Title:
OPEN HULL STABILIZER
United States Patent 3797440
Abstract:
A passive stabilizer operated directly by the action of the external waters in which a vessel floats. The port and starboard sides of the hull of a double hull water-going vessel are opened to the external waters in two positions. The uppermost and larger of the openings serve as inlets for the water in which the vessel floats; and the lower and smaller of the openings serves as drainage ports for the expulsion of the water. Behind each of the sets of openings, and between the inner and outer hulls of the vessel, a water-housing chamber is defined. During the roll of the vessel, caused by wave action, either the port or the starboard chamber fills with water. In these harnessed positions, the water serves to develop a moment tending to stabilize the vessel against roll. Apparatus is provided for adjusting the size of the hull openings. With such a tank system, the operating efficiency is optimized without having to tune both the water inlet and the water outlet.
US Patent References:
Ship stabilizer
Hort - December 1936 - 2066150
Drill ship
Knight, Jr. et al. - February 1968 - 3368509
SHIP STABILIZING SYSTEM
Halden et al. - April 1970 - 3504651
Anti-pitch systems
Hilliard - January 1968 - 3366087
Ship stabilizer
Hort - December 1936 - 2066150
Drill ship
Knight, Jr. et al. - February 1968 - 3368509
SHIP STABILIZING SYSTEM
Halden et al. - April 1970 - 3504651
Anti-pitch systems
Hilliard - January 1968 - 3366087
Application Number:
05/202257
Publication Date:
03/19/1974
Filing Date:
11/26/1971
View Patent Images:
Export Citation:
Assignee:
Flume Stablizations Systems, Inc. (Hoboken, NJ)
Primary Class:
Other Classes:
114/122, 114/121
International Classes:
B63B39/03; B63B43/06; B63B39/00; B63B43/00; B63B43/06
Field of Search:
114/125,121,122
Primary Examiner:
Reger, Duane A.
Assistant Examiner:
Kunin, Stephen G.
Attorney, Agent or Firm:
Fleit, Gipple & Jacobson
Claims:
I claim
1. The combination of a double hull ship and a passive roll stabilizer, the combination comprising: a ship having an inner hull and an outer hull; at least one water-housing chamber on the port side of said ship; at least one water-housing chamber on the starboard side of said ship; each water-housing chamber being defined between said inner and said outer hulls; a first set of openings in said outer hull, both on the port and starboard sides of said ship, said first set of openings being at least partially above the normal waterline and serving as entry ports for the admission of the water in which the ship floats into said chambers; a second set of openings in said outer hull, both on the port and starboard sides of said ship; said second set of openings being positioned below said first set of openings and below said waterline and serving as discharge ports for the exit of the water housed in said chambers; said first set of openings being positioned at a height whereby the external waters attempting to roll the ship may freely pass into alternate chambers during given portions of the ship's roll cycle.
2. The combination recited in claim 1, wherein: the rear wall of each chamber is defined by the inner hull of the ship; the front wall of each chamber is defined by the outer hull of the ship; the top of each tank is defined by a respective horizontal plate extending from the inner hull to the outer hull; the bottom of each tank is defined by a respective bottom plate extending from the inner hull to the outer hull; and the sides of each tank are defined by vertical plates extending from the inner hull to the outer hull and from the horizontal top plate to the bottom plate.
3. The combination recited in claim 2, wherein the bottom plate extends outwardly and downwardly with respect to the center line of the ship.
4. The combination of claim 3, wherein each bottom plate mates with the outer hull at the bottom of the respective discharge port.
5. The combination recited in claim 1, wherein each entry port and each discharge port is adjustable in area.
6. The combination recited in claim 5, wherein each entry port and each discharge port is made adjustable by means of vertical plates slidably mounted on the outer hull of the ship.
7. The combination recited in claim 6, wherein the area of each entry port is adjustable over a range whereby it is ensured that the external waters have free access to each water-housing chamber under all sea states; and wherein the area of each drainage port is adjustable over a range whereby the discharge of the water from the chambers can be made coperiodic with the roll of the ship.
8. The combination recited in claim 1, wherein said first and second sets of openings and said port and starboard chambers, are associated in such a manner that the external waters attempting to roll the ship enter alternate chambers and develop moments, when in the chambers, opposing the roll of the ship.
9. The combination of claim 8, wherein the openings and chambers are associated in such a manner that when the ship is substantially horizontal, the chambers on one side of the ship are being charged with water while the chambers on the opposite side of the ship are discharging water.
10. The combination recited in claim 1, wherein the area of said first set of openings ensures that the external waters rapidly and completely fill said water-housing chambers during the roll of the ship; and wherein the area of said second set of openings is such that the external waters completely drain from said water-housing chambers in a time slightly longer than one-half of the period of the ship.
1. The combination of a double hull ship and a passive roll stabilizer, the combination comprising: a ship having an inner hull and an outer hull; at least one water-housing chamber on the port side of said ship; at least one water-housing chamber on the starboard side of said ship; each water-housing chamber being defined between said inner and said outer hulls; a first set of openings in said outer hull, both on the port and starboard sides of said ship, said first set of openings being at least partially above the normal waterline and serving as entry ports for the admission of the water in which the ship floats into said chambers; a second set of openings in said outer hull, both on the port and starboard sides of said ship; said second set of openings being positioned below said first set of openings and below said waterline and serving as discharge ports for the exit of the water housed in said chambers; said first set of openings being positioned at a height whereby the external waters attempting to roll the ship may freely pass into alternate chambers during given portions of the ship's roll cycle.
2. The combination recited in claim 1, wherein: the rear wall of each chamber is defined by the inner hull of the ship; the front wall of each chamber is defined by the outer hull of the ship; the top of each tank is defined by a respective horizontal plate extending from the inner hull to the outer hull; the bottom of each tank is defined by a respective bottom plate extending from the inner hull to the outer hull; and the sides of each tank are defined by vertical plates extending from the inner hull to the outer hull and from the horizontal top plate to the bottom plate.
3. The combination recited in claim 2, wherein the bottom plate extends outwardly and downwardly with respect to the center line of the ship.
4. The combination of claim 3, wherein each bottom plate mates with the outer hull at the bottom of the respective discharge port.
5. The combination recited in claim 1, wherein each entry port and each discharge port is adjustable in area.
6. The combination recited in claim 5, wherein each entry port and each discharge port is made adjustable by means of vertical plates slidably mounted on the outer hull of the ship.
7. The combination recited in claim 6, wherein the area of each entry port is adjustable over a range whereby it is ensured that the external waters have free access to each water-housing chamber under all sea states; and wherein the area of each drainage port is adjustable over a range whereby the discharge of the water from the chambers can be made coperiodic with the roll of the ship.
8. The combination recited in claim 1, wherein said first and second sets of openings and said port and starboard chambers, are associated in such a manner that the external waters attempting to roll the ship enter alternate chambers and develop moments, when in the chambers, opposing the roll of the ship.
9. The combination of claim 8, wherein the openings and chambers are associated in such a manner that when the ship is substantially horizontal, the chambers on one side of the ship are being charged with water while the chambers on the opposite side of the ship are discharging water.
10. The combination recited in claim 1, wherein the area of said first set of openings ensures that the external waters rapidly and completely fill said water-housing chambers during the roll of the ship; and wherein the area of said second set of openings is such that the external waters completely drain from said water-housing chambers in a time slightly longer than one-half of the period of the ship.
Description:
BACKGROUND OF THE INVENTION
The stabilization of water-going vessels is becoming an increasingly popular field. One area of this field deals with the stabilization of vessels by controlling the flow of liquids in the vessel to be stabilized.
This area of the stabilizing art concentrates on moving a quantity of on-board liquid, frequently water, from one side of the ship to the other, and this is done either passively or actively. The active stabilization of vessels is inherently complex in design and relatively expensive. The problems involved with such systems are well known in the art. The passive stabilizers, on the other hand, are substantially less expensive to manufacture and are relatively cost free in operation. There are, though, many drawbacks associated with the passive stabilizers.
A common drawback facing both the passive and the active types of stabilizers employing stabilizing liquids is that a free-surface correction must be considered in the design of the ship and the stabilizing tanks. When a stabilizing tank is filled with liquid, the "initial stability" of the ship decreases. That is, the presence of the liquid adds to the static instability of the ship. When a free-surface correction for this reduction in initial stability is designed into the ship, the cost of the ship is correspondingly increased.
Another problem resulting from the use of several of the known passive tank stabilizers relates to the space required for the stabilizing tanks. In certain ships, such as container ships and tankers, space utilization is extremely important. Many of the well known passive tank stabilizers are incompatible with the space requirements of these ships.
There are two known tanks which, to some extent, alleviate the problems described above. These tanks are described in a paper entitled "Roll Stabilization by Means of Passive Tanks," presented by J. Vasta et al. during the Nov., 1961 meeting of the Society of Naval Architects and Marine Engineers. The first is illustrated in FIG. 2 of the Vasta et al. paper and relates to what is essentially a U-tube stabilizing tank with the crossover duct removed and with the bottom of the wing tanks open to the sea. A valved air cross-over duct connects the tops of the wing tanks and serves as a means for tuning the tank to the roll of the ship. The second tank is illustrated in FIG. 6 of the Vasta et al. paper and is very much like the first tank described above except for the elimination of the air duct interconnecting the tops of the wing tanks. In this tank, the tops of each of the wing tanks are air vented to the atmosphere.
While the two tanks described in the preceding paragraph eliminate the need for free-surface corrections, one severe drawback is introduced. This drawback relates to the fact that the opening between the tank and the external sea must admit and expel water at a frequency tuned to the frequency at which the ship rolls. Therefore, either the air venting must be adjusted or the size of the hull openings must be adjusted so as to carefully match the frequency of the tank water transfer to the rolling frequency of the ship.
It is toward the elimination of the above-enumerated problems that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention relates to a passive stabilizer particularly suited for use in container ships and tankers. The inventive stabilizer depends, for its operation, directly upon the wave action which rolls the ship and, in fact, uses this wave action to stabilize the ship--the waves are harnessed in chambers defined in the ship and define the mass responsible for opposing roll.
The stabilizer of the present invention is particularly suited for ships of double hull construction. A water-tight compartment is defined between the inner and outer hulls in both the port and the starboard sides of the ship. The water in which the vessel floats is allowed to enter one or the other of the two between-hull compartments by means of a relatively large opening cut into each side of the outer hull. Then, to allow for the drainage of one of the compartments when it is desired that the other compartment be filled with water, a relatively small drainage opening is cut into the hull below each charging opening. In operation, the stabilizer functions by ensuring that the filled compartment or compartments are always on the side of the ship where a stabilizing moment is developed.
As noted previously, the upper openings, or water inlets, are relatively large while the lower openings, or water outlets, are small. It is the provision of two openings on each side of the ship, rather than one, and the relative sizes of the two openings, which eliminates the tuning difficulties encountered in the prior art tanks described above. The water inlets are dimensioned so as to allow for complete water entry notwithstanding the frequency at which the ship rolls. The water outlets, on the other hand, are dimensioned so as to provide for substantially complete water ejection in a time slightly longer than one-half the roll period of the ship. In this manner, the tank on the "low side" of the ship rapidly takes on its full capacity of water while, simultaneously, the tank on the "high side" of the ship slowly performs its discharge function. In this manner, it is ensured that the effects of the tank liquid are maximized; it follows that the efficiency of the stabilizer is maximized.
Because the stabilizer of the present invention is fully passive in nature, there is relatively no expense involved in its operation and, because the inventive stabilizer fits between the existing inner and outer hulls of a ship, the loss of space is minimized. Further, due to its construction, no static free-surface exists in the inventive stabilizer, and, consequently, no free-surface correction is required.
Accordingly, it is the main object of the present invention to provide a fully passive roll stabilizer which may be provided in a ship without wasting valuable cargo cubic and which operates efficiently without requiring careful tuning procedures.
It is another object of the present invention to provide a passive roll stabilizer which requires no free-surface correction.
It is yet a further object of the present invention to provide a passive roll stabilizer which uses the same waves which roll the vessel to stabilize the vessel.
These and other objects of the present invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a schematic representation of a typical unstabilized ship rolling in response to the action of a wave;
FIG. 1b is a view similar to FIG. 1a but showing a ship equipped with a typical free-surface passive tank stabilizer;
FIG. 2 is a cross-section through the hull of a ship equipped with a stabilizer constructed in accordance with the teachings of the present invention;
FIG. 3 is a view similar to FIG. 1 showing the action of a ship equipped with a stabilizer such as that shown in FIG. 2; and
FIG. 4 is a graph comparing the moment response of the open-hull stabilizer forming a part of the present invention with an open hull stabilizer known to the prior art.
DETAILED DESCRIPTION OF THE DRAWINGS
With reference first to FIG. 1a, a wave is indicated, in phantom, at 10 and a ship rolling in response to the wave is indicated at 12. It should be appreciated that the ship sequence illustrated in this figure is simplied for purposes of description. While the relative positions between the wave and the ship are accurate, it is the wave which actually moves past the ship, thereby causing rolling action, with little translation of the ship occurring in practice. It should also be appreciated that for ease of description, the wave 10 is shown as an ideal sine wave of a fixed frequency. Such, of course, is highly hypothetical.
As can be seen in FIG. 1a, the wave 10 imparts energy to the unstabilized ship 12, thereby causing the ship to roll. Because of the large inertial forces associated with the ship, the roll of the ship lags the wave by 90°. This phenomenon is borne out by practical experience. The arrows 14 represent the instantaneous direction of the roll of the ship 12.
In the sequence shown in FIG. 1b, a ship 16 rides in a wave (not shown) identical with the wave illustrated in FIG. 1a. The ship 16 is, however, equipped with a free-surface passive tank stabilizer 18 containing a body of water indicated at 20. It can be seen that the stabilized ship of FIG. 1b rolls as does the unstabilized ship shown in FIG. 1a but at a substantially reduced amplitude.
In FIG. 1b, it can be seen that the average position of the water 20 lags the roll of the ship 16 by 90°. This phase lag is due to the inertial forces associated with the water mass. Because the ship lags the wave by 90° and because the stabilizing mass lags the ship by 90°, the stabilizing mass is 180° out of phase with the wave. In this manner, the stabilizing mass developes a moment which opposes the moment developed by the wave. Basically, this is the principle of operation of the typical free-surface passive tank stabilizer.
As stated previously, the free-surface stabilizing tank has proved quite successful in the past. However, because of the necessity for free-surface corrections and the "wasted" space occupied by the stabilizer, there are conditions under which such a stabilizer would be impractical. It was with this in mind that the open hull passive stabilizer of the present invention was designed.
With reference then to FIG. 2, the inventive open hull stabilizer will be described. The stabilized ship is shown generally at 22 and is equipped with a pair of water-housing chambers 24 located between the inner hull 26 and the outer hull 28 of the ship 22. The top of each chamber 24 is defined by a horizontal plate 30; and the bottom of each chamber is defined by a longitudinal plate 32 which is angled downwardly and outwardly with respect to the center line of the ship 22. The respective inner hulls 26 define the rear walls of each chamber 24. A pair of plates 34 and 36, vertically movable with respect to each outer hull 28, define the remainder of the front walls of the water-housing chambers 24. The side walls (not shown) of the chambers 24 are defined by respective vertical plates extending between the inner and outer hulls of the ship, and from the top to the bottom of the chambers. The vertical side plates may be spaced apart any desired distance depending, of course, upon structural considerations.
An entry port 38 extends between the top of the vertical plate 36 and the horizontal plate 30; and a drainage port 40 extends between the bottom of the vertical plate 34 and the angled plate 32. As is indicated by the respective arrows 42 and 44, the plates 34 and 36 can be moved in a vertical direction, thereby changing the area of the entry ports 38; and, similarly, the plates 34 can be moved to change the area of the drainage ports 40. Normally, the entry ports 38 will be substantially larger than the drainage ports 40. The reason for this will become clear with reference to FIG. 3.
The sequence illustrated in FIG. 3 takes a form similar to the sequence illustrated in FIG. 1b. This is because the effect of the stabilizer of the present invention, illustrated in FIG. 3, is substantially the same as the effect of the stabilizer shown in FIG. 1b. That is, the inventive stabilizer operates on the principle that a mass of water is, at all times, on the side of the ship developing a moment opposing the moment developed by the wave.
In FIG. 3, the ship is shown at 22', floating in a body of water and acted upon by a wave 46. One of the inventive stabilizing tanks is shown at 48 and the opposite stabilizing tank is shown at 50. The arrows 52 represent the instantaneous direction in which the ship rolls.
The ship shown at the extreme left hand side of FIG. 3, as indicated by arrow 52, is rolling in the counter-clockwise direction. At this time, due to inertial forces, the ship lags the wave by 90°, and, therefore, the wave 46 is at its maximum height with respect to the ship 22'. Therefore, water pours into the entry port 38 of the tank 50, this water action being indicated by arrow 54. Concurrently, the water which entered the tank 48 during a previous portion of the roll cycle, leaves the tank 48 through the discharge port 40 thereof as shown at 56. With the ship attempting to roll in the direction of the arrow 52, and with the tank 50 fully charged with water, a stabilizing moment is developed, owing to static forces developed by the water in the tank 50, tending to oppose the roll of the ship.
The maximum counterclockwise roll is illustrated in the ship occupying the next position in FIG. 3. At this time, it will be noted that the water in the tank 48 is approximately equal in volume to the water in the tank 50. The large mass of water once filling tank 50 has partially drained; and the wave action has partially filled tank 48.
Then, in the next position of FIG. 3, the ship 22' is again in its horizontal position. At this time, the wave 46 fills tank 48. Concurrently, the tank 50 is being drained of its water supply. Then, when the ship attempts to roll in the direction of arrow 52, the static moment developed by the weight of the water in tank 48 counteracts the action of the wave and therefore reduces the roll of the ship.
The operation of the tank continues as described above, thereby reducing the roll experienced by the ship. As noted previously, the fact that there is no static free-surface in the inventive stabilizer makes a free-surface correction unnecessary. And, as is evident when viewing FIG. 2, the location of the water-housing chambers minimizes the "wasted" space required for roll stabilization.
It is anticipated that the ship equipped with the inventive stabilizer will experience sea states which vary both in frequency and intensity. For this reason, and as shown in FIG. 2, the vertical plates 34 and 36 are provided. In this manner, both the entry port 38 and the drainage port 40 may be changed in area.
The area changes noted in the preceding paragraph would be made as follows. If the sea is relatively rough, that is, if the waves are high, then the entry ports 38 would be made the size shown in FIG. 2. However, if the sea is quite, with low waves, then the plates 36 would be lowered, thereby lowering the bottom of the entry ports 38. In this manner, water entry would be ensured under all sea states.
The drainage ports 40, on the other hand, are varied in area for a different reason. Naturally, it is desirable to maintain the water in the chambers where it will do most to stabilize the ship for a maximum time. This statement is based upon the fact that the ship tends to roll at a period defined both by the ship and the sea state. Therefore, if the period of the ship, when rolling in a given sea state, is long, the water should be held in the chambers 24 for a proportional time. This is accomplished by lowering the plates 34, thereby decreasing the area of the exit ports 40. If, on the other hand, the period of the ship is short, then the area of the exit ports 40 would be increased, thereby allowing rapid exit of the water from the chambers 24.
Now, with reference to FIG. 4, the increased effectiveness of the inventive stabilizer, when compared with the tuned open hull stabilizers known to the prior art, will be explained. In FIG. 4, the curve 60 represents the harmonic moment displayed by a typical open hull stabilizer known to the prior art. From the figure, it can be seen that curve 60 approximates a sine wave. This results from the fact that the prior art open hull stabilizers have but a single, tuned, hull opening communicating between the stabilizer tank and the external waters. Therefore, the entry port and the exit port are of identical areas and, as a consequence, sea water enters and exits at an equal rate.
The curve 62 illustrates the moment response of a stabilizer constructed in accordance with the teachings of the present invention. And, for purposes of illustration, a stabilized ship 22', following the same time scale as the curves 60 and 62, is depicted above the curves.
Beginning at time t 0 , with the ship 22'heeling in a maximum counterclockwise direction, tanks 48 and 50 contain substantially equal volumes of water. This has been explained with reference to FIG. 3. It is here, however, that the waves of the sea begin to fill the tank 48; and, because of the relatively large area of the entry port, the sea water rapidly fills this tank.
From FIG. 4, it should be readily apparent that there is a substantial difference in filling rates between the open hull tanks known to the prior art and the tank of the present invention. There is also a difference in the ultimate volume of water housed in the two types of tanks. With the inventive tank, having large entry ports, the wing tanks are filled rapidly and to their full capacity twice during each roll cycle of the ship. With the open hull tanks known to the prior art, because of the need to match both the water entry and the water exit to the roll of the ship, it is usual that the tank fills slowly and only partially during the roll cycle of the ship.
As a result of the two factors noted in the preceding paragraph, the inventive tank shows two substantial advantages not found in the tanks known to the prior art. These advantages are that the inventive tank has a larger maximum moment than the prior art tanks, and that the moment developed by the inventive tank acts over a time longer than the moment developed by the tanks of the prior art. This is evident when one notes the area under the moment curves shown in FIG. 4, this area being directly proportional to the effectiveness of the stabilizing tank.
In FIG. 4, it will also be noted that the inventive tank empties at a rate substantially tuned to the roll of the ship. As noted previously, this rate is adjusted at will, by acting upon the area of the exit ports. It should be remembered, however, that the entry ports remain always of an area ensuring rapid and total filling of each wing tank during the roll of the ship. Therefore, while the water expulsion may be slow, to match the tank to the ship, the water acceptance is always rapid.
Above, several embodiments of the present invention have been described. It should be appreciated, however, that these embodiments are described for purposes of illustration only and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is the intent that the invention not be limited by the above but be limited only as defined in the appended claims.
The stabilization of water-going vessels is becoming an increasingly popular field. One area of this field deals with the stabilization of vessels by controlling the flow of liquids in the vessel to be stabilized.
This area of the stabilizing art concentrates on moving a quantity of on-board liquid, frequently water, from one side of the ship to the other, and this is done either passively or actively. The active stabilization of vessels is inherently complex in design and relatively expensive. The problems involved with such systems are well known in the art. The passive stabilizers, on the other hand, are substantially less expensive to manufacture and are relatively cost free in operation. There are, though, many drawbacks associated with the passive stabilizers.
A common drawback facing both the passive and the active types of stabilizers employing stabilizing liquids is that a free-surface correction must be considered in the design of the ship and the stabilizing tanks. When a stabilizing tank is filled with liquid, the "initial stability" of the ship decreases. That is, the presence of the liquid adds to the static instability of the ship. When a free-surface correction for this reduction in initial stability is designed into the ship, the cost of the ship is correspondingly increased.
Another problem resulting from the use of several of the known passive tank stabilizers relates to the space required for the stabilizing tanks. In certain ships, such as container ships and tankers, space utilization is extremely important. Many of the well known passive tank stabilizers are incompatible with the space requirements of these ships.
There are two known tanks which, to some extent, alleviate the problems described above. These tanks are described in a paper entitled "Roll Stabilization by Means of Passive Tanks," presented by J. Vasta et al. during the Nov., 1961 meeting of the Society of Naval Architects and Marine Engineers. The first is illustrated in FIG. 2 of the Vasta et al. paper and relates to what is essentially a U-tube stabilizing tank with the crossover duct removed and with the bottom of the wing tanks open to the sea. A valved air cross-over duct connects the tops of the wing tanks and serves as a means for tuning the tank to the roll of the ship. The second tank is illustrated in FIG. 6 of the Vasta et al. paper and is very much like the first tank described above except for the elimination of the air duct interconnecting the tops of the wing tanks. In this tank, the tops of each of the wing tanks are air vented to the atmosphere.
While the two tanks described in the preceding paragraph eliminate the need for free-surface corrections, one severe drawback is introduced. This drawback relates to the fact that the opening between the tank and the external sea must admit and expel water at a frequency tuned to the frequency at which the ship rolls. Therefore, either the air venting must be adjusted or the size of the hull openings must be adjusted so as to carefully match the frequency of the tank water transfer to the rolling frequency of the ship.
It is toward the elimination of the above-enumerated problems that the present invention is directed.
SUMMARY OF THE INVENTION
The present invention relates to a passive stabilizer particularly suited for use in container ships and tankers. The inventive stabilizer depends, for its operation, directly upon the wave action which rolls the ship and, in fact, uses this wave action to stabilize the ship--the waves are harnessed in chambers defined in the ship and define the mass responsible for opposing roll.
The stabilizer of the present invention is particularly suited for ships of double hull construction. A water-tight compartment is defined between the inner and outer hulls in both the port and the starboard sides of the ship. The water in which the vessel floats is allowed to enter one or the other of the two between-hull compartments by means of a relatively large opening cut into each side of the outer hull. Then, to allow for the drainage of one of the compartments when it is desired that the other compartment be filled with water, a relatively small drainage opening is cut into the hull below each charging opening. In operation, the stabilizer functions by ensuring that the filled compartment or compartments are always on the side of the ship where a stabilizing moment is developed.
As noted previously, the upper openings, or water inlets, are relatively large while the lower openings, or water outlets, are small. It is the provision of two openings on each side of the ship, rather than one, and the relative sizes of the two openings, which eliminates the tuning difficulties encountered in the prior art tanks described above. The water inlets are dimensioned so as to allow for complete water entry notwithstanding the frequency at which the ship rolls. The water outlets, on the other hand, are dimensioned so as to provide for substantially complete water ejection in a time slightly longer than one-half the roll period of the ship. In this manner, the tank on the "low side" of the ship rapidly takes on its full capacity of water while, simultaneously, the tank on the "high side" of the ship slowly performs its discharge function. In this manner, it is ensured that the effects of the tank liquid are maximized; it follows that the efficiency of the stabilizer is maximized.
Because the stabilizer of the present invention is fully passive in nature, there is relatively no expense involved in its operation and, because the inventive stabilizer fits between the existing inner and outer hulls of a ship, the loss of space is minimized. Further, due to its construction, no static free-surface exists in the inventive stabilizer, and, consequently, no free-surface correction is required.
Accordingly, it is the main object of the present invention to provide a fully passive roll stabilizer which may be provided in a ship without wasting valuable cargo cubic and which operates efficiently without requiring careful tuning procedures.
It is another object of the present invention to provide a passive roll stabilizer which requires no free-surface correction.
It is yet a further object of the present invention to provide a passive roll stabilizer which uses the same waves which roll the vessel to stabilize the vessel.
These and other objects of the present invention, as well as many of the attendant advantages thereof, will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a schematic representation of a typical unstabilized ship rolling in response to the action of a wave;
FIG. 1b is a view similar to FIG. 1a but showing a ship equipped with a typical free-surface passive tank stabilizer;
FIG. 2 is a cross-section through the hull of a ship equipped with a stabilizer constructed in accordance with the teachings of the present invention;
FIG. 3 is a view similar to FIG. 1 showing the action of a ship equipped with a stabilizer such as that shown in FIG. 2; and
FIG. 4 is a graph comparing the moment response of the open-hull stabilizer forming a part of the present invention with an open hull stabilizer known to the prior art.
DETAILED DESCRIPTION OF THE DRAWINGS
With reference first to FIG. 1a, a wave is indicated, in phantom, at 10 and a ship rolling in response to the wave is indicated at 12. It should be appreciated that the ship sequence illustrated in this figure is simplied for purposes of description. While the relative positions between the wave and the ship are accurate, it is the wave which actually moves past the ship, thereby causing rolling action, with little translation of the ship occurring in practice. It should also be appreciated that for ease of description, the wave 10 is shown as an ideal sine wave of a fixed frequency. Such, of course, is highly hypothetical.
As can be seen in FIG. 1a, the wave 10 imparts energy to the unstabilized ship 12, thereby causing the ship to roll. Because of the large inertial forces associated with the ship, the roll of the ship lags the wave by 90°. This phenomenon is borne out by practical experience. The arrows 14 represent the instantaneous direction of the roll of the ship 12.
In the sequence shown in FIG. 1b, a ship 16 rides in a wave (not shown) identical with the wave illustrated in FIG. 1a. The ship 16 is, however, equipped with a free-surface passive tank stabilizer 18 containing a body of water indicated at 20. It can be seen that the stabilized ship of FIG. 1b rolls as does the unstabilized ship shown in FIG. 1a but at a substantially reduced amplitude.
In FIG. 1b, it can be seen that the average position of the water 20 lags the roll of the ship 16 by 90°. This phase lag is due to the inertial forces associated with the water mass. Because the ship lags the wave by 90° and because the stabilizing mass lags the ship by 90°, the stabilizing mass is 180° out of phase with the wave. In this manner, the stabilizing mass developes a moment which opposes the moment developed by the wave. Basically, this is the principle of operation of the typical free-surface passive tank stabilizer.
As stated previously, the free-surface stabilizing tank has proved quite successful in the past. However, because of the necessity for free-surface corrections and the "wasted" space occupied by the stabilizer, there are conditions under which such a stabilizer would be impractical. It was with this in mind that the open hull passive stabilizer of the present invention was designed.
With reference then to FIG. 2, the inventive open hull stabilizer will be described. The stabilized ship is shown generally at 22 and is equipped with a pair of water-housing chambers 24 located between the inner hull 26 and the outer hull 28 of the ship 22. The top of each chamber 24 is defined by a horizontal plate 30; and the bottom of each chamber is defined by a longitudinal plate 32 which is angled downwardly and outwardly with respect to the center line of the ship 22. The respective inner hulls 26 define the rear walls of each chamber 24. A pair of plates 34 and 36, vertically movable with respect to each outer hull 28, define the remainder of the front walls of the water-housing chambers 24. The side walls (not shown) of the chambers 24 are defined by respective vertical plates extending between the inner and outer hulls of the ship, and from the top to the bottom of the chambers. The vertical side plates may be spaced apart any desired distance depending, of course, upon structural considerations.
An entry port 38 extends between the top of the vertical plate 36 and the horizontal plate 30; and a drainage port 40 extends between the bottom of the vertical plate 34 and the angled plate 32. As is indicated by the respective arrows 42 and 44, the plates 34 and 36 can be moved in a vertical direction, thereby changing the area of the entry ports 38; and, similarly, the plates 34 can be moved to change the area of the drainage ports 40. Normally, the entry ports 38 will be substantially larger than the drainage ports 40. The reason for this will become clear with reference to FIG. 3.
The sequence illustrated in FIG. 3 takes a form similar to the sequence illustrated in FIG. 1b. This is because the effect of the stabilizer of the present invention, illustrated in FIG. 3, is substantially the same as the effect of the stabilizer shown in FIG. 1b. That is, the inventive stabilizer operates on the principle that a mass of water is, at all times, on the side of the ship developing a moment opposing the moment developed by the wave.
In FIG. 3, the ship is shown at 22', floating in a body of water and acted upon by a wave 46. One of the inventive stabilizing tanks is shown at 48 and the opposite stabilizing tank is shown at 50. The arrows 52 represent the instantaneous direction in which the ship rolls.
The ship shown at the extreme left hand side of FIG. 3, as indicated by arrow 52, is rolling in the counter-clockwise direction. At this time, due to inertial forces, the ship lags the wave by 90°, and, therefore, the wave 46 is at its maximum height with respect to the ship 22'. Therefore, water pours into the entry port 38 of the tank 50, this water action being indicated by arrow 54. Concurrently, the water which entered the tank 48 during a previous portion of the roll cycle, leaves the tank 48 through the discharge port 40 thereof as shown at 56. With the ship attempting to roll in the direction of the arrow 52, and with the tank 50 fully charged with water, a stabilizing moment is developed, owing to static forces developed by the water in the tank 50, tending to oppose the roll of the ship.
The maximum counterclockwise roll is illustrated in the ship occupying the next position in FIG. 3. At this time, it will be noted that the water in the tank 48 is approximately equal in volume to the water in the tank 50. The large mass of water once filling tank 50 has partially drained; and the wave action has partially filled tank 48.
Then, in the next position of FIG. 3, the ship 22' is again in its horizontal position. At this time, the wave 46 fills tank 48. Concurrently, the tank 50 is being drained of its water supply. Then, when the ship attempts to roll in the direction of arrow 52, the static moment developed by the weight of the water in tank 48 counteracts the action of the wave and therefore reduces the roll of the ship.
The operation of the tank continues as described above, thereby reducing the roll experienced by the ship. As noted previously, the fact that there is no static free-surface in the inventive stabilizer makes a free-surface correction unnecessary. And, as is evident when viewing FIG. 2, the location of the water-housing chambers minimizes the "wasted" space required for roll stabilization.
It is anticipated that the ship equipped with the inventive stabilizer will experience sea states which vary both in frequency and intensity. For this reason, and as shown in FIG. 2, the vertical plates 34 and 36 are provided. In this manner, both the entry port 38 and the drainage port 40 may be changed in area.
The area changes noted in the preceding paragraph would be made as follows. If the sea is relatively rough, that is, if the waves are high, then the entry ports 38 would be made the size shown in FIG. 2. However, if the sea is quite, with low waves, then the plates 36 would be lowered, thereby lowering the bottom of the entry ports 38. In this manner, water entry would be ensured under all sea states.
The drainage ports 40, on the other hand, are varied in area for a different reason. Naturally, it is desirable to maintain the water in the chambers where it will do most to stabilize the ship for a maximum time. This statement is based upon the fact that the ship tends to roll at a period defined both by the ship and the sea state. Therefore, if the period of the ship, when rolling in a given sea state, is long, the water should be held in the chambers 24 for a proportional time. This is accomplished by lowering the plates 34, thereby decreasing the area of the exit ports 40. If, on the other hand, the period of the ship is short, then the area of the exit ports 40 would be increased, thereby allowing rapid exit of the water from the chambers 24.
Now, with reference to FIG. 4, the increased effectiveness of the inventive stabilizer, when compared with the tuned open hull stabilizers known to the prior art, will be explained. In FIG. 4, the curve 60 represents the harmonic moment displayed by a typical open hull stabilizer known to the prior art. From the figure, it can be seen that curve 60 approximates a sine wave. This results from the fact that the prior art open hull stabilizers have but a single, tuned, hull opening communicating between the stabilizer tank and the external waters. Therefore, the entry port and the exit port are of identical areas and, as a consequence, sea water enters and exits at an equal rate.
The curve 62 illustrates the moment response of a stabilizer constructed in accordance with the teachings of the present invention. And, for purposes of illustration, a stabilized ship 22', following the same time scale as the curves 60 and 62, is depicted above the curves.
Beginning at time t 0 , with the ship 22'heeling in a maximum counterclockwise direction, tanks 48 and 50 contain substantially equal volumes of water. This has been explained with reference to FIG. 3. It is here, however, that the waves of the sea begin to fill the tank 48; and, because of the relatively large area of the entry port, the sea water rapidly fills this tank.
From FIG. 4, it should be readily apparent that there is a substantial difference in filling rates between the open hull tanks known to the prior art and the tank of the present invention. There is also a difference in the ultimate volume of water housed in the two types of tanks. With the inventive tank, having large entry ports, the wing tanks are filled rapidly and to their full capacity twice during each roll cycle of the ship. With the open hull tanks known to the prior art, because of the need to match both the water entry and the water exit to the roll of the ship, it is usual that the tank fills slowly and only partially during the roll cycle of the ship.
As a result of the two factors noted in the preceding paragraph, the inventive tank shows two substantial advantages not found in the tanks known to the prior art. These advantages are that the inventive tank has a larger maximum moment than the prior art tanks, and that the moment developed by the inventive tank acts over a time longer than the moment developed by the tanks of the prior art. This is evident when one notes the area under the moment curves shown in FIG. 4, this area being directly proportional to the effectiveness of the stabilizing tank.
In FIG. 4, it will also be noted that the inventive tank empties at a rate substantially tuned to the roll of the ship. As noted previously, this rate is adjusted at will, by acting upon the area of the exit ports. It should be remembered, however, that the entry ports remain always of an area ensuring rapid and total filling of each wing tank during the roll of the ship. Therefore, while the water expulsion may be slow, to match the tank to the ship, the water acceptance is always rapid.
Above, several embodiments of the present invention have been described. It should be appreciated, however, that these embodiments are described for purposes of illustration only and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. Accordingly, it is the intent that the invention not be limited by the above but be limited only as defined in the appended claims.