16 MANUAL OF CURRENT OBSERVATIONS 
is obtained for each 15° rhumb of ship’s heading by compass. Placing the vessel on 
different headings to obtain these deviations is called “swinging the ship.”’ 
36. There are a number of methods for swinging ship. The range method de- 
scribed here has been successfully employed in our work. This method requires the 
selection of two objects on shore which are definitely located on the chart and which 
are so situated as to be clearly visible from the ship. ‘To form a sensitive range, the 
front object should be as near as possible, while the back object should be at a distance. 
The selected range should be located at a place where it will be convenient to carry 
out the ship swing without being in the way of traffic. The procedure is to place the 
ship on a heading corresponding to one of the 15° rhumbs and come up on range slowly. 
When in line with the two objects on shore, a compass bearing of the range is observed. 
The difference obtained by subtracting this compass bearing from the known magnetic 
bearing of the range is the deviation corresponding to the ship’s head at the time of 
the observation. The operation is repeated for headings corresponding to other 15° 
rhumbs. 
37. If a ship on a current station is anchored with a single anchor and is free to 
swing with the current, the following method for preparing a deviation table may be 
employed. Although more time is required to obtain the deviations for all the required 
headings, the work can be carried on simultaneously with the current observations at 
the station. The method requires three or more objects on shore which can be used to 
determine the location of the ship by sextant angles and a reference object upon which 
to take bearings. The positions of these objects must be plotted on a chart. Two ob- 
servers are necessary, one to determine the position of the ship by sextant angles as 
it swings into different headings, and the other to note the ship’s head by compass and 
at each 15° rhumb to take a compass bearing to the reference object. The position of 
the ship being plotted for each observation, the true and magnetic bearings of the 
reference object can be readily obtained. By subtracting the simultaneous compass 
bearing from the magnetic bearing the corresponding deviation is obtained. With this 
method the observations at times must be taken very hurriedly when a strong current 
swings the vessel rapidly through any of its headings. 
38. With either method of swinging ship it is desirable that the results be checked 
by plotting. This may be done on cross section paper using the ship’s compass headings 
as abscissas and the corresponding deviations as ordinates. The plotted points should 
fall into a fairly smooth curve and any outstanding irregularity should be checked by 
additional observations. The final results are to be recorded in a deviation table, a 
form for which will be found in the front part of the book (Form 270) provided for the 
record of current observations. In this table the magntic heading of the ship is obtained 
by applying the deviation according to sign to the compass heading of the ship. 
39. In addition to the variation and deviation described above, a compass may be 
subject to error due to local attractions arising from extraneous influences in the vicinity 
of the vessel, especially when a ship is in port or in close proximity to other vessels. 
Local attraction may be encountered in certain shallow waters of the sea with underlying 
mineral deposits. In general it is not necessary to take into consideration this source 
of error in connection with current observations. The effect of such local conditions 
that might exist in the area where the ship is being swung may be expected to be included 
in the deviation error. Disregarding local attractions, the relation of magnetic and true 
azimuth to compass bearing may be expressed by the following formulas: 
Magnetic azimuth=compass bearing+ deviation. 
True azimuth=compass bearing +deviation-++ variation. 
