BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally concerns floats that move upon seas and other bodies of water including while (i) being towed while (ii) bearing loads.

The present invention particularly concerns a float for the suspension of a seismic gun towed in deep water during seismic surveys.

The present still more particularly concerns the hydrodynamic contours, and the fittings, of a towable float, especially a towable float that permits loads, such as a seismic air gun, to be maintained at desired depth below the float, or in air above the float, while the float is towed over the sea at high speeds during a broad range of sea conditions.

2. Description of the Prior Art 2.1 General State of the Towable Marine Float Art Deep water seismic surveys as are used, among other purposes, to prospect for oil entail pulling a suspended seismic gun attached to a float behind a ship. Until recently inexpensive inflatable floats have been used for flotation of the towed seismic equipments.

However, these floats have proven incapable of maintaining either (i) a constant attitude or (ii) a constant depth in the water at varying speeds, and are additionally prone to destruct at high speeds. A constant (i) attitude and (ii) depth for the towed seismic air gun is desired to derive the best quality seismic data.

An illustrative specialty flotation device for use in seismic surveys--which flotation device is different than surface-type flotation devices--is made by Flotation Technologies, P. O. Box 1171, Biddeford, ME 04005. The Flotation Technologies"lead-in floats"are claimed to be (i) rugged, (ii) possessed of low drag, and (iii) capable of surviving excursions down to 200 msw. The new float design of Flotation Technologies is in the substantial shape of a torpedo. Because no two towing vessels and tow gear configurations are the same, Flotation Technologies offers to design flotation to customer specifications.

The standard"lead-in floats"of Flotation Technologies have 2000 lbs. net buoyancy, a composite hull, a 200 meter depth rating, forward and aft lift points, an impact resistant FRP coating, a free-flooding tail section, an S. S. tail fin assembly, U. H. M. W. side slides, a stainless steel tow ball, and a cast urethane nose to protect against impact damage. This float of advanced construction and form is shown, circa 2000, on the world wide web at <http://www. flotec. com/flo25. html>.

Few previous floats are intended to hydroplane. When pulled sufficiently fast so as to rise to the surface of the water then these previous floats, and the seismic devices that they serve to support, have typically exhibited an undesirable jerky motion.

Moreover, the transition between subsurface and surface operation is typically very rough, with much spasmodic motion occurring and with widely variable forces experienced by the tow line.

Hydroplanning oceanic instrumentation floats are known. See, for example, the float shown on the world wide web circa 2000 at <http ://hahana. soest. hawaii. edu/hot/methods/towfish. html>. This float is used by the HOT program of the University of Hawaii to obtain a long time-series of physical and biochemical observations in the North Pacific subtropical sphere towards addressing the goals of the U. S. Global Change Research Program.

2.2 Discussion of the Requirements for Towable Marine Floats Because ocean seismic surveys are very expensive, typically ranging in cost from $10,000 to $15,000 U. S. per day circa 2000, it would be desirable if seismic guns and the like could be reliably towed faster, permitting seismic data gathering to be accelerated.

This would require flotation devices that would maintain, insofar as possible, (i) a smooth and equal resistance to forward motion over a wide range of. towing speeds, (ii) a constant buoyancy with level support being provided at a predetermined constant depth for all attached devices, and (iii) uniform performance over a broad range of sea states.

More generally there is a requirement for a towable marine float that has and maintains (i) a relatively constant buoyancy, or, equivalently, lift capacity for any attached load, at (ii) a relatively constant height of the float above the water or, equivalently, freeboard of the float within the water, preferably with (iii) a stable upper support surface (i. e., without roll or pitch or yaw) over (1) a broad range of tow speeds, normally starting at zero (i. e., stopped), and (2) sea states. It would also be useful if, while realizing these performance characteristics, that (iv) a drag presented by the towable float to its tow vessel should remain relatively constant.

One type of"float"device that meets characteristics (i) through (iii)--although most certainly not characteristic (iv)-- would be a huge barge, or ship. Such a structure would be both stable and of equal freeboard over a range of two speeds, However, a towable marine float, especially as presents an efficient (iv) drag that is also preferably substantially even over a range of speeds must realistically be much smaller than the ship that is towing it, and must commonly be of such size, shape and volume as becomes subject to hydrodynamic forces--both flotation (i. e., up/down) and drag (i. e., fore/aft)--of differing magnitudes during the course of being towed at various speeds. This is especially true if at some speed--which is the case with nearly any object towed sufficiently rapidly--the float will tend to"come up onto plane", meaning to skim (in worse cases, spasmodically bounce) across the surface of the water with increasing speed over the water. Such operation"on plane"is highly beneficial for the energy (per unit distance traversed) of the towing, but is, of course, inimicable to maintaining the float (and any load supported thereby) at equal depth/height in/above the water.

The solution to providing a marine float that may exhibit some stability in all axis while towed is, of course, to make the float of hydrodynamic contour. To some extend this is recognized by, and embodied within, all towable marine floats, which have"regular" contours that produce reasonable float stability under tow.

However, just like the famous"lifting bodies"of the U. S. National Aeronautical and Space Administration (NASA) developed for use in the atmosphere, the question presents itself as to which, or what, curves and contours, exactly, should a marine float have in order to best perform its normative roll of riding"smooth and level", "steady as she goes". The best shape of a marine float that moves through the water, normally under tow, is the subject of the present invention.

SUMMARY OF THE INVENTION The present invention contemplates a towable marine float of a new design that serves to provide even flotation forces osera broad range of tow speeds and sea conditions. In simplest terms, the height of the float above the water, and, equivalently, the freeboard of the float below the water, remain substantially constant.

The float is also stable, and is resistant to roll and pitch and yaw even as it may be pulled in an arc during a turn of the towing ship or watercraft. Finally, the float presents relatively even resistance force to the towing vessel over a range of tow speeds. This presents, among other possibilities, that the towing vessel may be of a smaller size and towing power relative to the float and its load than is customary without making that the changing resistance of the towed float to forward motion with changes in tow speed and/or sea state should greatly affect the towing vessel, causing it to lurch and/or to spasm.

The marine float in accordance with the present invention could be described as"very well behaved". It is especially useful to support a submerged object, normally a seismic air gun, in all phases of high-speed deep water towing (from rest to high speed to rest again), normally upon the ocean during seismic surveys.

The even flotation force makes that the towed seismic air gun or other instrument transits over the ocean floor, and with its transducer in the water under the ocean surface, at an even depth such as promotes gathering accurate seismic data. The fact that the float may permissively be towed at high speeds--normally at least as fast as 7 knots, with the float being usable at tow speeds at least up to 12 knots and often much higher dependent upon the supported device, sea state, etc.--makes that this seismic data may be gathered faster.

The new float in particular smoothly transitions from (i) a simple displacement mode to (ii) hydroplanning as tow speed is increased without incurring (i) any dramatic change in drag or (ii) any significant variation in the depth of the supported seismic air gun. Thus--although the terms describing the various regimes of operation are correct--the float of the present invention operates differently than a normal marine body, such a the hull of a recreational boat, where (i) increasing resistance to forward motion is encountered in displacement mode with increasing speed until the hull rises out of the water onto (ii) plane as speed is increased, at which time the hull sits higher in the water while drag is diminished. Comparing the float of the present invention to conventional behavior, height above (freeboard within) the water is not much changed at all speeds, while the towing force does not much increase as speed increases in what can be called"displacement mode"while, at some point, the towing force will not much decrease as speed still further increases in what can be called"hydroplaning mode".

With its broad and level operational curves of both (i) height (freeboard) in the water, and (ii) towing force, as functions of speed through the water, the new float permits towing the seismic gun with superior smoothness and depth control up to, and at, unprecedented high tow speeds. Towing operation is smooth, stable and consistent over a broad range of tow speeds, and during sea states from 0 to at least 6, including with a high following sea.

The stable high speed towing in particular permits seismic surveys--which use both expensive capital assets in the form of ships and extensive labor in the form of ship's crews--to be accomplished more quickly.

The float of the present invention, and the principles of its construction, have applicability to marine bodies that move through the water under any force, not necessarily limited to tow forces.

For example, the force of propulsion may be that of gravity, and the preferred float of the present invention, which normally possesses a fin and which already has some visual similarity to a surfboard, can be used as a surfboard. The float can also be used in river rafting or the like.

A number of floats variously positionally biased by an associated angular preset of the fin, or rudder, of each float will tow behind a powerboat or like watercraft in a dispersal pattern, with the several tow lines leading to each of several floats being splayed as in the fingers of the hand. The several spaced-parallel towed floats may then serve as platforms for such diverse purposes as supporting theatrical'props or performers in water shows, for themselves towing associated water skiers or kites, and/or for more mundane purposes such as for mounting or towing booms serving to sweep up spilled oil, or sensors for marine monitoring and detecting.

In simplest terms, a float in accordance with the present invention that performs well when used singly and when towed, also, quite logically, performs well when (i) used in parallel arrays, and/or (ii) moved in and upon water by forces other than tow forces.

1. Functioning of A Hydrodynamically-Shaped Body That Rises Smoothly onto Plane While Providing Even Drag and Flotation in Both Displacement and High Speed Planning Modes The present invention is embodied in a hydrodynamically-shaped body having, inter alia, a very particular shape.

The objective of the hydrodynamically-shaped body is not to produce the least drag at some particular speed or range of speeds at which it might be towed (or propelled), nor to provide the greatest lift (flotation) for some towing (or propelled) speed or range of speeds. Instead, the objectives of the new hydrodynamically-shaped body are these: The body will have commenced to exhibit slightly reduced towing force--an operational regime that is commonly known as"on plane"although this will not be visible in the height of the body in the water--at such tow speeds as are realizable by most ships, generally at speeds over 7 knots. In fact, the force required to tow the body will not much vary from slow to high speeds. The body thus tows reasonably efficiently at high speeds--although perhaps at the cost of requiring a greater than expected towing force at slowest speeds while failing to tow at low force at the very highest, planning regime, speeds.

Moreover, and perhaps more importantly, the flotation body will function through a broad useful range of tow speeds--normally from 3 to at least 12 knots--and in both its"displacement"and in "planning"modes (slight as the differences between these modes may be) to provide flotation to a connected and supported immersed object, normally a seismic air gun. The (i) constant equal flotation (equivalently, freeboard) and the (ii) even drag make for a smooth tow where an object connected and supported by the float (the air gun) will travel through the water at an even preset depth.

Because many marine craft or flotation bodies will, above certain speeds, plane upon the surface of the water, and will also float within the water at slower speeds or when stopped, it may initially be difficult to appreciate exactly what the present hydrodynamically-shaped flotation body is doing differently. The difference is this: a normal craft or hydrodynamic lifting body that is capable of planing--which, at a high speed, consumes the most propulsion energy--is normally directed to conserving energy on plane; i. e., to going as fast as possible for the energy expended. Very often this goal means that the same craft or body "lurches"onto plane, offering progressively more and more resistance to being pulled (or pushed) through the water until, finally breaking free of the surface and onto plane, a sharp break in the speed/energy profile is encountered. (High performance private watercraft often have trim tabs to control this phenomena.) The flotation body of the present invention functions oppositely. The flotation body of the present invention will, as a first matter, have an operational mode where, at speed, it may be said to"plane"upon the surface of the water, performing its flotation function in so doing. It will also, however, both enter and exit this planing mode very smoothly, with but minimal change in the resistance that it provides to being towed (or propelled) through the water. Moreover, there will be but minimal change to the (vertical axis) flotation force provided to any connected and supported immersed device, such as a seismic gun.

On the basis of the fact that the float does not rise in the water with increasing speed, some persons will deny that the float of the present invention"rises upon the surface of the water"at all, which is the dictionary definition of planning. However, at speed the float is upon the surface of the water, is starting to exhibit a diminished towing force (typical of any maritime body being"on plane"), it is simply that the float does not"rise"as high as is conventional. In fact, it barely"rises"at all. In fact, the float of the present invention could be called"a marine body operational over a broad range of speeds and in regimes associated with both displacement and with planning, where the body is distinguished that it is, in combination, so high in displacement and so low in plane that it intentionally varies in height above the water and, equivalently, in freeboard, but insignificantly." 2. Structure of A Hydrodynamically-Shaped Body That Rises Smoothly onto Plane While Providing Even Drag and Flotation in Both Displacement and High Speed Planning Modes According to its function, the present invention is embodied in a flotation body, towable from the fore, in the substantial shape of a hydrodynamic lifting body. The body exhibits both substantial fore-aft and substantial port-starboard symmetry. A"hydrodynamic lifting body"provides, by definition, lift by movement through the water. Most commonly, and in the case of the float of the present invention, such a hydrodynamic lifting body also provides flotation force when stationery."Substantial"symmetry is arbitrarily defined, for purposes of the present invention, to mean that immersion of the fore and aft, or the port and starboard, halves of the flotation body (as defined by axis of construction) will, by Archimedes law, shown no more than 10% difference in displacement.

The basic curves of the left-right symmetrical and fore-aft symmetrical float of the present invention (not including its fins and padeyes) are substantial half-ellipses in the cases of both the side-and front-views, different ellipses being used above and below the major axis. In the case of the plan view, a complete ellipse is circumscribed. For efficient use of construction materials, and in consideration of (i) the buoyancy of the preferred material of construction, and (ii) a nominal load of 200 pounds with a total volume displacement of 400+ lbs., the preferred float is about one foot high, two feet wide, and seven feet long. The major axis is about 20k down from the top. The fins are also elliptical in shape.

The ellipse is known to be an efficient aerodynamic/hydrodynamic shape, but the combination of ellipsoidal surfaces to realize the purposes of the present invention are not known to exist in the prior art.

Fins are affixed to the aft of the flotation body.

The preferred flotation body in accordance with the present invention intentionally exhibits, except for its rear fins, perfect (i) fore-aft and (ii) port-starboard symmetry (to the normal limits of manufacture). The body has as the fore-to-aft contour of its lower surface the substantial shape of a ellipse having a major axis of from 4-10 feet and most commonly about 7 feet. The portion of the ellipse, or the ellipsoidal surface, subtended by the float has a nominal thickness of from 6-14 inches and most commonly 10 inches.

The hydrodynamic aspect ratio is preferably in the range from 9-13 and most commonly approximately 11.4.

Meanwhile, the same floatation body has as the port-side to starboard-side contour of its lower surface the substantial shape of an ellipse with a major axis of from 1-3 feet and most commonly about 2 feet. The portion of this ellipse, or ellipsoidal surface, subtended by the float also, necessarily, has a nominal thickness of from 6-14 inches and most commonly 10 inches. The hydrodynamic aspect ratio in this, the transverse axis of the float, is preferably in the range from 2 to 4.4 and is most commonly approximately 3.2.

The top surface of the flotation body--which is relatively less important to its function at least while on plane--is preferably substantially flat.

The flotation body of the present invention is preferably constructed from laminated and shaped expanded polyethylene (EPE) foam. The volume of the float to the sides from the center profile is entirely of EPE; no aluminum or other structure is used. EPE foam is light yet durable. It also has the desirable property of shape memory: when dented or compressed, it will eventually assume its original shape.

A tough skin of, preferably, polyurethane coating is applied, preferably by spraying. The profile of the float is maintained by marine grade aluminum plate, which also serves for the fins and padeyes. The padeye bushings are preferably of stainless steel.

These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.

BRIEF DESCRIPTION OF THE DRAWINGS Referring particularly to the drawings for the purpose of illustration only and not to limit the scope of the invention in any way, these illustrations follow: Figure la is an x-ray view of the preferred embodiment of a hydrodynamically-shaped float in accordance with the present invention.

Figure lb is a side plan view of the preferred embodiment of a hydrodynamically-shaped float in accordance with the present invention previously seen in Figure la.

Figure 2a is a detail top plan view, and Figure 2b is a detail side plan view, of the preferred padeye bushings used on the preferred embodiment of a hydrodynamically-shaped float in accordance with the present invention previously seen in Figure 1.

Figures 3 and 4 are diagrammatic perspective views of the preferred embodiment of a hydrodynamically-shaped float in accordance with the present invention, previously seen ia Figure 1, on land in respective horizontal, and vertical, storage positions.

Figures 5 and 6 are diagrammatic perspective views of the preferred embodiment of a hydrodynamically-shaped float in accordance with the present invention, previously seen in Figure 1, on-board a ships deck with two cables attached, and in its operative, towed, position providing flotation to a suspended seismic gun.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following description is of the best mode presently contemplated for the carrying out of the invention. This description is made for the purpose of illustrating the general principles of the invention, and is not to be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

An x-ray view of the preferred embodiment of a hydrodynamically-shaped and-contoured float 1 in accordance with the present invention is shown in x-ray perspective view in Figure la, and in side plan view in Figure lb. The float 1 has a body 11 that is preferably made from laminated and shaped expanded polyethylene (EPE) foam. A skin 12 is preferably a polyurethane coating, preferably by spraying. The profile and shape of the float is maintained by marine grade aluminum bulkhead plates 13, which also serve to support the fins 14 and padeyes 15. The padeye bushings 16, shown in detail plan view in Figures 2a and 2b, are preferably made from stainless steel.

As may be observed in Figures la through ld, the body 11 of the float 1 shows complete fore-aft and port-starboard symmetry. The fore-to-aft contour of the lower surface 111 of the body 11 is in the substantial shape of an ellipse having a major axis (lengthwise on the float) of 7 feet, a minor axis (height) of 10 inches, and a hydrodynamic aspect ratio of 11.4. The port-side-to-starboard-side contour of the same lower surface 111 of the body 11 is in substantial shape of an ellipse having a major axis (transversely, or across, the float) of 1 foot, a minor axis (height) of 10 inches, and a hydrodynamic aspect ratio of 3.2. The top surface 112 of the flotation body 11, which is relatively less important to the function of the float 1 at least while the float 1 is on plane, is normally substantially flat.

The float 1 is illustrated on land in its horizontal storage position in Figure 3, and in its vertical storage position in Figure 4.

The float 1 is illustrated on the deck of a tow ship, with cables 2 attached, in Figure 5. The float 1 is illustrated in operation towed [tow ship not shown] supporting a seismic air gun 3 (shown in phantom line for not being part of the present invention) in Figure 6. A cable 2 connects the float 1 to the tow ship (not shown). Cables hang down to support the air gun 3. [The gun itself is towed by an umbilical package that contains the air supply hose, firing and hydrophone lines, as well as a strength member.] The position of the water line when the float 1 is at rest, operating in a displacement mode, is shown at A. The position of the water line when the float 1 is on plane at speed is shown at B.

Substantially because of its contour as a sort of hydrodynamic "lifting body", the float 1 transitions between its displacement and planning modes with a minimum of change. Accordingly, the water lines A and B are relatively close together.

Not shown in the figures are any undulations or variations in any of (i) depth relative to the ocean surface, (ii) attitude of the suspended gun 2, and/or (iii) varying force on the tow line 3, occurring when then float and its suspended devices are subjected to any of (a) different tow speeds, (b) different sea states, and/or (c) different wind conditions. Normally variations in any of the quantities (i) though (iii) typically do not exceed 5% over the usable operational range of the float from at least (a) 3 to 12 knots tow speeds during (b) sea states ranging from 0 to6 with (c) variable direction surface winds up to 25 knots. (The float performs well even should any of these quantities (a)- (c) vary outside their stated ranges: it is simply that variations within the stated ranges (a)- (c) typically--although not invariably, as the confluence of aperiodic sea and tow conditions cannot always be predicted--produce variations in any of (i)- (iii) a magnitude less than 5%.) In accordance with the preceding explanation, variations and adaptations of the hydrodynamically-contoured float in accordance with the present invention will suggest themselves to a practitioner of the marine and marine watercraft arts. For example, the numbers and-locations of the fins can be varied.

In accordance with these and other possible variations and adaptations of the present invention, the scope of the invention should be determined in accordance with the following claims, only, and not solely in accordance with that embodiment within which the invention has been taught.