A towline depressor (2) having forward and rearward ends with a main body portion (4) at the forward end. Wings (6) and (8) extend from the body with stabilizing fins (10) and (12) depending from the wings. A dorsal fin (14) having holes (16), (18) and (20) extends from the top center of the body for attaching the depressor to a towline. The holes permit attaching the towline at varying points relative to the center of gravity of the depressor.

FIELD OF THE INVENTION 
This invention relates to hydrodynamic equipment in general and, more 
specifically, to non-planar bodies which are employed to maintain an 
object which is being towed through water, at a predetermined depth. 
BACKGROUND OF THE INVENTION 
Such bodies are sometimes called "towed underwater instruments" or, when 
employed as fishing aids, as "towline depressors". They are used with 
downriggers in underwater research activity wherein scientific instruments 
such as seismic recording devices are towed, as well as in deep trolling 
for fish. 
One objective of the invention is to troll or drag an object through the 
water constantly at a predetermined depth below the surface but above the 
bottom. In the research fields the object being towed is generally a 
measuring instrument and in fishing it is a lure or bait. 
Traditionally, constant depth trolling or towing has been maintained by 
towing a sphere or ball which is usually made of lead. Because of its 
symmetry surface the lead ball provides no net "lifting" effect but is 
employed because of its extremely high density and its low drag 
coefficient. 
In this specification it should be understood that "lifting" or "lifting 
effect" really means "sinking" or "sinking effect" , that is, negative 
lifting or causing the object being towed to stay below the surface. 
One of the negatives involved in using a lead ball is its extreme weight. 
Another negative is the drag of the sphere through the water which reduces 
its effectiveness at depths where actually it would be desired to tow the 
instrument or bait. The drag on the ball is a direct function of its speed 
through the water and the operating depth is also a directed function of 
speed at which the ball is towed. When not in motion, the ball hangs 
vertically downward at the full depth of its towline and as towing speed 
increases the ball moves upwardly into a position directly behind the boat 
or vehicle which is towing it. The higher the towing speed the higher the 
ball rides in the water. This is a disadvantage. 
This action may be likened to the way a lighter than air balloon acts when 
tethered by a string. When there is no wind the balloon rises vertically 
(i.e., at ninety degrees to the ground). Its height is limited only by the 
length of its string. As the wind begins to blow, the angle of the string 
becomes less than ninety degrees and the balloon moves closer to the 
ground, still limited by its string which now is likened to a hypotenuse. 
It is an object of this invention to produce a towline depressor which, 
through its hydrodynamic design will, when towed through water, remain at 
a substantially constant depth throughout a range of towing speeds. 
Another object of this invention is to produce a towline depressor the 
towing depth of which may be determined solely by the length of the 
towline through a range of towing speeds.

SUMMARY OF THE INVENTION 
Invention resides in a towline depressor which has a main body portion 
located at its forward end. There are a pair of wings extending laterally 
from the main body portion and a stablizing fin depends from each wing at 
the rearward portion of the depressor. A towline attaching dorsal fin 
extends upwardly from the top center of the main body portion. 
The dorsal fin has a series of spaced holes for attaching a towline. The 
hole which is nearest to the forward end of the depressor is located 
forward of the center of gravity of the depressor. The center of gravity 
of the depressor itself is located between the hole nearest to the forward 
end and the hole which is next adjacent to it. The center of gravity of 
the depressor is also closer to the forward end than the theoretical 
center of pressure. 
Various dimentional relationships exist relative to the overall size of the 
depressor and location of the towline attaching holes. When the overall 
length of the depressor is 13 units of length, the hole nearest to the 
forward end is spaced approximately 2.625 units of length from the front 
end and with this same size depressor, the hole next adjacent to the hole 
nearest to the forward end is space approximately 3.25 units of length 
from the forward end. A third hole is spaced 4.125 units of length from 
the forward end. The center of gravity is spaced approximately 3 units of 
length from the forward end, i.e., between the first and second holes. 
The above and other features of the invention, including various novel 
details of construction and combinations of parts, will now be more 
particularly described with reference to the accompanying drawings and 
pointed out in the claims. It will be understood that the particular 
towline depressor embodying the invention is shown by way of illustration 
only and not as a limitation of the invention. The principals and features 
of this invention may be employed in varied and numerous embodiments 
without departing from the scope of the invention. 
DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, a towline depressor generally indicated 2, embodying 
the features of my present invention will be seen in perspective. While it 
is similar in external appearance to a towline depressor disclosed in my 
co-pending design patent application Ser. No. 895,748 filed Aug. 12, 1986, 
now U.S. Pat. Des. 308,558, issued Jun. 5, 1990, its fluid dynamic 
properties, size and weight differ substantially from the prior device. 
The towline depressor is also seen in detail in FIGS. 2 through 7. It has a 
main body portion 4, a pair of laterally extending wings 6 and 8 which 
come together centrally of the body 4 at a negative dihedral angle .alpha. 
(see FIG. 5). Depending from the wings 6 and 8 are stabilizing fins 10 an 
12, respectively. A towline attaching dorsal fin 14 having holes 16, 18 
and 20, extends upwardly from the top center of the main body portion 4. 
The underside of the body portion 4 together with the wings 6 and 8 from a 
concave surface 22 (FIG. 6). 
Superposed on FIG. 1 are three principal axes, X,Y, and Z, intersecting at 
the theoretical center of gravity CG/d of the depressor, or "wing" as it 
is sometimes called. The X, or lateral axis passes side-to-side through 
the depressor 2 and it is about this axis that the depressor would pitch 
when towed, thus it is also known as the pitching axis. The Y axis passes 
from front to rear axially through the center of the depressor and is the 
rolling axis. The Z axis passes vertically through the depressor and 
defines the yawing axis. 
As seen in FIGS. 7-12, located in the main body portion 4 of the depressor 
2 is a shaped weight 30. The dorsal fin 14, for attaching the towline 32, 
is intrical with the weight 30 is made preferably of non-corrosive 
material, such as stainless steel. In effect, the fin, only dorsal portion 
which is seen, is inverted "T"-shape. The weight 30, which is of lead or 
other equivalent material, is cast around the top of the "T" so that only 
the dorsal portion 14 extends outwardly and upwardly as shown. 
In plan view, as seen in FIG. 9, the weight is essentially triangular 
shaped having a nose 34. As viewed from the side in FIG. 8, the nose 34 
will be seen to be bulbous. The weight extends toward the right, as viewed 
in FIGS. 8 and 9, and terminates in an edge 36, which as seen from the 
rear in FIG. 10, is arcuate. The undersurface is concave as seen at 38. 
The weight is located within the depressor 2 by molding the plastic around 
the weight with the dorsal fin 14 extending upwardly and out of the 
plastic at the top of the depressor. 
Again referring to FIGS. 2 through 6, the leading edges 42, 44 of the 
depressor, which in part define the bulbous front portion 40, each form an 
angle of approximately 25.degree. with the longitudinal axis Y. 
The trailing edges 46 and 48 each form an angle of approximately 80.degree. 
with the longitudinal axis Y and are inclined downwardly and rearwardly at 
an angle of approximately 60.degree. (FIG. 3) with the upper edge 50 of 
the body 4 and with the longitudinal axis y with which the edge 50 is 
parallel. 
Depending from the wings (6 and 8) are a pair of stabilizing fins 52 and 
54. The leading edge of the fins are inclined at an angle of approximately 
22.degree. with the upper edge 50 and the axis y. The wings (6 and 7) are 
shaped to the reverse dihedral angle X which is approximately 23.degree. 
from the horizontal, as the X axis 5. The fins extend downwardly and 
outwardly, as seen in FIG. 4, at approximately 13.degree. from a plane 
passing vertically through the center of the depressor 2. 
As also seen in FIG. 4, the lower portion of the body slopes upwardly from 
the horizontal at approximately 23.degree.. 
It will be appreciated that two depressors embodying the features of the 
present invention may be made in many different sizes and weights, but it 
is the relationship of the size and weight which is important. 
Hereinafter, a size-weight relationship will be established in units of 
size and units of weight which may be increased or decreased, each 
proportional to the other. 
The overall length of the tow depressor from the nose 60 to the trailing 
tips 64 of the fins 52, 54 is 13 units of length. The maximum width of the 
tow depressor is 7 units. The maximum vertical dimension at its highest 
part is 2.5 units. The vertical dimension of the bulbous nose portion is 
1.5 units. The distance from the lower front of 62 of the fins 52, 54 to 
the trailing tips 64, is 5 units. 
As stated above, there are three attaching holes 16, 18 and 20 in the 
dorsal fin 14 to which the towline may be attached. The leading end of the 
dorsal fin is approximately 2.25 units from the nose 60. The center of the 
first hole 16 is 2.625 units from the nose; the second hole 18 is located 
3.25 units from the nose; and the third hole 20 is located 4.125 units 
from the nose. 
The relationship of the holes 16, 18 and 20 to the center of gravity of the 
weight CG/w 30 (FIG. 8) and the center of gravity of the CG/w depressor 2 
is important. The center of gravity of the weight CG/w 30 as seen in FIG. 
8, is located vertically beneath the hole 16. However, the center of 
gravity CG/d of the towline depressor 2 is located below the attaching 
holes 16 and 18, and substantially half-way between them, or 3 units from 
the front end 60. As a result, if a towline is attached to the forward 
most hole 16, the towline depressor will tip upwardly at the front or 
leading end 60. If attached in the hole 18, it will tip slightly 
downwardly at the front end 60 and if attached in the rearward most hole 
20, it will tip substantially downwardly at the front end 60. Thus, at a 
high trolling speed hole 16 is used and at a slower speed, the hole 20 is 
used. But, this towline length determines the depth of troll, regardless 
of speed. 
The towline depressor or "wing" as it is also called, is shown in Section 
along its longitudinal axis y in FIG. 7. The forward region or nose 
portion 4 includes a characteristic airfoil/hydrofoil shape designed to 
generate lift according to classical theory of lift (e.g., Bernoulli 
theory). The section shown reduces to a simple flat at 5 along the y axis 
and is convex as at 7. This area will generate lift (negative) both as 
described by lifting theory and due to forces applied by the fluid to the 
surface of the wing. 
It is usual to consider the total lift as acting at one theoretical point 
along the y axis or along the chord line, known as the center of pressure, 
CP/d. The resultant lift vector (downward) acting at this point is 
resolved into two components, lift acting normal to the free stream fluid 
flow and total drag acting in the direction of free stream flow. Total 
drag is comprised of several components due to the geometry and surface 
characteristics of the depressor 2, and includes induced drag due to lift. 
These forces, combined with the forces applied by the towline and the 
trailing rigging such as a fish line, and due to gravity, combine in a 
force balance that determines the equilibrium position of the device. FIG. 
1 describes the net forces acting on the device at the three axes that are 
naturally brought into equilibrium in operation. 
Lift (negative) increases with the angle of attack (FIG. 7) of the "wing", 
i.e., the angle between the free stream velocity vector and the chord line 
of the wing section. By shifting the towline attachment into each 
successive hole 16, 18 and 20, from front to rear, the angle of attack 
assumed under load will increase, thereby increasing negative lift or 
downforce on the towline. This added negative lift is sometimes required 
to achieve the same depth for increased trailing loads. Alternatively, for 
a given trailing load, the depth of operation may be selected in this 
manner. The location of the towline attachment holes 16, 18 and 20 is 
selected such that an angle of attack is never achieved that would stall 
the system or allow cavitation due to deparation of water flow from the 
surface of the wing. 
The wing does not include any control mechanisms. In order to assure 
positive, stable operation without control input, the wing or depressor is 
designed to provide inherent restoring action in the event of any 
perturbation from equilibrium conditions. Such perturbation may occur when 
wing speed changes due to changes in throttle setting, wave motion at the 
surface, turning accelerations, fish strikes, etc. 
The Wing is shown in FIG. 1 illustrating three primary axes of motion X, Y 
and Z and related displacement planes. 
The force balance described above relates primarily to stability in the 
pitching plane, and to stability in the other planes of motion (yaw and 
roll). In general, no direct loads are applied in yaw and roll, although 
water currents and side loads applied by striking fish, for example, can 
disturb the system in these planes. 
As shown in FIG. 7, the main body portion 4, which includes the weight 30 
has the weight located in relation to the rest of the system so as to 
place the system or depressors center of gravity, CG/d, under all loading 
conditions, ahead of the center of pressure CP/d of the system. Because 
the theoretical center of pressure CP/d is a function of the co-efficient 
of lift, it will shift along the chord line with changes in angle of 
attack. The center of pressure on an object in a liquid is the point where 
the application of a single support will balance the liquid pressure. The 
location of the towline attachment is, therefore, limited such that the 
angle of attack is constrained to an operating range that maintains a 
condition of longitudinal stability. 
The wing is designed to provide positive static longitudinal stability; any 
displacement of the longitudinal axis y in the pitching plane will alter 
the force balance such that a net restoring force returns the depressor to 
a stable equilibrium position. This imposes the condition that the net 
force due to gravity, resolved at the CG/d, in combination with the force 
due to lift, resolved at the CP/d, be forward of the point of application 
of towline force (point of attachment) in all cases, further ensuring 
longitudinal stability under all loading conditions. 
The broad, relatively flat area 5 provided outboard of the lifting section, 
as an extension of it, and trailing rearward from it, provides a dynamic 
restoring, pitching moment and damping sufficient to quickly reduce 
longitudinal dynamic oscillation due to changes in velocity, thereby 
restoring equilibrium under static stability. 
The depressor is designed to provide positive static stability in yaw; any 
displacement along the normal axis z in the yawing plane will alter the 
force balance in this plane, such that a net restoring moment returns the 
wing to a stable equilibrium position. Any vertical surface will 
contribute to the net restoring moment in this circumstance. This imposes 
the condition that the net hydrodynamic center of the vertical stabilizer 
is located sufficiently rearward of the system center of mass so as to 
ensure longitudinal stability under all loading conditions. 
When in yaw, the vertical stabilizers will be at some angle of attack 
relative to free stream and a lift will result along these surfaces. The 
position of the stabilizers relative to the center of mass of the system 
provides a net restoring moment that acts in a direction to restore 
alignment of the device with free stream. Factors affecting the magnitude 
of the restoring force dur to the stabilizers include the area, section, 
aspect ratio and sweep-back. 
The wing is designed to provide positive static stability in roll; any 
displacement of the lateral axis x in the rolling plane will alter the 
force balance in this plane such that a net restoring moment returns the 
Wing to a suitable equilibrium position. 
The principal contributions to overall lateral stability include dihedral 
angle .alpha. of the wings 6 and 8, the sweep-back of the wings, vertical 
location of the wings with respect to the body 4 of the wing or depressor, 
and the keel surface due to the vertical stabilizer and towline attachment 
place. 
The preferred embodiment includes the substantial dihedral angle .alpha. of 
the approximately 23.degree., as shown in FIG. 5. This contributes 
significant restoring force as the magnitude of the component of the lift 
vector that is aligned with gravity is greater for the lower wing than for 
the upper wing when the device is displaced in roll. 
Spanwise flow over the wing or towline depressor 2 will result in a 
stablizing, rolling moment when the entire unit, which includes the 
towline attachment fin 14 is above the lifting surface in a negative lift 
configuration. 
A small, stabilizing force will result from loads on the vertical surfaces 
of the fins 52 and 54 in sideslip. 
Directional stability and lateral stability are hydrodynamically coupled. 
It is desired that the depressor track behind the tow vehicle and not 
deviate laterally. Because of the coupling between roll and yaw in a 
hydrodynamic system, lateral stability is of some importance in the normal 
operation of the device.