CURRENT MEASUREMENTS FROM MOORED BUOYS 
by WILLIAM S. RICHARDSON 
Woods Hole Oceanographic Institution 
Woods Hole, Massachusetts 
The history of the direct measurement of 
currents in the deep water of the open ocean is 
a rather short one. Indirect methods based on 
the distribution of various chemical and physi- 
cal properties provide us with a general picture 
of the deep circulation and calculations based 
on the distribution of density provide quantita- 
tive data which it would be desirable to check 
by direct measurement. 
Many techniques for direct current measure- 
ment have been proposed but few have been used 
to any great extent. Perhaps the most powerful 
and widely used has been the Swallow float. 
This technique, developed by Dr. John C. Swallow 
of the National Institute of Oceanography, 
utilizes a neutrally buoyant drifter which can 
be adjusted to drift at any desired depth. The 
float is tracked acoustically from a ship and 
may be followed for a period of days or even 
weeks. This technique in the hands of Swallow, 
Volkmann, Knauss and others has given us our 
first glimpse of the details of the movement of 
the deep water and has contributed greatly to 
our knowledge of several major current systems. 
Deep water measurements by this technique in 
areas well removed from major current systems 
have shown the currents to be swifter and more 
variable than had been expected. This points 
to the necessity for longer time series of 
current measurements over more extended areas 
than are easily undertaken with Swallow floats. 
To provide such measurements, a program of 
current observations from anchored buoys has 
been undertaken and as an initial effort a line 
of stations between Martha's Vineyard and 
Bermuda has been set. (see Figure 1) 
The stations themselves are rather simple. 
The surface float is a foam filled fiberglass 
doughnut eight feet in diameter with a three f 
foot hole. It has about 6000 pounds of buoyancy 
which is sufficient to part the mooring warp 
if the mooring strain builds up excessively. 
The float carries a ten foot high tripod tower 
on which is mounted a light, a low powered 
radio beacon for location purposes and a 
recorder for wind speed and direction. The 
float is connected to the warp by means of a 
3 point, 45° bridle and a 30 foot leader of 
1/2 inch galvanized chain. The warp itself is 
polypropylene rope about 1/2 inch in diameter 
and having a breaking strength of about 5000 
pounds. The rope is somewhat positively buoy- 
ant in water and therefore contributes no dead 
weight strain to the moor. It will stretch 
about 40% before breaking and this permits the 
205 
mooring to be set with little or no scope, 
this being provided by stretch as required. 
The warp is provided in 500 meter lengths 
with eyes spliced in each end; instruments 
are inserted as links between the lengths and 
must be capable of supporting the full tensile 
load of the warp. They can therefore be located 
at any 500 meter mltiple in depth or at other 
depths if special lengths are made up. The 
bottom end of the deepest length of the warp 
is connected to a weak link having a strength 
of about 4000 pounds. This link should part 
during recovery of the mooring if the anchor 
is fouled. The ground tackle consists of a 
cast iron clump weighing 800 pounds followed 
by 200 feet of 1/2 inch chain (600 pounds) 
and a 90 pound Danforth anchor. 
Because the mooring warp is ordinary rope 
there is no electrical connection between the 
various instruments and the surface float. 
Therefore each instrument must be designed 
to record internally and the mooring must be 
pulled in order to retrieve the records. In- 
dividual instruments are designed to provide 
about four months recording so recovery three 
or four times per year should suffice if the 
stations last well. The Bermuda line, if fully 
in place, involves about 150 instruments of 
various types, current meters, wind recorders, 
inclinometers, depth recorders and tension 
recorders. Each of these is capable of storing 
about 10,000 readings of the variable being 
measured. This leads to an ultimate data re- 
duction problem with a potential load of 
1,500,000 readings, most of which involve more 
than one measurement, i.e. a current measure- 
ment is both speed and direction and the direc- 
tional measurement is made up of two parts, the 
orientation of the instrument in the earth's 
magnetic field (compass) and the relative 
direction of the current as detected by a vane. 
Because of this potentially large data reduc- 
tion problem the recordings in each instrument 
are made in a digital format on photographic 
film. The film can then be scanned photo- 
electrically and the data buffered to a com- 
puting machine for processing. Photographic 
film was selected as the recording medium 
because many parallel data channels are re- 
quired and power consumption is less for this 
type of recording than for magnetic tape or 
other media. 
As an example of the instrumentation we 
may consider the current meter. This is shown 
in Figure 4. The instrument is cylindrically 
