thus an initially level water surface is subjected to unequal pressures at- various places and becomes humped into small waves as the wind moves over it. When such waves are produced they, in turn, increase the turbulence of the wind by opposing obstructions to it. Thus eddies in the air make waves in the water, and the latter make bigger eddies in the air, which increase still further the size of the waves

It is the eddies among the waves that enables gulls to soar more easily over water than over land, and the presence of eddies is also revealed by the fact that a sailing vessel, in the lee of a hig wave, sometimes has her sails taken aback by a momentary reversal of wihd direction.

pn the open sea the waves, though they may race along at the speed of an express train, do not carry the surface water far with them. A floating log, as a wave sweeps under it, is seen to make little or no advance, but merely to rise and fall with a small movement to and fro. It is the form and not the substance of the wave that travels along the surface. Waves tend to occur in groups, with comparatively calm water between the successive groups. Watch the leading wave of a group and you will note that it soon dies out, the one next behind it taking the lend.. At the same time new waves rise one after another at the rear of the group, so that the general appearance of the assemblage of waves remains unchanged. The group as a whole advances only about half as fast as the individual waves composing it. There is an ancient belief that every tenth wave—or some say every ninth —is exceptionally big. A different idea has grown up among sailors to the effect that big waves generally come in groups of three. German fishermen speak of such groups as “ the mother with her two daughters,” the middle one, supposed to be the highest, being the “ mother.” The grain of truth in such notions is that two or more systems of waves are often running at one time in different directions and at different speeds over the sea, leading to various combinations. Two intersecting waves produce a big one; two troughs unite to form a deeper trough; and there are all gradations between these extremes.

This combining of waves belonging to different series leads to an endless variety of shapes as well as sizes, and thus the pattern of mounds and hollows assumed by the surface of the sea is often extremely intricate. Greatly exaggerated ideas have always prevailed on this subject. Waves of impossible dimensions are often seen in paintings. “ Mountainous ” waves have been described by writers of all ages. Ovid, bound for a land of exile, shuddered at the billows whose summits seemed to touch the stars and whose valleys appeared within measurable distance of the infernal regions. Hardly less exuberant accounts of monstrous seas said to have been by ships are found in contemporary newspapers. TREMENDOUS BILLOWS. One reason why. an untrajnel observer often over-estimates the height of waves as seen from a ship is that the vessel may meet a wave while pitching forward, as her stern is lifted by the preceding wave, so that normally lofty objects, such as the crow’s nest, are relatively near the water. Another reason is that the breaking of a wave against an obstacle, such as the boW or side of a ship, throws the water to a far greater height than tl)6 unbroken wave could attain. The usual non-instrumental method of measuring the height of waves from shipboard is for the observer to climb the rigging or otherwise place himself at such an elevation that as the wave advances his eye will be just on a level with its crest and the horizon when the ship is in the hollow. The height of the eye above the ship’s waterline is then taken ns the height of the wave. Many series of measurement!! made by this rough-and-ready method are embodied in the literature of oceanography. The billow's of a severe Atlantic gale were thus measured in 1848 from a station on the top of the paddle-box of the steamship Hibernia by the llev William Seoi'esby, w’ho found that many wore more than forty feet high, while the crossing of two waves sometimes sent up a sharp peak of water to an estimated height of fifty or fiftyfive feet.

The most assiduous present-day observer of waves is Dr Vaughan Cornish, who has studied and measured those of every sort and size, from the ripples of the Round Pond in Kensington Gardens to the heaviest mid-ocean swells. He finds that in a strong Atlantic gale some waves may reach heights around forty-three feet and momentarily, at intersections, shoot up as high as sixty feet. In rare cases the Atlantic is believed to be capable of producing even greater waves. Cornish accepts as plausible the estimate of between eighty and ninety feet for the height of storm waves observed from the White Star liner Majestic on that ocean in December, 1922; but it is unlikely that much taller billows have ever been heaved up on any ocean, unless, perhaps, with the aid of ’a submarine earthquake. The length of a wave, measured horizontally from crest to crest or from trough to trough, is supposed to depend upon the strength' of the wind producing it and the extent of open sea, called the “ fetch,” over which the wind acts in one direction. If the “ brave west winds ” of the Roaring Forties in the Southern Hemisphere actually blow, as mariners once supposed, in an unbroken sweep from west to cast around the globe, we might credit the accounts in seafaring narratives of waves a mile or two long in those latitudes. We now know, however, that these winds are really a succession of blasts curving around the northern borders of eastward-moving cyclones, so that their direction is far from constant, and the unlimited “fetch ” of that region is illusory. It is true that, on an average, the waves of the Southern Ocean are the longest on tho globe, but it happens that the longest wavo supposed to have been measured anywhere with reasonable accuracy was one observed on the Atlantic a little north of the Equator by Admiral Mottea, of the French Navy. Its length was about 2,700 ft.

“Sea” consists of waves set up by wdnds blowing at the time and place of their occurrence. Swell consists of rhythmic undulations caused either by winds ou some other part of the ocean or by those that have prevailed at some previous time at the place of observation. Thus a ship encounters a heavy sea only w’hen she is also exposed to high winds, but she may encounter a heavy swell in perfectly windless weather. The one exception to this terminology is the turmoil of confused waters, so dreaded by sailors, at the calm centre of a tropical cyclone, which is always described as “ sea.” Here, without a breath of wind stirring, a vessel may be Tacked to pieces by the waves.

The swell from a storm travels far and wide over the ocean and often gives the first notice of the storm’s approach; especially in the Tropics, where cyclones move slowly, however sw'iftly the winds may blow around their centres. Swell as the portent of a coming storm is an old story; a newer one is the prediction of the arrival of swell upon a coast from reports of the far-away storm producing it. This is a matter of practical importance, because swell when it reaches shore may become terrific surf. PROTECTION FROM THE SEA. Waves wage incessant war against shores and beaches, alter coastlines, batter and undermine walls, jetties, and buildings. One of the tasks of the engineer is to devise means of protection against these inroads, and in order to do so he must gauge the strength of the waves and study their modes of attack As long ago as the middle of the last century Thomas Stevenson, the inventor of the marine dynomo-meter, was pioneering in this field, yet to-day his successors are still busily seeking new facts about wave action on coasts and coastal structure.

Some 6f the most remarkable of these investigations are now in progress in the United States. At one Massachusetts Institute of Technology and elsewhere have been installed miniature oceans, equipped with waveibAkofs, for observing under controlled conditions tbe erosive action of surf. The Beach Erosion Board, attached to the Army Engineer Corps, lias carried out a number of novel exploits in studying the mechanism of destructive surf. Current meters installed under the water measure its to-and-fro movements at different depths; gauges dropped from ocean piers measure the amount of sand churned up during storms; divers observe directly the effects produced on the sca-bottorn by the impact Of the waves. Motion picture photography has been pressed into service to record the evolutions of the incoming breakers. Artificially-coloured sand reveals the transportation of beach material along the foreshore. Such are some of the wars in which restless science is putting questions to the waves of the sea.