GB2173462A

GB2173462A - A depressor

Info

Publication number: GB2173462A
Application number: GB08508290A
Authority: GB
Inventors: Dr James Frederick Henderson
Original assignee: SOUTH WESTERN IND RES
Current assignee: SOUTH WESTERN IND RES
Priority date: 1985-03-29
Filing date: 1985-03-29
Publication date: 1986-10-15
Also published as: GB2173462B

Abstract

This invention relates to a depressor for towed cables. A depressor 12 comprises, in one embodiment, a sheet metal body 13, nose ballast 14 and a suspension arm 15. The body 13 is curved out of an isosceles triangular blank about an axis of generation 16 and has fins 17 to provide directional or weathercock stability. The construction is such that all hydrodynamic forces acting on the body 13 act normally to the surface thereof and hence through the axis of generation 16 of the body. The suspension point 18 of the depressor is provided on or close to the axis 16 so that little or no movement is created by these forces. <IMAGE>

Description

SPECIFICATION A depressor This invention relates to depressors for towed cables.

When a cable is towed in a fluid it interacts hydrodynamically with the fluid and there can be problems in ensuring that the cable extends downwardly to a predetermined depth below the towing vehicle or vessel. To overcome this problem it is known to suspend a depressor from the end of the cable. Such depressors normally have hydrodynamically interacting portions in the shape of inverted aircraft wings. These induce a downward force on the cable, but they are often unstable in roll because the hydrodynamic forces create unbalanced moments about the suspension point. In addition the size of these forces is such that the depressor has no sense of "up" and/or "down" and hence it can become flipped by turbulence or flow separation and will not right itself.

It is an object of the present invention to provide a depressor which reduces or removes at least one of these problems.

From one aspect the invention consists in a depressor for a towed cable or the like having a body and means for suspending the body on the cable wherein at least part of the body is curved upwardly and wherein the suspending means suspends the body on the cable at a point lying on or close to the axis of generation of the curved part.

In this arrangement the hydrodynamic forces acting on the depressor exert negligible rolling moment about the suspension point, because they must act normally with respect to the surface of the curved part and hence act directly through the suspension point. There are therefore little or no moments about the suspension point and roll can be significantly reduced or even eliminated. Another benefit of this arrangement is that the rolling moment due to unbalanced hydrodynamic forces no longer masks the force of gravity on the depressor and the depressor will therefore tend to maintain its orientation due to the forces of gravity.

In a preferred embodiment the suspension point is in a fore and aft position such that in combination with the depressor weight and towing cable force the depressor is hydrodynamically stable in pitch. Additionally or alternatively the suspension point may be in a position such that the depressor has weathercock stability.

The depressor may further comprise ballast mounted on the body and means may be provided for releasably mounting the ballast so that it can be adjusted in accordance with operating conditions.

The depressor may have two or more curved parts spaced along its length, in which case their curvature is preferably approximately the same. Alternatively the body of the depressor may be formed from a single curved sheet, which may for example be triangular if flattened into a plane. In this case the triangle is preferably an isosceles triangle with the base corners of the triangle upstanding to form fins to provide direction and weathercock stability.

It will be appreciated that these curved structures are much cheaper and simpler to form than an aerofoil section.

From another aspect the invention consists in a depressor for a towed cable or the like having a body incorporating an upwardly curved portion of constant radius.

From a further aspect the invention consists in a depressor having a body shaped to be hydrodynamically balanced when suspended from a cable such that the hydrodynamic rolling moments are at least in order of magnitude less than the gravitational restoring moment in roll.

Although the invention has been defined above, it is to be understood that it includes any inventive combination of the features set out above or in the following description. Objects of the invention have been stated, but the invention is not limited to depressors for filling those objects.

The invention may be performed in various ways and specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows diagrammatically a cable being towed with a depressor suspended therefrom; Figure 2 is a schematic view of a first construction of depressor; Figure 3 is a cross-section through the depressor of Fig. 2 (not to the same scale); and Figure 4 is a schematic view of an alternative embodiment of depressor.

As mentioned above, and as can be seen in Fig. 1, when a cable 10 is towed by a ship 11, a depressor 12 may be suspended from the cable. For the reasons described above current depressors are unsatisfactory.

Referring to Figs. 2 and 3 one embodiment of the applicant's depressor comprises a sheet metal body 13, nose ballast 14 and a suspension arm 15. The body 14 is curved out of an isosceles triangular blank about an axis of generation 16. This is a particularly simple method of manufacture and provides an integral body of the requisite shape to provide a downward force and with fins 17 formed from the upturned base corners of the triangle.

These fins give the depressor directional or weathercock stability.

As can be seen in Fig. 3 the downward hydrodynamic forces acting on the body 13 all act normally to the surface of the body 13 and hence are directed through the axis of generation 16. Accordingly if the suspension point 18 of the depressor 12 is on or close to the axis of generation 16 there will be little or no moment created on the body by these hydrodynamic forces and therefore the depressor will not be subjected to significant roll.

Another advantage is that the effective negation of rolling moment effect of these forces means that they do not, as is the case in traditional depressors, swamp the effects of gravity and so gravity becomes a significant force on the body 13. The result is that the body will always try to maintain this equilibrium position under gravity and this adds to the stability of the device.

The ballast 14 may be releasably mounted on the body 13 by releasable fasteners or other devices so that the correct ballast can be selected in the light of proposed operating conditions.

It is not necessary that the whole body be curved and an alternative arrangement is shown in Fig. 4, wherein the depressor has two spaced curved portions 18 mounted on a torpedo like chassis 19. Again this should be suspended about a point close to or on the axis of generation.

The exact suspension point in a fore and aft sense may be also selected to provide stability in pitch and yaw and typically it should be in front of the hydrodynamic centre of the device.

It will be appreciated that the depressor of the invention may be configured in many different ways and has advantages even if the suspension point is not related to the axis of generation. This is because the curved structures proposed are much simpler to manufacture than the traditional aerofoil sections and provide inherent weathercock reaction.

The depressor is suitable for use in any fluid.

Claims

1. A depressor for a towed cable or the like having a body and means for suspending the body on cable wherein at least part of the body is curved upwardly and wherein the suspending means suspends the body on the cable at a point being on or close to the axis of generation of the curved part.

2. A depressor as claimed in claim 1, wherein the suspension point is in a fore and aft position such that in combination with the depressor's weight and the towing cable force, the depressor is hydrodynamically stable in pitch.

3. A depressor as claimed in claim 1 or claim 2, wherein the suspension point is in a position such that the depressor has weathercock stability.

4. A depressor as claimed in any one of the preceding claims further comprising ballast mounted on the body.

5. A depressor as claimed in claim 4 further comprising means for releasably or adjustably mounting the ballast on the depressor.

6. A depressor as claimed in any one of the preceding claims having two or more curved parts spaced along its length.

7. A depressor as claimed in claim 6 wherein the curvature of the parts is approximately the same.

8. A depressor as claimed in any one of claims 1 to 5 wherein the body is formed from a single curved sheet.

9. A depressor as claimed in claim 8, wherein the sheet would be triangular if flattened into place.

10. A depressor as claimed in claim 9, wherein the triangle is an isosceles triangle and the base corners thereof upstand to form fins to provide direction and/or weathercock stability.

11. A depressor for a towed cable or the like having a body incorporating an upwardly curved portion of constant radius.

12. A- depressor substantially as hereinbefore described, with reference to the accompanying drawings.