BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates generally to the design of seagoing vessels and in particular to a vessel having a hull which when fully loaded with bulk cargo such as fresh water, is then fully submerged to a degree where its deck is substantially flush with the surface of the sea, so that it is slightly below this surface.

STATUS OF PRIOR ART Vessels: As used herein the term"seagoing vessel"refers to a ship, boat or barge capable of transporting a bulk cargo over ocean, sea, lake or river waters deep enough to accommodate the vessel. And while the preferred embodiments disclosed herein are towed barges adapted to transport a fresh water cargo, it is to be understood that a vessel in accordance with the invention may be self-powered and that its bulk cargo may be mineral oil, grain or other flowable material that can be stored in the hold or tank of the hull.

Seagoing vessels are of two distinct types: surface vessels and submarines.

A surface vessel operates at the interface of seawater and the atmosphere, the lower part of the vessel being immersed below the surface of the sea, while the upper part which includes a deck projects above this surface. The distance between the surface of the sea and the deck is referred to as the"freeboard".

A submarine begins and completes its voyage as a surface vessel. But after its departure, a submarine becomes fully submerged by filling its ballast tanks with seawater. In order for the submarine to again surface, the ballast water must be expelled and replaced by air which renders the submarine buoyant.

The attitude of a submarine is governed by means including rudders for controlling pitch and heel. In a vessel in accordance with the invention which includes a fully submerged hull, the attitude of the hull is automatically stabilized, as will later be explained, by pontoons mounted above the hull deck.

A vessel in accordance with the invention is provided with a hull which when fully loaded is then submerged in the sea to an extent at which the deck of the hull is then nearly flush with the sea surface. Since the extent to which a hull is immersed depends on the interaction of buoyancy and gravitational forces, it is vital that the nature of these forces and how they interact with each other and with other forces be clearly understood.

Buoyancy and other forces: Buoyancy is the lifting effect of a fluid upon a wholly or partially-submerged body. A buoyancy force acts vertically upwards in opposition to a gravitational force exerted on the body which seeks to pull the body downwards. The magnitude of the buoyancy force is equal to the weight of fluid the body displaces (Archimedes Principle). The line of action of a buoyancy force is through the centroid of the displaced volume, known as the"center of buoyancy". This buoyancy force is equal to the displaced volume of the fluid multiplied by the specific weight of the fluid (weight per unit volume).

A body floating on a static fluid has vertical stability. A small upward displacement decreases the volume of fluid displaced, thereby decreasing the buoyancy force and leaving an unbalanced force tending to return the body to its original position. Likewise a small downward displacement gives rise to a greater buoyancy force, thereby producing an unbalanced upward force.

A body has rotational stability when a small angular displacement sets up a restoring couple that tends to return the body to its original position. When the center of gravity of the floating body is lower than its center of buoyancy, it will

always have rotational stability. A floating ship has its center of gravity above its center of buoyancy. Whether such a ship is rotationally stable depends on its shape.

When it floats in equilibrium, its center of buoyancy and center of gravity are in the same vertical line. When the ship is tipped, its center of buoyancy shifts to the new centroid of the displaced fluid and exerts its force vertically upward, intersecting the original line through the center of gravity and center of buoyancy at a point called the metacenter. A floating ship is rotationally stable if the metacenter lies above the center of gravity. The distance of the metacenter above the center of gravity is the metacentric height and is a direct measure of the stability of the ship.

A towed vessel has directional stability when it does not deviate from the direction in which it is being pulled. When floating in still water, the weight of the ship which includes everything it carries, is equal to the displacement. The weight of the ship itself is called the lightship weight. This weight includes the weight of the hull structure, fittings, equipment, propulsion machinery, piping and cargo-handling equipment. The load carried by the ship, in addition to its own weight, is called the deadweight. This includes cargo, passengers and crew as well as fresh water for the boilers in the case of steam propelling machinery, and other weights which may be part of the ship's operational load. The sum of all of these weights plus the lightship weight gives the total displacement. Hence the displacement equals lightship weight plus deadweight.

A ship at sea is subjected to various forces arising from the action of the waves, the motion of the ship, and the cargo and other weights which are distributed throughout the length of the ship. These forces produce stresses, and the structure of the ship must be of adequate strength to withstand these stresses. The types of structural stresses experienced by a ship riding waves at sea are caused by the unequal distribution of the weight and buoyancy throughout the length of the ship. The structure, as a whole, bends in a longitudinal plane, with the maximum bending stresses being found in the bottom and top of the hull girder.

The depth of a vessel is the vertical distance between the bottom of the ship and the highest point on its deck. Depth is an important consideration in ship

design, for as depth is increased, less material is required on the deck and in the bottom shell.

In a hull for a vessel in accordance with the invention, it is essential that no voids exist between the fresh water or other cargo and the sea water in which the hull is immersed. Hence if the hull is of double shell construction and the hull is fully loaded, it is essential that the inner space between, the shells be filled with fresh or sea water. But when the hull is empty of cargo, it is then essential that the inner-space between the shells be empty.

Also to be taken into consideration is the specific gravity of the cargo and that of sea water. In the case of a fresh water cargo, its specific gravity is 1.000 while that of typical sea water is 1.027. Hence the specific gravity of sea water is 2.7 percent greater that that of fresh water. But in some instances, the bulk cargo may be lighter than sea water. Thus the specific gravity of a crude mineral oil cargo of North American origin varies between 0.813 and 0. 921.

Practical Considerations: Because of water shortages experienced in many regions of the world, a need has arisen for vessels capable of transporting a cargo of fresh water from a site having a plentiful supply thereof, to a region remote from the site suffering from a dearth of potable water.

Useable for this purpose are oil tankers or towed barges capable of transporting an enormous quantity of fresh water rather than oil. However, the cost of manufacturing a typical oil tanker or a towed barge suitable as a cargo vessel for transporting fresh water are considerable. As a consequence, the expenses to be incurred by a country that wishes to purchase fresh water from a remote source thereof and to then transport it to a home base by means of oil tankers may be so great as to be prohibitive.

The reason for these high costs is that a surface vessel having a hull loaded with a fresh water cargo will in the course of its voyage be subjected to powerful bending forces as a result of wave activity. To be able to withstand and survive these stresses, the structure of the vessel must incorporate girders and other hull

reinforcing members. These not only increase manufacturing costs but they also to some degree reduce the cargo capacity of the hull.

Of prior art background interest is the method of transporting fresh water by sea disclosed in the Hsia et al. US Patent 5,657, 714 (1997). In this method a light weight bag fabricated of flexible synthetic plastic material is loaded with fresh water at a fill location, the floating bag being towed by a tub boat to a water removal site having a sump into which is fed the fresh water from the bag. This relatively weak bag offers little protection against floating debris encountered in the course of a voyage.

Hence, the need exists for a seaworthy vessel that can be manufactured at a reasonable cost, yet be capable of safely transporting a cargo of fresh water or other flowable bulk cargo over long distances, thereby making it commercially feasible to transport fresh water.

SUMMARY OF THE INVENTION In view of the foregoing, the main object of this invention is to provide a seaworthy vessel having a submergible hull for transporting fresh water or other bulk cargo, the hull including a substantially planar deck.

More particularly, an object of this invention is to provide a vessel of the above type which dispenses with the need to incorporate in its hull structure reinforcing members to resist bending forces, yet is capable of safely transporting a massive bulk cargo under sea conditions that a surface vessel lacking such members could not survive.

A significant feature of a vessel in accordance with the invention is that when its hull is fully loaded with a bulk cargo, the hull is then submerged so that its deck is nearly flush with the surface of the sea ; hence its freeboard is slightly negative.

Also an object of this invention is to provide a seagoing vessel having a submergible hull which is substantially less costly to manufacture than a surface vessel having the same bulk cargo capacity. Hence the costs incurred in

transporting enormous quantities of fresh water from a region in which the water is plentiful to a country suffering from a dearth of fresh water is in an affordable range.

Yet another object of this invention is to provide a seagoing vessel adapted to transport a bulk cargo which is a liquid whose specific gravity exceeds that of fresh water.

Briefly stated, these objects are attained in a seagoing vessel adapted to transport a bulk cargo such as fresh water over sea or other navigable waters. The vessel includes a hull within which the cargo is stored having a substantially planar deck. Mounted above the deck and distributed thereover is an array of pontoons which function as a stabilizer for the vessel.

The structure of the vessel is such that when its hull is fully loaded with the cargo, the hull is then submerged so that its deck lies in a horizontal plane slightly below the surface of the sea, and the array of pontoons are then just above this surface. When in the course of a voyage, rolling, pitching and other forces act to upset the equilibrium of the vessel, the deck then tilts so that one or more of the pontoons become immersed in the sea, the buoyant forces exerted by these pontoons acting to restore the submerged hull to its state of equilibrium.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention as well as other objects and features thereof, reference is made to the annexed drawings, wherein Fig. 1 is an isometric view of a seagoing barge in accordance with a preferred embodiment of the invention ; Fig. 2 shows the prow of the barge ; Fig. 3 is a side view of the prow section of a barge that differs from the prow shown in Fig. 2; Fig. 4 is an end view of a pontoon mounted on the deck of the barge ; and Fig. 5 is a side view of the pontoon.

DETAILED DESCRIPTION OF THE INVENTION Referring now to Figs. 1 and 2, shown therein is a seagoing vessel in accordance with the invention in the form of a towed barge which can be towed in either direction. Hence what constitute the prow and stern of the barge depends on the direction in which it is being towed.

The barge includes a generally-rectangular hull 10 having a generally planar deck 11 above which is mounted a symmetrical array of cylindrical pontoons 12.

When empty these pontoons function as floats to stabilize the vessel and to resist forces imposed on the barge which seek to upset its equilibrium.

Pontoons are normally employed by the military as floats for supporting a temporary bridge. They are also used in seaplanes. Pontoons are available in high-strength metal cylinders in various sizes. These pontoons, instead of being placed under the deck of a temporary bridge, are in the present invention securely mounted on the deck of a hull.

Hull 10 is divided internally by suitable bulkheads to create compartments to carry a flowable bulk cargo. When the cargo is fresh water, the compartments function as tanks, but when the cargo is grain or flowable material, these compartments then function as holds.

The prow 13 of the barge which acts to plow through the sea is tapered both with respect to the longitudinal and transverse axes of the barge, and it is symmetrical with respect to the midship plane. The stern 14 of the barge has a tapered formation identical to that of the prow. The hull is symmetrical with respect to the midship plane of the vessel as well as to the centerline and the horizontal plane passing through the center of buoyancy. In practice, vertical fins may be mounted on the stern to maintain directional stability.

As previously noted, the hull design is such that when the hull is fully loaded it is then submerged so that its deck 11 lies in a horizontal plane slightly below the still water surface of the sea, and its array of pontoons 12 mounted above the deck are just above this water surface.

Because of the pontoons mounted on the upper deck, the center of gravity of the barge may be situated above the center of buoyancy, this being at the center of volume of hull 10. The barge would therefore be somewhat unstable.

However, when in the course of a voyage the barge is subjected to forces which seek to tilt deck 11 from its normal horizontal attitude parallel to the still water surface of the sea, then one or more of the pontoons 12 which function as floats are immersed by the tilted deck into the sea to an extent determined by the degree to which the orientation of the deck deviates from a horizontal plane. The buoyancy of the immersed pontoons produces a vertical upward force whose line of action is displaced from the horizontal site of the center of gravity. It therefore exerts a redressing moment of force which serves to restore the deck of the barge to its proper horizontal orientation.

The range of static stability of a fully loaded barge in accordance with the invention is broader than that of a conventional surface vessel. It is at least 90 degrees, for even when heeling 90 degrees, the barge remains stable and will return to its initial horizontal attitude.

When fully loaded, the total weight of the barge including cargo and ballast, equals its displacement. The volume occupied by the plating of the bottom of the hull, the deck and sides of the hull is negligible as compared with the volume of the hull tank or hold. On the other hand, the ratio of specific gravities of seawater (1.027) and fresh water (1.000) indicates that the empty weight is at most 2.7 percent of the cargo weight. In a barge in which the empty weight is less than 2.7 percent of the cargo weight, fresh cargo is added to cargo pontoons so that the deficiency below the 2.7 percent is made up. This added cargo increases the deadweight so that 1.000 to 1.027 is the ratio of hull cargo to displacement. Space for this additional cargo is provided in some of the pontoons specifically intended for the purpose. The cargo pontoons must be so distributed over the deck so as not to disturb its horizontal attitude.

Should the barge negotiate part of its voyage in fluvial fresh water, then while sailing in fresh water, all or some of its pontoons will be immersed to

overcome the absent 2.7 percent of displacement. Any pontoon filled with fresh water affords no buoyancy when immersed in fresh water. To illustrate this, consider a detached metallic tank filled with fresh water and immersed in fresh water. The fresh water filling just supports itself, whereas the shell being heavier than water, sinks to the bottom with its filling.

Hence the total buoyancy provided by the buoyancy pontoons must be in excess of 2.7 percent of the deadweight. In a preferred embodiment, the buoyancy of the buoyancy pontoons is between 4 and 5.5 percent of the deadweight.

When intended for travel in sea water, any specific design of a barge according to the invention should specify the maximum height of waves in an agitated sea that the barge can expect to encounter along the route of scheduled voyages, the duration of these voyages and the weather forecast. In this context the height of the wave is the distance from the base of the wave trough to the peak of the wave crest. If a weather forecast predicts for the duration of the voyage, wave heights higher than that for which the barge is designed, that voyage should be interdicted.

It will be seen in Fig. 1 and Fig. 2 that pontoons 12 are supported by pylons 15, arranged in three distinct directions. As best seen in Figs. 3,4 and 5 each pylon has a vertical leg LI, a leg L2 slanted in the transverse direction of the pontoon and a leg L3 slanted in the longitudinal direction of the pontoon. Pylons 15 supporting the buoyancy pontoons are hollow and watertight so that they contribute to the buoyancy which act to restore the barge to its equilibrium state when subjected to heaving, rolling and pitching motions. These pylons 15 supporting pontoons carrying cargo are hollow and communicate with the hull tank or hold at their lower end, and serve as feeders to the hull tank or hold.

In a preferred embodiment of the invention, the combination of the height of pylons 15 and the dimension of pontoons 12 is such that the top of the pontoons is at an elevated level above the deck equal to the anticipated maximum height of the waves to be encountered. This relationship ensures that except for heaving and rolling motions, it will never happen that all pontoons are immersed concurrently.

This also ensures that under all possible conditions, any bending moments which arise involve less than the total buoyancy of the buoyancy pontoons.

Overflow pipes 16 make it possible to completely fill the hull tank or hold.

The overflow pipes 16 communicate with the hull tank or hold at their lower end.

However, the overflow pipes 16 do not necessarily pass through cargo pontoons, as shown.

During filling the hull tank or hold or a cargo pontoon, air contained in the tank or hold escapes through the overflow pipes. Filling is halted as soon as the deck is fully immersed. By that time, the barge might lose its static stability when horizontal, in which case it will heel and pitch over until the base of the first pontoon reaches the surface of the water. By now, filling continues into the cargo pontoons, to gradually restore the barge to a horizontal stable attitude. Freeboard is <BR> <BR> now negative, i. e. , the deck is completely immersed. In these embodiments of the invention in which some of pontoons are cargo pontoons, overflow pipes pass through these pontoons and communicate with the interior at the top of the pontoons.

In a preferred embodiment of the invention, all overflow pipes 16 are mounted on the centerline, thus ensuring that until there is a roll angle of 90 degrees, no ambient water can enter spaces intended for cargo. Overflow pipes 16 mounted on the centerline are shown on Figs. 1 to 5. However, this is not essential to the invention.

The outlet 17 of the overflow pipes 16 must be high enough above the level of deck 11 so that no ambient water can enter the space intended for cargo, irrespective of the state of the sea and of pitching rolling and heaving activity.

Outlet 17 is preferably shaped as a circular arc.

Conventional seagoing surface vessels have positive freeboard. Generally, the longitudinal distribution of weights of the vessel does not conform to the longitudinal distribution of the buoyancy. A consequence of the uneven distribution of weights and buoyancy is bending of the hull. This effect is amplified in an agitated sea because the buoyancy might be increased or reduced amidships due to

the available freeboard, i. e. , due to the possibility that the crest of a wave may be closer to the deck than the formal freeboard, or because a wave trough may be more distant from the deck than the still water freeboard.

In a barge according to the invention, when the barge is fully loaded in still sea water and has zero or negative freeboard, it is then not difficult to make both the longitudinal distribution of buoyancy and the longitudinal distribution of weight virtually uniform, thereby conforming to each other. As a result, the barge hull is not subjected to bending moments and does not bend. The barge therefore does not require expensive reinforcing members to withstand bending stresses.

In an agitated sea, the distribution of buoyancy might not remain uniform due to the exposure of the deck at some places (local buoyancy loss) and the immersion of buoyancy pontoons in other places (local buoyancy gain). Due to their uniform distribution on the horizontal plane of the deck it is not possible for all buoyancy pontoons to be immersed simultaneously. Any departure from uniform buoyancy distribution will involve the buoyancy of less than the totality of the buoyancy pontoons and this is a small fraction of the deadweight (say less than 2.7 percent).

Meanwhile, the distribution of weights remains unaffected, for the weight of the waves flooding the deck which are free to flow off the deck, is not supported by the buoyancy of the barge. As a consequence, any bending of the barge hull will involve 2.7 percent of the deadweight at most.

When sailing empty, the total weight of the barge is equal to its empty weight which does not exceed 2.7 percent of the deadweight, as previously indicated. Hence any bending of the hull involved will correspond to 2.7 percent of the deadweight at most, as is the case with a fully loaded barge. But the barge cannot be allowed to sail partially loaded, i. e. , with positive freeboard.

The most costly element of any seagoing barge is that of the hull, this being practically proportional to its weight. Other factors remaining equal, the weight of the hull is substantially proportional to its bending moment. In conventional seagoing vessels, the maximum bending moment entails a multiple of the

above-mentioned 2.7 percent of the deadweight corresponding to that of barges according to the present invention. This means that the bending resistance, and hence the cost of barges made according to the present invention can be significantly lower than that of existing seagoing vessels.

The bending resistance depends on the thickness of the bottom, the sides and the main deck plating of any vessel. As previously explained, the thickness of bottom, sides and main deck plating of barges made according to the present invention can be as small as needed to resist the pressure differential between seawater on the hull exterior and the interior of the hull containing fresh water. This necessitates that no empty space be permitted to exist between the cargo and ambient water. Such empty space would create an intolerable pressure differential between the empty space and the cargo on one side and between the empty space and the ambient water on the other side. This mandates a complete filling of the cargo tanks to their very tops so that no empty space exists between the upper surface of the cargo and ambient water flooding the deck.

By way of example, let us assume that when fully loaded, the bottom of the barge is 10 meters below the water surface, and that the ambient specific gravity of seawater is 1.027. Hence the pressure differential between the outside and the inside of the hull is as low as 0.027 kilogram per square centimeter. The weight of a structure adapted to resist such pressure differential is very low compared with the structural weight of conventional seagoing vessels which are normally subjected to heavy bending forces.

For barges beyond a certain barge size limit, keeping the weight of the hull below 2.7 percent of the deadweight is readily feasible. At the limit size, the empty weight is 2.7 percent of the deadweight consisting now entirely of cargo weight (specific gravity of seawater is 2.7 percent more than that of fresh water).

As required by environment protection regulations or by other regulations, certain cargo vessels, such as oil tankers are provided with a double bottom as well as double sidewalls and double decks. The purpose of these double shells is to

prevent cargo from escaping to the sea when the damage is limited to the outer shell.

When a submergible hull in accordance with the invention including double shells intended for transport of say Persian Gulf crude oil, the specific gravity of the cargo is 0.90. This value is 12.37 percent less than that of seawater. In that case, it becomes necessary to limit the empty weight of the barge to same value of 12.37 percent of the displacement. The inside shell nearly doubles the weight of the barge but keeping the empty weight below 12.37 percent of the displacement presents no difficulty.

It is imperative that the space between the outer and the inner shells be filled with ballast water during loaded voyages so as to conserve a low pressure-differential between the two sides of both the inner and the outer shells.

This is necessary for maintaining a low lightship weight. Filling this space with sea water does not affect the requirement that in case of damage inflicted on the outer shell, no cargo will then be spilled. No harm is done if the water inside and outside the outer shell are mixed.

Let us now consider by way of example the case of empty weight equivalent to 5 percent of the deadweight. This means that the displacement is equivalent to 105 percent of the deadweight. Let us also assume a ballast weight in the space between inner and outer shells to be 15 percent of the deadweight. This leaves 85 percent of the deadweight available for cargo. Obviously, the volume available in the hull for cargo, or the volume of the hull tank, is 85 percent of the hull volume.

The hull tank is occupied by a liquid of 0.90 specific gravity. Therefore, the weight of cargo in the hull tank is 76.5 percent of the deadweight while the share of the cargo in the deadweight is 80 percent as said above. The remaining 3.5 percent are to be carried in cargo pontoons significantly more than was the case with fresh water cargo. Either the number or the size of the pontoons, or both number and size will be greater than was the case for fresh water cargo.

On empty voyages, it is imperative to empty the double shell of its water ballast, thereby reducing the pressure differential between the inside and outside of

the double shell and the buoyancy to a minimum. Minimum buoyancy means a minimal bending moment, thereby reducing to a substantial degree, the cost of the barge.

When a vessel of the type disclosed above is fully loaded with a fresh water cargo, having a specific gravity of 1.000, the deadweight of the loaded vessel is such that the deck of its submerged hull is then slightly below the surface of the sea and its array of stabilizer pontoons are then just above this surface. Hence when, in the course of a voyage, the vessel is subjected to forces which seek to tilt the deck and submerge some of the pontoons, the buoyancy force exerted by these pontoons acts to restore the submerged vessel to a state of equilibrium.

When however, the cargo is a liquid having a specific gravity exceeding that of seawater such as a saturated brine taken from the Dead Sea which has a specific gravity above 1.3, then the increase in deadweight is such that when the hull is only partially loaded, it will become submerged so that its deck is then slightly below the sea surface. And when the hull is fully loaded with the saturated brine cargo, its deck will then be well below the sea surface and pontoons on the decks will become submerged and no longer function as stabilizers.

In order therefore to accommodate the vessel to a fluid cargo having a specific gravity exceeding that of fresh water, the total capacity of buoyant pontoons must be enlarged to account for the increased deadweight.

The increase in the capacity of the pontoons must be such that when the hull is fully loaded with a fluid having a specific gravity higher than that of fresh water, its deck is then slightly below the sea surface while its pontoons are just above the surface so that they can function properly as stabilizers.

In practice, the arrangement may be such that when fully loaded with a liquid having a specific gravity higher than that of fresh water, the hull is then submerged to a degree in which the pontoons mounted above the deck are then partially submerged, yet retain a sufficient buoyancy force to stabilize the vessel.

While there have been disclosed preferred embodiments of a vessel having a submergible hull in accordance with the invention, it is to be understood that many changes may be made therein without departing from the spirit of the invention.