A New Concept in Steerable Tow-Fish Design

Abstract

INTRODUCTION The study of how fish behave in relation to fishing gear has become increasingly important. The Danish Fisheries Technology Institute (DFTI) is engaged in a major research project on this theme, with particular emphasis on video observation of fish in relation to cod ends, including the survival of small fish that escape through the meshes, to help in the design of more selective fishing gear. Also the effect of fishing gear on the sea bed IS being studied. For this project DFTI needed a steer able tow-fish carrying a video camera with ±180° horizontal and ±90°vertical line of sight capability and able .to be steered into any position near the gear or even In the mouth of the trawl. The vehicle was required to be stable and highly maneuverable in the speed range 1 to 6 knots.. With 500 m towing cable it should be capable of flying up to 80 m on either side of the track of the tow Ship, from the surface to the sea bed in a water depth of up to 200 m. At the same time it had to be compact, light and robust for ease of handling from a small fishing boat, and to be "snag free" after coming into contact with trawls or nets. Market research showed that no existing vehicle would satisfy the specifications set up by .DFTI. However, the Danish Maritime Institute (DMI), In an badvisory capacity, felt confident that a vehicle that would meet the specification could be developed. DMI was then commissioned to develop and subsequently to build a prototype vehicle. The experience with the prototype shows that it is easy to handle, highly maneuverable and very stable in flight. These qualities make the new type. of tow fish very relevant for various offshore applications, especially pipeline inspection. DEVELOPMENT OF DESIGN CONCEPT Hydrodynamic Principles During initial concept discussions it had been agreed, that in addition to meeting the demands for stability and control over a large speed range all for high "stand-off" capability, it would be beneficial If the new tow-fish would also:Maintain nearly constant attitude in the water whether flying deep or high, following the ship track or off-set from it.Have independent control of horizontal and vertical position (for simplicity and easy manual control). Simple reasoning, as well as previous experience with Para vanes and several tow-fish, convinced the Author that the following rules should then be followed, for the main reasons indicated:The vehicle should be nearly neutrally buoyant. Reason: If not fulfilled the "stand-off" capability at low speed would be severely restricted.The centre of buoyancy, B, should be located considerably higher than the centre of gravity, G, to give good roll and pitch stability. Reason: With no active control of roll and pitch, a high "static" roll and pitch stability is essential.Considerable effort should be made to ensurethat the vehicle has "hydrodynamic symmetry" about a vertical and a horizontal plane of symmetry. The tow-point should be on the forward end of the vehicle on the intersection line between these planes of symmetry.