designers of aircraft also have a great interest in the surface of the sea. Water-based 

 aircraft are moving out of the bays and rivers further into the open sea. 



Generally, aircraft are too fragile to operate in all sea conditions. The sea 

 becomes too rough (both in a local and overall sense) once in a while, but on the 

 other hand the hybrid nature of the water-based aircraft enables us to avoid the worst 

 conditions. On the water, at rest or slowly taxiing, the seaplane experiences a long 

 sequence of waves representing the full spectrum of the sea. However, in the transition 

 conditions of landing and take-off, which are critical for the aircraft design, only a 

 very small portion of the whole sea wave spectrum is encountered. 



Thus, we may be looking for somewhat different information than ship designers. 

 We are both concerned with the occurrence of high water loads and seaworthiness as 

 they may be related to the energy spectrum for the seaways. Yet because of the short 

 transient nature of the seaplane takoff and landing, a further analysis of the sea spec- 

 trum may be profitably employed. 



For example, as the various components of the sea add to give the larger waves 

 — do high crests generally follow immediately the deep troughs? The extent to which 

 this may occur on a probability basis would be helpful in determining the conditions 

 for which the hull loads should be obtained. 



Another problem suggested by the type of operation of water-based aircraft is 

 the description of the sea so that small areas at a time could be defined in terms of the 

 overall spectrum existing at the time. Along this line we may be able to contribute 

 through the measurement of the sea surface from the air. Although airborne direct 

 wave measuring systems have not yet been satisfactorily demonstrated, there are several 

 now under development which have considerable promise. Measurements of the sea 

 roughness by aircraft will not only provide a tremendous increase in data over what can 

 be obtained at fixed locations but also provide quick data for portions of the total sea 

 spectrum for small transient areas. 



W . J. Pier son, Jr. 



I should like to add to Dr. Munk's remarks a very brief summary of Project 

 SWOP (Stereo Wave Observation Project), a joint effort of the Office of Naval 

 Research, U. S. Navy Hydrographic Office, Woods Hole Oceanographic Institution, and 

 many operating arms of the Navy. A final report on the project is soon to appear under 

 the title 'The directional spectrum of a wind-generated sea as determined from data 

 obtained by the Stereo Wave Observation Project." 



The plan of the project was to take simultaneous stereo photographs and wave 

 pole records of the sea surface. The directional spectrum could be computed from the 

 stereo photographic data, and the frequency spectrum could be computed from the 

 wave pole data. Theoretical considerations led to the conclusion that the total volume 

 under the directional spectrum should equal the total area under the frequency spec- 

 trum and thus a check of the two methods would be obtained. After the computation 

 of the directional spectrum and the wave pole spectrum, it was discovered that the 

 two quantities given above did not agree. Several sources of discrepancy have been 

 discovered and eliminated from the data. First of all, the wave pole spectrum did not 

 have enough area under it to represent the true wave record. The spectral compo- 

 nents for many important frequencies were attenuated because the wave pole had a 

 tendency to move up under the crest of passing waves and move down in the troughs. 

 A correction was derived for this effect, and brought the two results into closer 

 agreement. 



The stereo data unfortunately were obtained from topographic-base film instead 

 of stereo-base film and there was some differential shrinkage in the record. The errors 

 in the stereo data are due to this differential shrinkage, to a background curvature of the 

 plane of the sea surface, and to spot height reading error. They all combine to make 

 the volume under the directional spectrum quite a bit greater than the area under the 



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