SCIENCE AND THE SEA 



Swell, on the other hand could only be forecast in this scheme by 

 means of mathematical filters. These slowly tuned through the 

 spectrum of the wind-born sea as a function of the dimensions of the 

 storm source, the strength of the winds in the source, and the distance 

 from the source of the point for which the swell forecast was to be 

 made. 



These approaches to forecasting waves and swell were built into 

 an operational forecasting system — by Neumann, R. W. James, and 

 me— that is in current use by the US Navy and most other maritime 

 agencies. 



In the effort to develop better ways to predict waves and swell 

 many different spectral forms and other procedures have been pro- 

 posed. Large areas of disagreement developed among oceanographers 

 concerning the precise form of the wave spectrum of a "fully de- 

 veloped" sea for a particular wind speed. But more recent data 

 appear to be resolving these disagreements. Improved calibration of 

 wave-recording instruments— of all things— has brought the formerly 

 widely divergent results of different investigators into closer agree- 

 ment. And formerly overlooked considerations of how the wind speed 

 varies with height seem to account for most remaining discrepancies. 

 The forecasting rules developed by Neumann, James, and myself will 

 have to be corrected in the light of recent results. But they were 

 surprisingly close to reality in most respects, especially considering 

 how "primitive" the hand-held stop-watch data were, upon which 

 they were based. 



Lota of new ways to measure waves 



Study of wave spectra for all purposes will be helped a good 

 deal by more measurements, of higher accuracy, of real waves in the 

 ocean. In general, waves are recorded on graphs like those of Fig. 5, 

 which show the varying height of the water surface at a fixed point 

 as a function of time; needed analyses are performed later. Such 

 records ignore the sometimes significant question of which way the 

 waves travel, but there are a few recently developed approaches to 

 making this difficult measurement that we'll mention below. 



Wave measurement is simpler near the shore in shallow water 

 where suitable structures, or the bottom, are available for anchoring 

 instruments. On the open sea of course there is no fixed frame of 

 reference to which a wave recorder can be fastened. It was so very 

 difficult to get time histories of waves under open-sea conditions, in 

 fact, that an instrumental way to measure waves from a ship was 

 developed only a decade ago, at Great Britain's National Institute of 

 Oceanography (NIO). This method, diagrammed below, right , uses 

 two identical instruments mounted on opposite sides of the ship, and 

 their outputs are averaged to compensate for differences in wave 

 height on either side. Each instrument consists of a pressure sensor 

 and a vertical accelerometer. In principle, the pressure is a measuj-e 

 of water height above the instrument, and the doubly integrated 

 accelerometer value yields displacement of the instrument above some 

 chosen reference level; the sum of the two in the form of a voltage 

 is the wave record. With this instrument, storm waves and swells on 

 the open sea in all of their rampant variety have finally been meas- 

 ured accurately, and although it has only been used for a relatively 

 short time on British weather ships the data gained have been of 

 exceptional value. Recorders using essentially the same principle 

 may soon be installed on all US Coast Guard weather ships. The NIO 

 instrument is already on the Atlantis II research vessel of the Woods 

 Hole Oceanographic Institution. 



Pisfhcement 

 Ml—u/a*e recarJers 



^ffrence Uva I 



Other methods for measuring waves at sea do not use a ship's hull 

 as an anchor and reference point — instead they use as a reference 

 the quiet water that lies below the depths agitated by waves. One 

 such method suspends from a buoy, a large flat plate (or "drogue") 

 that is designed to respond to the passage of the waves with as little 

 lift as possible. To the buoy is attached a measuring pole (see 

 marginal sketch) against which wave fluctuations are recorded either 

 mechanically or electrically. Such methods are generally unsatisfac- 

 tory and quite costly. 



Pire-cfiBv, of 



Flo3+i<i5 -^^ ujind 



buoy 



Uet 





Ship -borne 



uJaVe RecorSer 



f. lot-facrs of 



The problem of high cost in obtaining large numbers of nondirec- 

 tional wave records may be eased by a floating-buoy device — dubbed 

 the "Splashnik"— recently developed at the David Taylor Model Basin 

 in Washington, D. C. Said to cost about $150, the device converts the 

 output of a vertical accelerometer mounted on a raft, with a trans- 

 mitter, to an FM signal whose frequency varies with acceleration; it 

 transmits the signal to a nearby ship where the familiar double inte- 

 gration to get wave height is performed. 



Another new and promising way to measure waves on the deep 

 sea is to use a highly stable platform, like the Scripps Institution of 

 Oceanography's new "ship-on-end" — dubbed FLIP. Any wave re- 

 cording device would work well on FLIP. In recent trials in waves 

 40 ft high, FLIP moved a remarkably small 6 in. ! Anyone who's 

 spent any time at sea will appreciate what this means. 



Nondirectional devices on stable platforms like this could be used 

 in suitably spaced arrays to yield useful data about wave directions, 

 if such platforms cost less. Using wave recorders in this way would 

 be similar in principle to using antenna arrays for direction-finding 

 in radio astronomy. 



Waves on the open sea with lengths shorter than one thousand 

 feet or so also can be measured by stereophotogrammetry. With a 

 long ship and cameras pointed horizontally at each end, the profiles 

 of several waves can be measured. Aircraft can also obtain stereo- 

 wave photographs, and these can be used— though at the cost of much 

 time in stereoanalysis and computation — to get the directional wave 

 spectrum. 



A more direct approach to getting directional information on 

 waves in a wind sea, developed at the British NIO, uses a single float- 

 ing buoy (Fig. 8) that's kept in constant alignment with the wind by 

 means of an attached pellet and drogue, as sketched in the margin. 

 Inside the buoy are an accelerometer and two gyroscopes and 

 associated electronics. The assemblage gives data on pitch, roll, and 

 heave (or vertical displacement) of the buoy, from which some 

 features of the angular (azimuthal) distribution of energy in each 

 frequency band of the wave spectrum can be derived. 



Regardless of the direction taken by waves and swell on the open 

 sea, their ultimate fate is certain. Sooner or later either they die at 

 sea, as the poorly understood processes which dissipate their energy 

 operate to destroy them, or they reach one of the world's coastlines. 

 There, frequently, they expend their energy in one last sometimes 

 destructive burst before they die. 



14 



