Depth-recording with Plankton-nets

Abstract

THE concise account given by Mr. F. S. Russell in NATURE of April 25, of the behaviour of ring-trawl nets when towed, enables an attempt to be made to solve the paradox of his diagrams. The shape of the warp during each haul may be compared with an imaginary catenary of reference. For this purpose the resistance of the net may be replaced by a horizontal force acting at the lowest point of an imaginary warp constituting a true catenary, and use may be made of Mr. Russell's observation that it is the practice to keep the angle of entry of the warp into the water constant at 40° With a constant angle of 40°, the ratio of bight to dip of a true catenary is 5.4954, and the ratio of span to dip is 4.9955, bight and span being measured between supports at a common level. In terms of these two constants, and of the particulars of the wire-rope, may be calculated the tension at the winch, the tension at the lowest point of the catenary, and the resistance of the warp through the water. Moreover, from the given particulars of the net an estimate may be made of its resistance for any required speed. Then, by equating the expression for this resistance, to the tension at the lowest point of the catenary, the speed through the water may be estimated. At high speeds there would be disturbances; but as Mr. Russell states that the engine was run “dead slow”, there is sufficient probability of an approach to a solution that will not encroach too far into the region of piscatorial credulity, to justify this method of interpretation.