38 MANUAL OF CURRENT OBSERVATIONS 
97. The selection of the floating equipment will depend upon the locality and 
upon the availability of vessels for hire. Among the factors to be considered are the 
strength of current, liability to danger of collision, exposure to heavy seas, sleeping 
accommodations required, ground tackle necessary, and deck space available for setting 
up gear. For harbor work the observing vessels need have little power but the tender 
should have a speed of 8 knots or more to enable the work to be carried on efficiently. 
In a waterway where the currents have considerable velocity and dense traffic intro- 
duces a factor of danger from collision, a flat-bottom barge which may be towed from 
station to station by the tender may be found desirable; while in other waterways 
where the currents are moderate and the traflic light, a power launch 60 to 75 feet 
long and capable of housing the complement of the unit may be found more economical. 
In some sheltered locations smaller boats may be used if other provisions for housing 
the observers can be conveniently made. 
98. Standard instrumental equipment for observing includes current poles, log 
lines, current meters, stop watches, compasses, peloruses, and sextants. Necessary 
drawing instruments include three-arm protractors for plotting positions. A sufficient 
number of spare instruments and parts must be provided to insure against any interrup- 
tion to the observations as a result of loss or the failure of an instrument to function. 
Sheaves, reels, and winches for handling the observing apparatus, and the equipment 
necessary for navigating and anchoring the boat and for feeding and housing the 
observing party must also be provided. When installing equipment on the observation 
units, all operating gear should be placed so as to reduce to a minimum any lost motion 
in setting out gear after anchoring on station. As far as possible gear should be under 
cover so that it may be kept dry and the observations carried on without interruption 
in rainy weather. After the party has arrived on the working grounds, the compasses 
which are to be used by the several observing units must be checked for deviation and 
tables of corrections prepared. (See pars. 35-88.) 
Operating Procedure From an Anchored Vessel 
99. Each observing unit proceeds as near as practicable to its assigned station. 
When possible the vessel should be anchored fore and aft so that its position will 
remain as nearly fixed as possible without swinging with the current. This is especially 
important in narrow waterways where a small change in position might mean a con- 
siderable difference in the current. When this mooring is impracticable and a single 
anchor is used, the chain should be as short as possible to maintain position without 
dragging. 
100. After the vessel has been moored, its exact position must be determined. 
This may be done by compass bearings, by sextant angles or by radar. By the first 
method at least 2 charted reference objects are necessary and by the second method 
there must be 3 or more such objects. When practicable the reference objects should 
be selected so that the angles subtended will be fairly large, as angles less than 30° 
cannot usually be expected to give strong positions. When the position of the station 
is to be determined by sextant angles, it must be kept in mind that this method will 
fail if the station occupied happens to lie in the circumference of a circle that also passes 
through all the reference objects. The bearings or angles must be immediately recorded 
in the current record book, and the position of the station plotted on a chart or work 
sheet. 
