Removable Hydrofoils Which Permit Surface And Bottom Sampling With A Single Otter Trawl

A rapidly installable and removable hydrofoil permits a single otter trawl to be used for both surface and bottom sampling. The hydrofoil is airplane-wing shaped in cross section and is three feet long, ten inches wide with a two inch maximum thickness. It is mounted on a fifteen degree wedge and secured to the top edge ofeach otter board. Conversion of doors from one mode of sampling to the other can be done in about one minute. As part of the Maryland anadromous fish studies, I young-of-the-year c1upeids and other species were collected by otter trawling on the bottom and surface of the nursery area studied. The same net and doors were used for both surface and bottom sampling. This paper details the attachment of hydrofoils to the otter boards to permit the gear's use in surface collection. An alternative method has been derived by Trent (1967), however, he did not indicate whether the same boards could be used for both surface and bottom work. Materials: The trawl employed was a 16-foot headrope, semi-balloon, modified shrimp (otter) trawl. The headrope carried four 1-12" x 2-12" ark floats as furnished; five additional floats were added to prevent sagging after experimentation. The dry weight of the net is 17 pounds. Each mahogany otter board, shod with a steel runner, measured 24" x 12" x I", and weighted 25-* pounds including its bridle chain and hydroplane. The boat used for collection was a 16-foot Boston Whaler powered by a 40 horsepower Johnson outboard. The boat made three knots at full throttle while towing. Construction of Hydrofoils: Each hydrofoil or plane was milled from a dressed plank measuring 2" x 10" x 3". It was shaped to approximate and airplane wing in cross section (See Figure I). Tips of each plane were left square. A 15 degree wedge was attached to the bottom of each plane at its center. The apex ofthe wedge was flush with the trailing edge of the plane. Holes were bored through the plane and wedge to accomodate 5/ 16" x 6" lag bolts. The holes were spaced approximately two inches apart, with the forward one about two inches behind the leading edge of the plane. (See Figure I for details) Using the holes in the plane and wedge for guides. slightly smaller holes, snug for the bolts, were bored in the top edge of the otter boards. The heads on the lag bolts were sawed off and the tops of the bolts threaded to receive butterfly or wing nuts. The after bolts, being inserted through the thinner, trailing wedge had to be threaded farther down the shaft than the forward bolts. When the wedge and plane are joined by nails, they can be put on or removed from the door as a unit. Butterfly nuts and washers hold the assembly firmly on the door. Use of the planes for surface trawling: Time required to apply or remove planes and thus convert the trawl from one mode of sampling to the other is about one minute.

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