Patent Publication Number: US-2010115820-A1

Title: Perforated slat trawl door

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to trawl doors, and, more particularly, to trawl doors adapted for stable, more efficient operation at high angles of attack. 
     BACKGROUND ART 
     A trawl is a large net generally in the shape of a truncated cone trailed through a water column or dragged along a sea bottom to gather marine life including fish. Methods and apparatuses for spreading a trawl trailed behind a moving towing vessel, frequently identified as “trawl doors,” are well known. Usually, a trawl door attaches to a towing vessel by a single main towing warp or other towing line secured to the trawl door near or at the trawl door&#39;s midpoint. The trawl then attaches to the trawl door by a pair of towing bridles, i.e. an upper and a lower towing bridle, respectively secured to the trawl door at or near opposite ends thereof. Trawl doors are also identified by other names, most commonly including “otter boards” and “doors”. Trawl doors, when used in the seismic industry are often referred to as “deflectors,” and may have several main “wings”, main “plates” and/or “slats.” 
     While a towed trawl door having a particular shape may operate stably throughout a range of angle of attack, when towed through water at a high angle of attack most trawl doors exhibit instability and/or low efficiency, i.e. high drag. It is important to note that usage and meaning of the term “high angle of attack” varies from fishery to fishery. Furthermore, trawl doors otherwise configured for a certain angle of attack when aboard ship ultimately fish at different angles of attack depending upon the lengths respectively of the sweep and/or bridles coupled to the trawl door. Similarly, the lengths respectively of a trawl&#39;s footropes and headropes can affect a trawl door&#39;s angle of attack while being towed through water. Moreover, how the towing vessel maneuvers can vary a trawl door&#39;s angle of attack. Lastly, the preceding factors that affect a towed trawl door&#39;s actual angle of attack do not do so independently. Rather, these factors act in concertedly in affecting a towed trawl door&#39;s actual operating angle of attack. 
     At a high angle of attack such as over thirty degrees) (30°), and especially at over thirty-five degrees (35°, most trawl doors exhibit instability and/or low efficiency, i.e. high drag. However, trawl doors commonly operate at such high angles of attack to create enough drag induced directional forces on the trawl doors so as to impart sufficient stability to the trawl door system to thereby maintain the trawl doors in a workable orientation. For example, when a towing vessel turns the inboard trawl door can become almost stationary relative to the water. As is readily apparent, slowing a trawl door down in relationship to the water reduces its spreading force, i.e. the trawl door&#39;s drag induced directional force. A similar situation can arise when a trawl door experiences a strong side current. Another condition which can cause trawl door instability occurs when some portion of the trawl contacts the sea floor. As is readily apparent, a trawl contacting the sea floor increases the force applied to the trawl door through the lower towing bridle in comparison with the force applied through the upper towing bridle. Stabilizing trawl doors when they operate under conditions such as those described above usually requires that the trawl doors operate at a high angle of attack. 
     A significant handicap of known trawl doors is that trawling vessels using trawl doors operating at a high angle of attack, such as in the Alaskan Pollock fishery, rarely make a “gear down” turn. Rather some trawl operators retrieve the trawl doors at or near the surface before making an efficient direction changing turn. If the trawl doors are not at or near the surface during a turn they tend to stall, i.e. loose their ability to spread and thus keep separate from one another. When the trawl doors lose their ability to spread they may tangle with each other, a phenomenon known as “crossing the doors”. Because remedying “crossed trawl doors” is a dangerous, and because it is also a time consuming procedure, some trawl operators prefer to retrieve the trawl doors at or near the surface before making a turn rather than risk “crossing the doors”. 
     It is well known that a particular species of fish usually concentrates at a certain ocean depth. Thus fishing at the certain ocean depth at which the fish species concentrates tends to avoid catching a significant quantity of unwanted fish species, i.e. by-catch. A drawback associated with retrieving trawl doors in order to turn efficiently is that the trawl correspondingly rises from the particular ocean depth at which the desired fish species concentrates. Thus, trawl door retrieval tends to catch unwanted species of fish (by-catch) while the trawl first ascends and then descends through various ocean depths during and after trawl door retrieval. Furthermore, many trawl operators find retrieving trawl doors in order to turn a tiresome affair. Such operators, therefore, often avoid turning, but rather remain on a course through portions of the ocean where the desired fish species are less concentrated. Unfortunately, towing a trawl through a less productive area of an ocean also tends to increased by-catch. For the preceding reasons, there exists a long felt need for a trawl door that operates stably and efficiently, e.g. exhibits lower drag, and/or generally exhibits a better lift constant “u” at high angles of attack, e.g. thirty degrees (30°) or more. 
     The instability exhibited by trawl doors when operating at a high angle of attack can be attributed to a phenomenon frequently referred to as “dynamic stall.” An airfoil or hydrofoil stalls when fluid flowing past the airfoil or hydrofoil separates therefrom. Stall may be a steady type wherein the location at which the flow separates from the airfoil or hydrofoil is essentially stationary. Alternatively, flow separation may be of an unsteady type wherein the separation location with respect to the airfoil or hydrofoil varies with time and flow conditions. In the scientific literature for fluid dynamics, dynamic stall of helicopter rotor blades and rotating stall of axial compressor blades provide well recognized examples of undesirable consequences resulting from unsteady flow separation. If unchecked, large oscillatory forces and moments produced in both types of stall can result in severe structural damage and erratic performance of such devices. 
     As described in “Evaluation of Turbulence Models for Unsteady Flows of an Oscillating Airfoil” by G. R. Srinivasan, J. A. Ekaterinaris and W. J. McCroskey, Computers &amp; Fluids, vol. 24, no. 7, pp. 833-861, the term dynamic stall usually refers to the unsteady separation and stall phenomena of aerodynamic bodies or lifting surfaces. As described in U.S. Pat. No. 6,267,331 (“the &#39;331 patent), a dominant feature characterizing dynamic stall on an airfoil or hydrofoil is a strong vortical flow, which begins near the leading-edge, enlarges, and then travels downstream along the foil. When a airfoil or hydrofoil reaches fairly high angles of attack, past the static stall angle limit, the resulting unsteady flowfield is characterized by massive separation and large-scale vortical structures. One important difference between this flowfield structure and that generated by the static stall is the large hysteresis in the unsteady separation and reattachment process. When dynamic stall occurs maximum values of lift, drag, and pitching-moment coefficients can greatly exceed their static counterparts, and not even the qualitative behavior of these conditions can be reproduced by neglecting the unsteady motion of the airfoil&#39;s or hydrofoil&#39;s surface. Typically, the problem of dynamic stall is addressed by some form of airfoil geometry modification (e.g. leading-edge slat), or boundary-layer control (e.g. blowing or suction), where these changes are geared specifically to the leading-edge region where the vortex originates. The &#39;331 patent states that all methods of dynamic stall control that have been attempted heretofore have been less than satisfactory. There is thus a widely recognized need for, and it would be highly advantageous to have, a more satisfactory method of dynamic stall control for airfoils and hydrofoils than methods now known in the art. 
     DEFINITIONS 
     ASPECT RATIO: means the Trawl Door Height relative to the Trawl Door Width. For example, a trawl door having a height of two (2) meters and a width of one (1) meter has an Aspect Ratio of 2:1 (two to one).
 
PROFILE: means the cross-sectional shape of a trawl door, or of a portion of a trawl door, viewed in a plane that is oriented perpendicularly across the trawl door&#39;s vertical dimension.
 
TRAWL DOOR: means any of a variety of essentially rigid structures having generally rigid deflectors (e.g. not formed of a foldable fabric as a kite) that is adapted for deployment in a body of water behind a towing vessel. More specifically, trawl door means any generally wing shaped and/or fin shaped device used to spread either a fishing net, such as a trawl, or to spread a seismic surveillance array and/or seismic array, such as used in making acoustic surveillance of a sea floor and sub-sea-floor, or to spread apart any other towed item, whether in air or sea. A trawl door usually attaches at a fore end to a terminal end of a main towing warp or other towing line depending from the towing vessel, and at an aft end to at least one other line itself ultimately attached to another towed item. In operation, trawl doors convert a portion of forward motion and/or energy imparted by the towing vessel into horizontally directed force for the purpose of spreading in a generally horizontal direction a trawl, seismic surveillance towed array complex, paravane line or the like.
 
TRAWL DOOR HEIGHT: the height of a trawl door is defined by the shortest distance between the trawl door&#39;s upper edge and the trawl door&#39;s lower edge. The Trawl Door Height measurement generally does not include any part of a purely weight shoe, wear plate, or the like, but rather relates to the portion of the trawl door&#39;s structure that is capable of efficiently generating lift and/or thrust.
 
TRAWL DOOR WIDTH: the width of a trawl door is defined by the shortest distance between the trawl door leading and trailing edges as taken from a profile of a portion of the trawl door. For trawl doors with straight leading and trailing edges, the width is generally the same everywhere along the vertical dimension of the trawl door. For a trawl door with a “swept back” configuration, the trawl door&#39;s width also may be expressed as an average of a sum of several trawl door width measurements taken at various profile locations located at varying positions along the vertical dimension of the trawl door, as such trawl doors typically have narrower widths at their extremities than at the middle thereof.
 
     DISCLOSURE 
     An object of the present disclosure is to provide a more stable trawl door. 
     Yet another object of the present disclosure is to provide a trawl door that operates more efficiently at a high angle of attack, such as at greater than thirty degrees (30°), and particularly greater than thirty-six degrees (36°) including forty degrees (40°). 
     Briefly, an improved trawl door adapted for being towed through water includes at least one main deflector. The main deflector has a profile formed by inner and outer surfaces. The profile of the main deflector spans a chord that extends between the main deflector&#39;s leading and trailing edges, and has a maximum thickness. The improved trawl door is characterized by including a permeable structure for bettering, in comparison with the trawl door lacking the permeable structure, at least one trawl door efficiency characteristic selected from a group consisting of:
         1. trawl door stability when the trawl door is towed through water at a high angle of attack;   2. trawl door drag;   3. a numerical value obtained by dividing a lift coefficient measured for the improved trawl door by a drag coefficient measured for the improved trawl door; and   4. noise generation.
 
At least a portion of the improved trawl door&#39;s permeable structure is situated adjacent to and separated from the outer surface of the main deflector, and between the main deflector&#39;s maximum thickness and its trailing edge.
       

     In one embodiment, a perforated slat, having a plurality of apertures formed therethrough, provides the permeable structure. Thus, the perforated slat permeable structure establishes a porous surface adjacent to the main deflector&#39;s outer surface. In another embodiment, a plurality elongated strips of solid material that are separated by a longitudinal gap therebetween provides the permeable structure. The elongated solid material strips, which have both a length and a width, have their length oriented mainly parallel to water flowing past the towed trawl door&#39;s main deflector. Correspondingly, the elongated solid material strips&#39; widths are oriented mainly orthogonal to water flowing past the towed trawl door&#39;s main deflector. 
     Advantages provided by a trawl door that employs a permeable structure in accordance with the present disclosure when operating at a high angle of attack, such as at greater than thirty degrees (30°) and particularly greater than thirty-six degrees (36°) including greater than forty degrees (40°), is that trawl door stability increases, the trawl door&#39;s angular operating range increases, and attainable trawl door lift and consequently trawl-mouth spreading force increases in comparison with the same characteristics exhibited by a conventional trawl door when configured for operation at a correspondingly high angle of attack. 
     Another advantage of the improved trawl door structures is less noise generation in comparison with conventional trawl doors. The improved trawl door structure produce significantly less wake turbulence compared to conventional trawl door structures. Less wake turbulence corresponds to less noise generation which is particularly advantageous when towing paravanes included in seismic surveillance arrays. Seismic surveillance uses arrays of microphones towed behind a vessel for collecting acoustic data for subsequent processing to produce images of underwater structures. As is readily apparent, paravane noise generation compromises the quality of underwater seismic surveillance images. 
     These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective drawing illustrating one embodiment for a trawl door in accordance with the present disclosure that includes only one (1) main deflector and that has straight leading and trailing edges, the disclosed trawl door includes a porous perforated slat disposed adjacent to, separated from, and supported from an outer surface of the main deflector; 
         FIG. 2  is a cross-sectional diagram taken along the line  2 - 2  in  FIG. 1  illustrating a profile of the trawl door depicted in that FIG.; 
         FIG. 3  is a plan view illustrating part of the trawl door depicted in  FIG. 1  taken along the line  3 - 3  in  FIG. 2 ; 
         FIG. 4  is a cross-sectional diagram that corresponds to the illustration of  FIG. 2  and that illustrates a specific configuration for the trawl door&#39;s perforated slat and main deflector providing detailed information about relative sizes for various curved components included in the trawl door; 
         FIG. 5  is a plan view similar to the illustration of  FIG. 3  that illustrates a specific configuration for the trawl door&#39;s perforated slat having a plurality of elongated, rectangularly-shaped perforations formed therethrough with the longest dimension of the perforations oriented parallel to the chord of the main deflector; 
         FIG. 6  is a plan view similar to the illustration of  FIG. 3  that illustrates another specific configuration for the trawl door&#39;s perforated slat having a plurality of elongated, rectangularly-shaped perforations formed therethrough with the shortest dimension of perforations through the perforated slat oriented parallel to the chord of the main deflector; 
         FIG. 7  is a plan view similar to the illustration of  FIG. 3  that illustrates yet another specific configuration for the trawl door&#39;s perforated slat having a plurality of circularly shaped perforations formed therethrough rather than rectangularly shaped perforations as depicted in  FIGS. 5 and 6 ; 
         FIG. 8  is a plan view similar to the illustration of  FIG. 3  that illustrates yet another specific configuration for the trawl door&#39;s perforated slat having a plurality of elongated, rectangularly-shaped perforations formed therethrough with some of the perforations having their longest dimension oriented parallel to the chord of the main deflector while others of the perforations have their shortest dimension oriented parallel to the main deflector&#39;s chord; 
         FIGS. 9A through 9C  are plan views of portions of the perforated slat illustrating respectively alternative round-shaped, prong-shaped and pointed-shaped ends for the elongated, rectangularly-shaped perforations formed through the perforated slat depicted in  FIGS. 3 ,  5 ,  6 , and  8 ; 
         FIGS. 10A and 10B  are perspective drawings respectively illustrating a top surface and under surface of a Vee-shaped (dihedral) trawl door in accordance with the present disclosure that includes two (2) abutting main deflector bodies joined together at the middle of the trawl door, and the trawl door also includes two (2) perforated slats which are respectively disposed adjacent to and separated from the outer surface of the respective main deflector bodies; 
         FIG. 11  depicts relationships existing among  FIGS. 11A ,  11 B,  11 C and  11 D, the combined  FIGS. 11A-11D  forming a spreadsheet that provides detailed technical information useful in constructing trawl doors in accordance with the present disclosure; 
         FIG. 12  is a perspective drawing illustrating a paravane adapted for inclusion in seismic surveillance array that includes four (4) main deflectors only one (1) of which includes a perforated slat; 
         FIG. 13  is a cross-sectional diagram taken along the line  13 - 13  in  FIG. 12  illustrating a profile of the paravane depicted in that FIG.; 
         FIG. 14  is a perspective drawing illustrating a paravane adapted for inclusion in seismic surveillance array that includes four (4) main deflectors each of which includes a perforated slat; and 
         FIG. 15  is a cross-sectional diagram taken along the line  15 - 15  in  FIG. 14  illustrating a profile of the paravane depicted in that FIG. 
     
    
    
     BEST MODE FOR CARRYING OUT THE DISCLOSURE 
     The perspective drawing of  FIG. 1  illustrates an improved trawl door in accordance with the present disclosure referred to by the general reference character  20 . The trawl door  20  includes a main deflector  22  having a leading edge  24  and a trailing edge  26 , best illustrated by the profile of the trawl door  20  depicted in  FIG. 2 . In the embodiment of the trawl door  20  illustrated in  FIGS. 1-3 , a cambered steel plate forms the main deflector  22 . For the particular profile illustrated in  FIG. 2 , the main deflector  22  has a maximum thickness  28  that is located approximately half way between the leading edge  24  and the trailing edge  26 . The steel plate forming the main deflector  22  has a cambered inner surface  32  and a cambered outer surface  34  which respectively span a chord  36  of the main deflector  22  that extends between the leading edge  24  and the trailing edge  26 . The trawl door  20  also preferably includes a leading edge lift enhancing structure consisting of a pair of cambered leading edge slats  42 A and  42 B that, similar to the main deflector  22 , are formed by cambered steel plates. A leading edge  44 B of the leading edge slat  42 B disposed furthest from the leading edge  24  of the main deflector  22  forms a leading edge of the trawl door  20 . A leading edge  44 A of the leading edge slat  42 A is disposed between the leading edge  44 B and the leading edge  24 . 
     In addition to the main deflector  22  and the leading edge slats  42 A and  42 B, the trawl door  20  also includes lower and upper end plates  48 A,  48 B. Opposite ends of to the main deflector  22  and the leading edge slats  42 A and  42 B are respectively fastened to the lower and upper end plates  48 A,  48 B, e.g. by welding, to establish and maintain the relationship among various parts of the trawl door  20 . Except for any mention of a permeable structure, the structure of the trawl door  20  as disclosed thus far is conventional and well known in the art. 
     The improved trawl door  20  further includes a permeable structure depicted in  FIGS. 1-3  and called a perforated slat  52  that is disposed adjacent to and separated from the outer surface  34  of the main deflector  22 . The perforated slat  52  extends from a trailing edge  58 , that is located near the trailing edge  26  of the main deflector  22 , part way over and separated from the outer surface  34  toward the leading edge  24  of the main deflector  22  to a leading edge  59 . Preferably, at least a portion of the perforated slat  52  is situated adjacent to and separated from the outer surface  34  of the main deflector  22  between the maximum thickness  28  of the main deflector  22  and the trailing edge  26  thereof. As indicated for the profile of the particular trawl door  20  depicted in  FIGS. 2 and 4 , the letter parameter “f” indicates the measure of the maximum thickness  28  extends from the chord  36  of the main deflector  22  to the main deflector  22 . 
     Similar to the main deflector  22  and the leading edge slats  42 A and  42 B, the perforated slat  52  depicted in  FIGS. 1-3  is formed by a cambered steel plate. Also similar to the main deflector  22  and the leading edge slats  42 A and  42 B, opposite ends of the perforated slat  52  are respectively secured to the lower and upper end plates  48 A,  48 B, e.g. by welding. The perforated slat  52  depicted in  FIG. 3  differs from the main deflector  22  and the leading edge slats  42 A and  42 B by being pierced by a plurality of elongated, rectangularly-shaped perforations  54 . Furthermore, due to the rectangularly-shaped perforations  54  piercing the sheet material of the perforated slat  52 , to make the trawl door  20  structurally sound the perforated slat  52  is preferably secured to the main deflector  22  via support structures welded at selected locations along its length. In the particular embodiment of the perforated slat  52  depicted in  FIGS. 1-3 , the rectangularly-shaped perforations  54  are arranged in parallel rows with their longer dimension oriented within thirty degrees (30°) of parallel to the chord  36  of the main deflector  22 , and preferably within 20 degrees (20°) and even more preferably within fifteen degrees (15°). 
     The cross-sectional diagram of  FIG. 4  illustrates a specific configuration for the perforated slat  52  and the main deflector  22  providing detailed technical information about relative sizes for various cambered components included in the trawl door  20 . Specific design information for the main deflector  22  and the perforated slat  52  appearing in  FIG. 4  scales from the length (“L”) of the chord  36  of the circular arc of the cambered main deflector  22 . The symbol “+” appearing between two numerical values in expressions in  FIG. 4  and subsequent FIGs. indicates a range of values that extends from the first numerical value to the second numerical value. A single asterisk (“*”) in  FIG. 4  denotes a value for the particular parameter so marked that has been empirically determined to yield the best improvement in lift for the main deflector  22  and perforated slat  52  having the structural relationships appearing in  FIG. 4 . The best values determined empirically for particular parameters when a ratio of the height of the circular arc-shaped main deflector  22  to the length of its chord  36  is in the range of 0.23 to 0.25 are tabulated below. 
     L W =(0.70 to 0.80)L where L is the length of the chord  36   
     h 1 =(0.045 to 0.075)L 
     h 2 =(0.040 to 0.075)L 
     h 4 ≧h 2    
     ΔL=(0.24 to 0.33)L 
     A double asterisk (“**”) in  FIG. 4  indicates the maximum permissible thickness for the perforated slat  52 . 
     The plan views of  FIGS. 5-8  illustrate various different configurations for apertures formed through the perforated slat  52  for the specific arrangement of the trawl door  20  depicted in the cross-sectional diagram of  FIG. 4 . The plan view of  FIG. 5  provides parametric values for a specific configuration of the rectangularly-shaped perforations  54  having the longest dimension of the rectangularly-shaped perforations  54  oriented parallel to the chord  36  of the main deflector  22 . The plan view of  FIG. 6  provides parametric values for a specific configuration of the rectangularly-shaped perforations  54  having the shortest dimension of the rectangularly-shaped perforations  54  oriented parallel to the chord  36  of the main deflector  22 . The plan view of  FIG. 7  depicts an embodiment of the perforated slat  52  having circularly-shaped perforations  56  formed through sheet material of the perforated slat  52 , and provides parametric values for such circular apertures. The plan view of  FIG. 8  provides parametric values for a specific configuration of the rectangularly-shaped perforations  54  some of which have their longest dimension oriented parallel to the chord  36  of the main deflector  22  while others have their shortest dimension oriented parallel to the chord  36 . 
       FIGS. 1-8  depict rectangularly-shaped perforations  54  or circularly-shaped perforations  56  arranged in parallel rows to provide the porous surface located adjacent to the outer surface  34  of the main deflector  22 . As depicted in  FIGS. 5-8 , forming the rectangularly-shaped perforations  54  with a length to width ratio in a range of 10:1 to 15:1 can be advantageous. However, in accordance with the present disclosure apertures formed through the perforated slat  52  may have shapes other than the rectangularly-shaped perforations  54  and/or circularly-shaped perforations  56 , and which differ in size, orientation and arrangement relative to the chord  36  and/or to the leading and trailing edges  24 ,  26  of the main deflector  22 . The plan views of  FIGS. 9A through 9C  depict portions of the perforated slat  52  illustrating, respectively, alternative shapes for short ends of rectangularly-shaped perforations  54  formed therethrough.  FIG. 9A  illustrates a rectangularly-shaped perforation  54  having a short end formed with a round-shape  112 .  FIG. 9B  illustrates a rectangularly-shaped perforation  54  having a short end formed with a prong-shape  114 . And  FIG. 9C  illustrates a rectangularly-shaped perforation  54  having a short end formed with a pointed-shape  116 . In general, a configuration selected for a particular embodiment of the trawl door  20  including the main deflector  22  and the perforated slat  52  and of the apertures which make the perforated slat  52  porous must be determined empirically, preferably by experimentally testing models of the trawl door  20  in a flume tank. 
     The perspective drawings of  FIGS. 10A and 10B  illustrate an improved Vee-shaped (dihedral) trawl door in accordance with the present disclosure referred to by the general reference character  60 . The trawl door  60  includes a upper trawl door section  62  and a lower trawl door section  64 . Individually, the upper trawl door section  62  and lower trawl door section  64  depicted in  FIGS. 10A and 10B  are very similar in structure to the trawl door  20  depicted in  FIGS. 1-3 . The upper and lower trawl door sections  62 ,  64  abut each other along a lower edge  62 LE of the upper trawl door section  62  that faces an upper edge  64 UE of the lower trawl door section  64  along a center plate  72 . Similar to the trawl door  20  depicted in  FIG. 1 , the trawl door  60  includes a lower end plate  48 A and an upper end plate  48 B. Corresponding exterior surfaces of the upper trawl door section  62  and lower trawl door section  64  respectively lie in different planes thereby providing the trawl door  60  with its Vee-shape, i.e. dihedral. Preferably, leading and trailing edges of the trawl door  60  are straight, i.e. not ‘swept back.’ 
     The upper trawl door section  62  includes an upper main deflector  22 U formed by a cambered steel plate, and that has an upper leading edge  24 U and an upper trailing edge  26 U. The upper trawl door section  62  also preferably includes a leading edge lift enhancing structure consisting of a pair of upper leading edge slats  42 AU and  42 BU that, similar to the upper main deflector  22 U, are formed by cambered steel plates. The upper leading edge slat  42 BU has an upper leading edge  44 BU that is disposed furthest from the upper leading edge  24 U of the upper main deflector  22 U. 
     The lower trawl door section  64  includes a lower main deflector  22 L formed by a cambered steel plate, and that has a lower leading edge  24 L and a lower trailing edge  26 L. The lower trawl door section  64  also preferably includes a leading edge lift enhancing structure consisting of a pair of lower leading edge slats  42 AL and  42 BL that, similar to the lower main deflector  22 L, are formed by cambered steel plates. The lower leading edge slat  42 BL has a lower leading edge  44 BL that is disposed furthest from the lower leading edge  24 L of the lower main deflector  22 L. The combined upper leading edge  44 BU of the upper leading edge slat  42 BU and lower leading edge  44 BL of the lower leading edge slat  42 BL form a leading edge  44 ′ of the trawl door  60 . Similarly, the combined upper trailing edge  26 U of the upper main deflector  22 U and lower trailing edge  26 L of the lower main deflector  22 L form a trailing edge  26 ′ of the trawl door  60 . Except for any possible description of a perforated slat, the structure of the trawl door  60  depicted in  FIGS. 10A and 10B  and as disclosed thus far is conventional and well known in the art. 
     The center plate  72  of the trawl door  60  depicted in  FIGS. 10A and 10B  is part of a load bearing frame that transmits towing loads from the towing vessel to the towed trawl or other item. Accordingly, when the trawl door  60  is assembled into a trawl system, a main towing warp  74  attaches to the trawl door  60  at any one of several different locations fore and aft along the center plate  72 . Similarly, a lower towing bridle  76 L attaches to one of several backstrop holes  78  that pierce the lower end plate  48 A of the trawl door  60  while an upper towing bridle  76 U attaches to one of several backstrop holes  78  that similarly pierce the upper end plate  48 B. 
     Note that the illustration of the trawl door  20  in  FIG. 1  omits the main towing warp  74 , and depicts only the lower towing bridle  76 L and the upper towing bridle  76 U. Note further that instead of the lower towing bridle  76 L attaching to the lower end plate  48 A and the upper towing bridle  76 U attaching to the upper end plate  48 B, for the trawl door  20  depicted in  FIG. 1  the lower towing bridle  76 L and the upper towing bridle  76 U both attach to backstrop holes  78  formed through plates which project outward from the outer surface  34  of the main deflector  22  and through the perforated slat  52  respectively near opposite ends thereof. 
     Similar to the trawl door  20 , the upper trawl door section  62  of the trawl door  60  further includes both a perforated upper perforated slat  52 U disposed adjacent to and separated from an outer surface  34  of the upper main deflector  22 U, and a perforated lower perforated slat  52 L disposed adjacent to and separated from an outer surface  34  of the lower main deflector  22 L. The upper perforated slat  52 U and the lower perforated slat  52 L respectively extend from near the trailing edge  26 ′ of the trawl door  60  partway over and separated from the outer surfaces  34  respectively of the upper main deflector  22 U and lower main deflector  22 L toward the upper leading edge  24 U and lower leading edge  24 L thereof. Similar to the upper main deflector  22 U, lower main deflector  22 L, the upper leading edge slats  42 AU and  42 BU and the lower leading edge slats  42 AL and  42 BL, the lower perforated slat  52 L and the upper perforated slat  52 U depicted in  FIGS. 10A and 10B  are formed by cambered steel plates. Furthermore, due to apertures piercing the sheet material of the lower perforated slat  52 L and upper perforated slat  52 U, to make the trawl door  60  structurally sound the lower perforated slat  52 L and upper perforated slat  52 U are respectively secured to the lower main deflector  22 L and upper main deflector  22 U at selected locations  82  along their respective lengths. The lower perforated slat  52 L and upper perforated slat  52 U both being pierced by apertures provide a porous surface adjacent to the outer surfaces  34  respectively of the lower main deflector  22 L and upper main deflector  22 U. 
     When during normal use trawl doors, particular Vee-shaped (dihedral) trawl doors, contact the side of an undersea cliff, canyon wall, or lean over from improper setting or an extremely strong side current, nearly all impact damage occurs near tips of the trawl door&#39;s leading edge. The perspective view of  FIG. 10A  best illustrates leading edge wear plates  86  that may be included in a trawl door immediately inboard of the lower and upper end plates  48 A,  48 B of the trawl door  60 . The wear plates  86  are formed by a second layer of steel laminated onto the material forming upper leading edge slats  42 AU and  42 BU and the lower leading edge slats  42 AL and  42 BL. Equipping the trawl door  60  and/or trawl door  20  with the wear plates  86  at distal ends thereof adjacent to the lower and upper end plates  48 A,  48 B increases the trawl door&#39;s useful service life. 
     The trawl doors  20 ,  60  may also include a mass weight plate, not illustrated in any of the FIGs, that attaches to the lower end plate  48 A. Addition of amass weight plate increases the stability of the trawl doors  20 ,  60  during field operations by permitting selecting an appropriate amount of weight for the intended trawl door altitude in the water column. 
     In accordance with the present disclosure, when the trawl door  20  or  60  is towed through water at a high angle of attack, the trawl door  20  or  60  operates stably and exhibits less drag than the trawl door  20  without the perforated slat  52 , or the trawl door  60  without the upper perforated slat  52 U and lower perforated slat  52 L. 
     INDUSTRIAL APPLICABILITY 
     A spreadsheet assembled by juxtaposing  FIGS. 11A-11D  in the manner depicted in  FIG. 11  provides detailed technical information useful in constructing trawl doors in accordance with the present disclosure. The spreadsheet formed by juxtaposing  FIGS. 11A-11D  includes numbered vertical columns  1 - 22  that extend from left to right. The bottom of column  1  at the left hand side of the spreadsheet depicts two (2) alternative shapes for apertures formed through the perforated slat  52  of trawl door  20 , or formed through the upper perforated slat  52 U and lower perforated slat  52 L of the trawl door  60 . These illustrations of shapes for apertures formed through the perforated slat  52 ,  52 U or  52 L include technical details about those particular shapes that are used elsewhere in the spreadsheet in providing additional detailed structural information. Column  2  in the spreadsheet depicts different profiles that may be used for the main deflector  22  of the trawl door  20  or for the upper main deflector  22 U and lower main deflector  22 L of the trawl door  60 . Similar to the illustrations in column  1 , these illustrations of shapes for the main deflector  22 ,  22 U or  22 L include technical details about those particular shapes that are used elsewhere in the spreadsheet in providing additional detailed structural information. 
     Beginning in column  3  and extending horizontally across the spreadsheet to column  22  are three (3) rows one above the other respectively labeled  1 ,  2  and  3  downward in FIG.  11 A′s column  3 , and similarly labeled adjacent to the left hand edge of  FIGS. 11B-11D . In columns  3  through  22  these three (3) rows provide technical details pertinent to the alternative perforation shapes illustrated in column  1  for the two (2) different types of profiles depicted in column  2 . Specifically, horizontal rows  1  and  2  in columns  3  through  22  provide technical details pertinent to the alternative perforation shapes illustrated in column  1  for two different configurations of the cambered plate profile depicted in the middle of column  2 . In columns  3  through  22  horizontal row  3  provides technical details pertinent to the alternative perforation shapes illustrated in column  1  for the complicated profile depicted at the bottom of column  2 . 
     Columns  4  through  11  in rows  1  through  3  provide ranges for relationships of preferred lengths to preferred widths for apertures formed through the perforated slat  52  of trawl door  20 , or formed through the upper perforated slat  52 U and lower perforated slat  52 L of the trawl door  60  with respect to the chord  36  and to the camber of the main deflector  22 ,  22 U or  22 L. As disclosed in columns  4  and  5  of  FIG. 11A , forming the rectangularly-shaped perforations  54  with a length to width ratio in a range of 20:3 to 50:3 can be advantageous. The notation “NP” appearing in columns  9  and  11  indicates that, presently, no definitive value has been ascertained for those particular parameters. 
     Column  12  of  FIG. 11B  in rows  1  through  3  provides a preferred range of porosities for the perforated slat  52  of trawl door  20  or the upper perforated slat  52 U and lower perforated slat  52 L of the trawl door  60  relative to the area of the cambered surface respectively of the main deflector  22 ,  22 U or  22 L. In general, it has been found that a total area for rectangularly-shaped perforations  54  and/or circularly-shaped perforations  56  formed through the perforated slat  52  of trawl door  20  or the upper perforated slat  52 U or lower perforated slat  52 L of the trawl door  60  that is between twenty percent (20%) and forty percent (40%) of the overall area of the perforated slat  52  of trawl door  20  or the upper perforated slat  52 U or lower perforated slat  52 L of the trawl door  60  achieves this disclosure&#39;s objectives and provides the advantages thereof. Particularly preferred for achieving this disclosure&#39;s objectives and providing its advantages is when the total area for rectangularly-shaped perforations  54  and/or circularly-shaped perforations  56  is between twenty percent (20%) and thirty percent (30%) of the overall area of the perforated slat  52 , upper perforated slat  52 U or lower perforated slat  52 L. 
     Similar to column  12 , column  13  provides a preferred range of porosities for the perforated slat  52  of trawl door  20  or the upper perforated slat  52 U and lower perforated slat  52 L of the trawl door  60  relative to the area OF the trawl door  20  including the main deflector  22  and the leading edge slats  42 A and  42 B, and the area of the trawl door  60  including the upper main deflector  22 U, the upper leading edge slats  42 AU and  42 BU, the lower main deflector  22 L and the lower leading edge slats  42 AL and  42 BL relative to the area of the cambered surface respectively of the main deflector  22 ,  22 U or  22 L. 
     Column  14  in  FIG. 11B  and column  16  in  FIG. 11C  provide information about a preferred range of distances parallel to the chord  36  from the leading edge  24  of the main deflector  22 ,  22 U or  22 L to the leading edge  59  of the perforated slat  52 ,  52 U or  52 L. In general, it has been found that a distance between the leading edge  24  respectively of the main deflector  22 ,  22 U or  22 L and the leading edge  59  respectively of the perforated slat  52 ,  52 U or  52 L parallel to the chord  36  of the main deflector  22 ,  22 U or  22 L that is between fifteen percent (15%) and sixty-five percent (65%) of a length of the chord  36  respectively of the main deflector  22 ,  22 U or  22 L achieves this disclosure&#39;s objectives and provides the advantages thereof. Particularly preferred for achieving this disclosure&#39;s objectives and providing its advantages for a cambered plate having the characteristics specified for row  1  of the spreadsheet is when the distance between the leading edge  24  respectively of the main deflector  22 ,  22 U or  22 L and the leading edge  59  respectively of the perforated slat  52 ,  52 U or  52 L parallel to the chord  36  of the main deflector  22 ,  22 U or  22 L is between twenty-five percent (25%) and thirty percent (30%) of the length of the chord  36  respectively of the main deflector  22 ,  22 U or  22 L. Particularly preferred for achieving this disclosure&#39;s objectives and providing its advantages for a cambered plate having the characteristics specified for row  2  of the spreadsheet is when the distance between the leading edge  24  respectively of the main deflector  22 ,  22 U or  22 L and the leading edge  59  respectively of the perforated slat  52 ,  52 U or  52 L parallel to the chord  36  of the main deflector  22 ,  22 U or  22 L is between twenty percent (20%) and thirty-five percent (35%) of the length of the chord  36  respectively of the main deflector  22 ,  22 U or  22 L. Particularly preferred for achieving this disclosure&#39;s objectives and providing its advantages for a complicated profile having the characteristics specified for row  3  of the spreadsheet is when the distance between the leading edge  24  respectively of the main deflector  22 ,  22 U or  22 L and the leading edge  59  respectively of the perforated slat  52 ,  52 U or  52 L parallel to the chord  36  of the main deflector  22 ,  22 U or  22 L is between thirty percent (30%) and sixty percent (60%) of the length of the chord  36  respectively of the main deflector  22 ,  22 U or  22 L. Similar to columns  14  and  16 , column  15  in  FIG. 11B  and column  17  in  FIG. 11C  provide information about a preferred range of distances parallel to the chord  36  from the leading edge  44 B of the leading edge slat  42 B,  42 BU or  42 BL to the leading edge  59  of the perforated slat  52 ,  52 U or  52 L. 
     Rows  1  through  3  of columns  18  through  20  provide information about a separation distance between the outer surface  34  of the main deflector  22 ,  22 U or  22 L and the perforated slat  52 ,  52 U or  52 L. Column  18  in rows  1  through  3  provides preferred ranges for those separation distances. Column  19  provides information for angles of attack less than 35 degrees (35°) indicating that the separation distances between the perforated slat  52 ,  52 U or  52 L and the outer surface  34  of the main deflector  22 ,  22 U or  22 L are preferably the same both at the leading edge  59  and trailing edge  58  of the perforated slat  52 ,  52 U or  52 L. However, as presented in column  20 , for angles of attack equal to or exceeding 35 degrees (35°) the separation distances between the perforated slat  52 ,  52 U or  52 L and the outer surface  34  of the main deflector  22 ,  22 U or  22 L can be:
         1. identical at the leading edge  59  and trailing edge  58  of the perforated slat  52 ,  52 U or  52 L; or   2. the distance at the leading edge  59  can exceed that at the trailing edge  58 .
 
In general, it has been found that a spacing between an inner surface  92  of the perforated slat  52 ,  52 U or  52 L at the trailing edge  58  thereof to the immediately adjacent outer surface  34  of the main deflector  22 ,  22 U or  22 L that is between seventy-five percent (75%) and one-hundred and fifteen percent (115%) of the spacing between the inner surface  92  at the leading edge  59  of the perforated slat  52 ,  52 U or  52 L to the immediately adjacent outer surface  34  of the main deflector  22 ,  22 U or  22 L achieves this disclosure&#39;s objectives and provides the advantages thereof.
       

     Column  21  provides preferred ranges for the area of the cambered surface perforated slat  52 ,  52 U or  52 L relative to the total area of all cambered surfaces of the trawl door  20  or the trawl door  60 . Similarly, column  22  provides preferred ranges for the area of the cambered surface perforated slat  52 ,  52 U or  52 L relative to the area of the cambered surface main deflector  22 ,  22 U or  22 L. 
     All detailed technical information appearing in  FIGS. 4-8  and in the spreadsheet appearing of  FIGS. 11A-11D  is hereby incorporated by reference as though fully set forth here. Accordingly, it is deemed that the detailed technical information appearing in appearing in  FIGS. 4-8  and in the spreadsheet appearing of  FIGS. 11A-11D  appears at this point in this patent application thereby providing a comprehensive disclosure of such information. 
     Rather than focusing on characteristics of perforations  54 ,  56  piercing the perforated slat  52 ,  52 U and  52 L, a description of the trawl door  20  or  60  which complements that set forth above is one which characterizes solid material of the perforated slat  52 ,  52 U and  52 L. For the illustrations of  FIGS. 2 and 5 , this complementary description of the perforated slat  52 ,  52 U and  52 L focuses on a plurality elongated strips  102  of solid material each of which extends between immediately adjacent columns of rectangularly-shaped perforations  54  from the leading edge  59  to the trailing edge  58 . For this characterization of the perforated slat  52 ,  52 U and  52 L, the strips  102  are:
         1. disposed adjacent to and separated from the outer surface  34  of the main deflector  22 ; and   2. have both a length and a width.       

     In the illustration of  FIG. 5 , the length of the strips  102  is oriented mainly parallel to water flowing past the main deflector  22  when towing the trawl doors  20 ,  60  through water, and the width of the strips  102  is oriented mainly orthogonal to that water flow. Considering in this way the strips  102  depicted in  FIG. 5 , the strips  102  have the following longitudinal gap separating them, length and width. 
     Gap d=(0.010÷0.015)L where L is the length of the chord  36  of the main deflector  22   
     Length the distance between the leading edge  59  of the perforated slat  52 ,  52 U and  52 L and the trailing edge  58  thereof 
     Width Δd=(1.5÷2.0) where d=(0.01÷0.015)L 
     Width Δd=(0.015÷0.030)L 
     A corresponding complementary description of  FIG. 7  in which circularly-shaped perforations  56  pierce the perforated slat  52 ,  52 U and  52 L is also possible. However, for such a description of the strips  102  their width is probably most conveniently characterized by the distance between immediately adjacent circularly-shaped perforations  56  while the longitudinal gap between immediately adjacent strips  102  is the diameter of the circularly-shaped perforations  56 . 
     Gap d=(0.015÷0.025)L where L is the length of the chord  36  of the main deflector  22   
     Length the distance between the leading edge  59  of the perforated slat  52 ,  52 U and  52 L and the trailing edge  58  thereof 
     Width Δd=d where d=(0.015÷0.025)L 
     Width Δd=(0.015÷0.025)L 
     Correspondingly, detailed technical information appearing in the spreadsheet formed by  FIGS. 11A-11D  characterizes other aspects of the strips  102  in this complementary description of the improved trawl doors  20 ,  60  provided by this disclosure. 
     Equipping a trawl doors  20 ,  60  with the strips  102  betters at least a numerical value obtained by dividing a lift coefficient measured for the improved trawl doors  20 ,  60  when towed through water by a drag coefficient measured concurrently for the improved trawl doors  20 ,  60  in comparison with a corresponding numerical value obtained for a trawl door:
         a. having a main deflector shaped identical to that of the improved trawl doors  20 ,  60 ; and   b. lacking the strips  102 .       

     Yet another complementary perspective for describing the perforated slat  52 ,  52 U and  52 L is to note that the strips  102  together with interconnecting pieces of solid material  104  which span between immediately adjacent pairs of the strips  102  form a mesh. Accordingly, instead of describing the permeable structure depicted in  FIGS. 1 through 8 ,  9 A and  9 B as a perforated slat  52 , it would be equally proper and equivalent to describe it as a mesh. 
     Pairs of  FIGS. 12 and 13 , and  14  and  15  respectively depict two ( 2 ) different configurations for paravanes that are adapted for use in spreading seismic surveillance arrays respectively referred to by the general reference characters  120  and  140 . Those elements of the paravanes  120 ,  140  depicted in  FIGS. 12 through 15  that are common to the trawl doors  20 ,  60  as depicted in  FIGS. 1 through 8 ,  9 A and  9 B carry the same reference numeral distinguished by a prime (“′”) designation. As depicted in  FIGS. 12 and 14 , a pair of bridles  124 A couple fore and aft locations on an upper end plate  48 B′ respectively of the paravanes  120 ,  140  to a main towing warp  74 ′. Similarly, a pair of bridles  124 B couple fore and aft locations on a center plate  72 ′ respectively of the paravanes  120 ,  140  to the main towing warp  74 ′. And finally a pair of bridles  124 C couple fore and aft locations on a lower end plate  48 A′ respectively of the paravanes  120 ,  140  to the main towing warp  74 ′. 
     In  FIGS. 12 and 13 , the paravane  120  includes four (4) upper main deflectors  22 UA′,  22 UB′,  22 UC′ and  22 UD′ that are located between the upper end plate  48 B′ and the center plate  72 ′. The paravane  120  also includes four (4) lower main deflectors  22 LA′,  22 LB′,  22 LC′ and  22 LD′ that are located between the center plate  72 ′ and the lower end plate  48 A′. As depicted in  FIGS. 12 and 13 , only the upper main deflector  22 UD and lower main deflector  22 LD of the paravane  120  respectively have a perforated slat  52 UD′ and perforated slat  52 LD′ situated adjacent to and separated from the outer surfaces  34 ′ of the upper main deflector  22 UD and lower main deflector  22 LD respectively. Alternatively, as depicted in  FIGS. 14 and 15 , each of the upper main deflectors  22 UA′,  22 UB′,  22 UC′ and  22 UD′ included in the paravane  140  has a perforated slat  52 UA′,  52 UB′,  52 UC′ and  52 UD′ respectively situated adjacent to and separated from the outer surfaces  34 ′ of the upper main deflectors  22 UA′,  22 UB′,  22 UC′ and  22 UD′ respectively. Similarly, each of the lower main deflectors  22 LA′,  22 LB′,  22 LC′ and  22 LD′ included in the paravane  140  has a perforated slat  52 LA′,  52 LB′,  52 LC′ and  52 LD′ respectively situated adjacent to and separated from the outer surfaces  34 ′ of the lower main deflectors  22 LA′,  22 LB′,  22 LC′ and  22 LD′ respectively. While trawl doors  20 ,  60  usually include only a single main deflector  22 ,  22 U,  22 L, in principle the trawl doors  20 ,  60  could include several main deflectors  22 ,  24 U,  24 L similar to those depicted for the paravanes  120 ,  140 . 
     Although the present disclosure has been described in terms of presently preferred embodiments, it is to be understood that such descriptions are purely illustrative and are not to be interpreted as limiting. The trawl door  20  illustrated respectively in  FIGS. 1-8  and  10 A and  10 B is a pelagic (midwater) trawl door. However, a trawl door in accordance with the present disclosure may be a bottom trawl door, or a deflector used in seismic surveillance, where high angles of attack are common for the trawl door or deflector. Generally, a trawl door in accordance with the present disclosure may be similar to any trawl door construction known in the art with the addition of perforated slat  52 ,  52 U and  52 L. Accordingly, a trawl door in accordance with the present disclosure may be either Vee shaped or straight, and may, as well, include or omit one or both of the leading edge slats  42 A and  42 B, or include more than two (2) leading edge slats. Similarly, the main deflector  22  of a trawl door in accordance with the present disclosure may have a wing shape cross-sectional profile, and may include or omit mass weight plates, etc. 
     The disclosed improved trawl doors  20 ,  60  have more outboard weight than conventional trawl doors. To accommodate the greater outboard weight, the trawl doors  20 ,  60  must have the connection point for the main towing warp  74  positioned differently along the center plate  72  than for a conventional trawl door so improved trawl doors  20 ,  60  remain an upright with a lesser mass weight plate. 
     Furthermore, the position of backstrop holes  78  must be properly located so the trawl doors  20 ,  60  operate at a desired angle of attack, usually approximately thirty-seven degrees (37°) to forty degrees (40°). Because the trawl doors  20 ,  60  when operating at a high angle of attack increases trawl-mouth spreading force in comparison with the same characteristics exhibited by a conventional trawl door, correspondingly the larger trawl mouth opening applies more force to the backstrop holes  78  via the towing bridles  76 L,  76 U. Therefore, configuring the trawl doors  20 ,  60  to operate at a desired angle of attack requires properly positioning the backstrop holes  78  to compensate for the greater force applied to the trawl doors  20 ,  60  via the towing bridles  76 L,  76 U. 
     Consequently, without departing from the spirit and scope of the disclosure, various alterations, modifications, and/or alternative applications of the disclosure will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the disclosure.