RESTRICTED 



24 



6. APPENDIX ON OCEANOGRAPHIC METHODS 



6.1 Current measxirements 



Measurements of surface currents were obtained by the cur- 

 rent pole method. The poles v/ere 12 to 16 feet long. Some of 

 them vfere four inches square; others were made of smaller stri-Ds 

 of wood with aluminum fins .inserted to increase the cross-section- 

 al area. They were weighted so as to hang vertically with about 

 one foot exposed above the surface of the v/ater. A light alum- 

 inum staff v/as rigged to the upper end of each pole, bearing some 

 device to aid in sighting the pole. Various methods used at one 

 time or another or in combination were pennants, life jacket dye 

 marker (bags of fluorescein, which left a trail of green dye in 

 the water), lights, and radar reflectors. Three or four poles 

 were used simultaneously. They were set out one to two miles 

 apart, and their position was determined every few hours by com- 

 ing alongside and taking bearings on beacons or other landmarks. 

 At night, and at considerable distances from land, radar ranges 

 were used for obtaining fixes. 

 f 



Along the reefs and in the channels, the currents v/ere 

 studied by dropping dye bombs from an airplane and photograph- 

 ing the dye patches at frequent intervals over a period of I5 

 to 30 minutes. It vras largely the surface currents that were 

 measured by this method, but packages of dye lashed to the bombs 

 left a trail in the v/ater as the bombs sank, permitting some con- 

 clusions as to subsurface ciorrents. The dye bomb method proved 

 to be particularly useful on the reefs and in any circumstance 

 in which a high degree of variability required a large number 

 of nearly simultaneous observations. 



Vertical profiles of current velocity were obtained with 

 a Von Arx current meter. It consisted of a propeller mounted 

 in a tube oriented to the current by means of a vane on one end. 

 Each turn of the propeller induced a small electrical potential 

 that was used to determine the nuiaber of revolutions, from which 

 the current velocity was computed. The current direction was 

 determined by observation of the vane throxigh a water glass. 

 The current was meas\ired at depth intervals or 5 to 20 feet from 

 the surface to bottom. The direction could be determined to a 

 depth of about I50 feet, the limit of visibility in the lagoon 

 waters. An tmderwater floodlight was used for night stations. 

 It was necessary to exercise considerable care in the current 

 meter work since swinging of the ship at anchor introduced an 

 error, Uye marker and the cable angle were used to determine 

 the times when valid measxirements could be made. 



