CURRENT AND DRIFT OF THE SHIP 125 



CURRENT AND DRIFT OF THE SHIP 



It was essential to the efficacy of our observations that the ship should cover long 

 distances in the shortest possible time, and we were thus unable to include detailed 

 current measurements in our programme ; we were, however, able to record the com- 

 bined effects of wind and current on the drift of the ship in a direction parallel with the 

 coast which is here a component of major interest. 



As explained on p. 120, the ship was steered regardless of lateral drift when steaming 

 from one station to another in a direction normal to the coast. The track chart is there- 

 fore a graphic representation of the resultant effect upon the ship's course, of the com- 

 ponents parallel to the coast, of wind, tide and current. The amount of this drift off 

 different parts of the coast has been summarized in Table I. The drift is expressed in 

 miles per day and is calculated from positions that had been fixed by Mr Davies, the 

 ship's navigator, from observations of sun, stars or land bearings: dead reckoning 

 positions have not been used in this calculation. 



Northerly drift was experienced at irregular intervals over the whole surveyed region, 

 but southerly drift only inshore and far out to sea off Peru; frequently no drift was 

 noticeable. We may inquire whether it is possible to find out from any of these observa- 

 tions how much of the drift was referable to current and how much to wind. 



On approaching Cape Carranza from the southward no wind was met, but a south- 

 westerly set of 3 knots, possibly tidal, was recorded close inshore : the ship does not seem 

 to have been under its influence when farther from shore between Sts. WS 593 and 596 

 (Fig. 7). The southerly drift experienced from 24 to 44 miles is almost certainly the 

 result of northerly winds reaching force 4. Upon a change of wind to east-south-east at 

 St. WS 599 the southerly drift changed to northerly. The conclusion is that there can 

 have been no appreciable current off Cape Carranza. 



On the second line, off Pichidanque Bay, no drift at all was recorded on the run 

 westward, from which we can infer there was no current (Fig. 6). The northerly leeway 

 experienced between Sts. WS 607 and 610 may be due to southerly wind, which during 

 Sts. WS 608 and 609 reached a force of 5-6. These observations are of particular interest 

 in showing that a mean drift of 20 miles a day can result from the direct action of 

 winds in a region where before their advent no flow of surface water was apparent. 



The track chart (Fig. 5) shows that the east and west courses off Caldera were sub- 

 ject to varying amounts of northerly and southerly drift, and this may be loosely related 

 to changes of wind. This may be seen in Fig. 8, in which the amounts of northerly and 

 southerly drift are plotted against wind records during the period from noon on June 4 

 to noon on June 6. The distance from shore at which the observations were made is also 

 indicated. Northerly drift usually attended southerly wind and southerly drift usually 

 attended northerly wind, but a strong southerly drift against the wind within 2 miles of 

 the coast is evidence of a counter-current inshore. At 1 1-27 miles from shore northerly 

 ■ drift was recorded during calm weather following a period of northerly winds : while this 



3-S 



