Oceanographic vessels are sometimes anchored in deep water for periods ranging from part of a day to as much as two weeks for the purpose of measuring currents at subsurface levels and for obtaining repeated observations on the vertical distribution of properties at a single locality. Such observations are made to confirm the validity of currents computed from the distribution of density and to obtain measurements of the water movements associated with the tides, internal waves, and other periodic and aperiodic disturbances. Knowledge of the fluctuations in the vertical distribution of properties is valuable in establishing the significance of single sets of observations and in the analysis of the character of internal disturbances.
For the accurate measurement of currents a fixed point of suspension is required, but this condition cannot be fulfilled by a vessel anchored in depths of several thousand meters. Not only does the vessel swing in a relatively large arc, but it also tends to ride up on the cable and then fall away again. In addition, the anchor usually drags somewhat in the soft bottom. So far, it has not proved practical to anchor in deep water with more than one cable. In order to eliminate as far as possible the effect of the vessel's movements on the current measurements, the tension on the cable is noted, detailed records are made of the direction and velocity of the wind and surface currents, and numerous astronomic fixes are taken. The movements of the anchored vessel are generally not significant when making repeated serial observations to
The first vessel to be anchored successfully at great depths was the U.S. Coast and Geodetic Survey steamer Blake, which in 1888–1889, under the command of Lieutenant J. E. Pillsbury, carried out studies of the Gulf Stream (Pillsbury, 1891). The Blake was anchored at thirty-nine localities in depths as great as 4000 m, where currents were measured and subsurface temperatures were determined. Under the direction of Helland-Hansen the Michael Sars and Armauer Hansen have been anchored at great depths in the North Atlantic, and other vessels have since been anchored at great depths elsewhere. The Meteor (Spiess, 1932b), Willebrord Snellius (Perks, in Pinke, 1938), and Atlantis (Seiwell, 1940) have all anchored in depths greater than 4500 m for periods of from several hours to as much as two weeks. The greatest depth in which a vessel has been anchored is about 5500 m, at which depth the Meteor anchored for two days.
Because of the weight of the wire rope suspended in the water, extremely strong hoisting gear and deck fittings are required. The wire ropes are generally tapered multistrand steel ropes of a special nonkinking type. Some have hemp cores for part of their length. The ropes used on the Blake, Meteor, and Snellius had a diameter at the free end of about 3/8 inch, which increased to about 5/8 inch at the winch end. Near the anchor, manila rope, chain, or special cable is used to avoid kinking the wire rope on the bottom. Various types of anchors have been used, either singly or in pairs, which weigh between 400 and 500 lb. The anchors are either of standard patterns with somewhat enlarged flukes or they are of the mushroom type. The weight of the anchors is apparently not very important. The E. W. Scripps has anchored successfully in depths of about 1600 m using a Danforth anchor weighing only 40 lb. The hoisting is usually done with a steamor power-driven capstan head of large diameter, the wire rope being wound under tension on a winch drum. As the strains may exceed several tons, all fittings must be of stout construction. The wire rope is usually led through an accumulator to ease sudden strains, over a dynamometer, through a braking device, and then over a large pulley or roller mounted on the bow. A meter wheel is necessary to measure the amount of rope payed out. The equipment and its use are described in the references already cited.
When anchored in the strong current of the Gulf Stream, the Blake used a scope (ratio between the amount of wire rope payed out and the depth of water) of two or three, but for anchoring in the open sea scopes between 1.1 and 1.6 have been used. The smaller the scope, the less will be the movement of the vessel, but the surface current and the wind and sea will determine the needed scope.