Ships – Towing or pushing – Submerged object
Reexamination Certificate
2001-12-13
2004-11-23
Swinehart, Ed (Department: 3617)
Ships
Towing or pushing
Submerged object
Reexamination Certificate
active
06820568
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to hydrofoil apparatus and, more specifically, this invention relates to hydrofoil apparatus for inclusion in any towed arrangement which, in order to fulfill its function, requires hydrodynamic lift as a component of the force that opposes the towing effort.
BACKGROUND OF THE INVENTION
Known hydrofoil apparatus for towing fall generally into two use categories, but may also fall into both at once. The first category of use includes a wide variety of activities that require an object, or different types of equipment, to be towed through the water by a vessel or other towing point for purposes of, for example, performing special measurements, catching or positioning something. It is often important that the object or equipment being towed should not follow directly behind the point of tow but be pulled out by a diverter to one side or another, pulled downwards by a depressor, or even pulled upwards by an elevator, if the towing point is beneath the water surface. Examples of hydrofoil apparatus that can perform some of these roles have been variously referred to as paravanes; vanes; mono-wings; diverters; doors; otter boards or just otters; deflectors; depressors; elevators and kites.
The second category of use includes all those arrangements in which the effort generated by the hydrofoil apparatus is used to effect the towing point or vessel in some desirable way. These might, for example, include the role of a sea-anchor, when used to give some direction to a vessel's drift; the role of a stabilizer used to stabilize a vessel in roll; the role of providing lateral resistance in a sailing arrangement such as that of the more conventional waterborne vessel that supports a rig of sails, or a more unusual airborne arrangement of aerofoil such as, for example; an autogyro; a hang-glider, kite or other winged craft; a paraglider; or a displacement vessel such as an airship or balloon.
A water-air interface is an extremely complex and difficult environment in which to operate towed hydrofoil apparatus. Typically, on reaching or breaking the water surface, most hydrofoil apparatus for towing will become unstable and cease to function as desired.
For those bodies and types of equipment that are required to be towed at or close to the water surface, a common practice has been to ensure that the hydrofoil apparatus remains fully immersed and at the desired running depth by means of a float on the surface. However, this does little to stabilize the hydrofoil apparatus in yaw and can itself be disruptive in rough water, so additional means are usually employed. Where towed equipment produces enough drag force, this can be used to stabilize the hydrofoil apparatus in yaw, but it is not usually desirable to introduce a constant drag force unnecessarily. Also, the capacity of a float to exercise control remains fixed, while the disruptive dynamic forces over which it is required to prevail grow in proportion to the square of the water speed. The size of float required, as speeds increase, would therefore grow out of all proportion producing excessive drag and becoming potentially unmanageable and dangerous to handle. Furthermore, the use of a float does not avoid the inevitability that the hydrofoil surfaces will become partially unwetted during their launch and recovery; a condition that is usually unstable and can produce difficult if not dangerous handling.
Many bodies and other types of equipment are not required to be towed at or close to the water surfaces when fully deployed, so would be hampered by a float. Nevertheless, they too must usually pass through the surface conditions during their launch and recovery, and in many cases it is also desirable that this can be carried out at some speed and through rough water.
For sailing it is desirable that a hydrofoil apparatus is towed at the water surface and often at considerable speed through rough water. It is also desirable that the same apparatus may be operated on either tack, which can be difficult to arrange if a float is employed for surface sensing.
To assist or take over the functions of a float completely, known hydrofoil apparatus have therefore been constructed with anhedral, to sense the water surface in a simple dynamic way. These known apparatus have a lower portion of hydrofoil surface which is orientated to give a depressing component of lift and an upper portion orientated to give an elevating component of lift. These two lift components therefore act in parallel and opposite directions away from each other. The apparatus can then be adjusted in roll through bridle adjustments, until the elevating and depressing lift components are in balanced opposition, while a part of its elevating portion pierces the water surface to remain in reserve. Should there then be a gain or loss in wetted surface area, the resultant of the lift components provides a restoring force that works to restore the apparatus to the desired running depth.
Unfortunately though, the opposing lift components also tend to form couples that seek to turn the apparatus in the direction in which it is moving at any one moment, during its surface sensing depth corrections. If, therefore, the apparatus is responding to either an elevating or depressing lift resultant, it tends to turn upward, or downwards, respectively, towards the water surface. Further to this, the elevating and depressing portions experience changes in their angles of incidence which are accompanied by a variety of possible alterations in their lift to drag ratios. These can have the effect of redistributing its lift and drag such that the lift and towing force resultant, and the drag vector are separated across the direction of movement at any one moment. This gives rise to further couples that either work with or against the opposed lift component couples, depending on the angles of incidence at which the hydrofoil surfaces had been working. A common result is that of repeated and alternating turns towards the surface, sometimes developing into a marked or even violent “porpoising” action.
Furthermore, if the apparatus should suffer disorientation due, for example, to an acquired drag force from weed, debris or from grounding, the surface sensing capability can be overcome, due to large changes, of an opposite nature, between the angles of incidence of the elevating and depressing portions. This can cause the apparatus to turn and jump from the water, dive beneath the vessel or, in the case of grounding, dive precipitously into the bottom.
The addition of a short stabilizing tail works with the opposed lift component couples to support any turns towards the water surface, which is unhelpfull. However, as the tail is lengthened, this support becomes increasingly less, tending more to support the maintainance of a fixed orientation, with respect to the general direction of advance of the hydrofoil apparatus, so obliging it to execute its surface sensing with a more side-slipping action. The longer the tail, therefore, the more turns towards the surface and “porpoising” are suppressed. However, while this modification of behaviour is appropriate, it is found in practice to be insufficient, unless the tail is unacceptably long.
A further disadvantage can arise if a bridle member parts and, as a consequence, the apparatus adopts completely the wrong orientation. Due to the anhedral relationship of the hydrofoil surfaces, the apparatus can then behave much as a spinner does on a fishing line, causing considerable entanglement and further loss or damage.
For many uses it is important that the drag of a towed hydrofoil apparatus be reduced as much as possible, and particularly so for sailing. An immersed tow-line and bridle members can generate excessive drag if not satisfactorily faired. However, the effectiveness of known cable fairings that are used in a much wider context than just the present invention is limited because they are designed to feather freely about a towed cable. Consequently, a bridle member, tow-line or towed cable that is
Iandiorio & Teska
Swinehart Ed
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