Heave damping device for ships
United States Patent 3896755
This invention relates to a ship heave stabilization apparatus having horizontally rotatable fins in selective, operable connection with a ship's hull. The fins preferably are balanced about the hull and may be deployed on more than one hull in a multi-hull ship.
US Patent References:
Devices for damping the rolling movement of ships
Kollenberger - August 1958 - 2848971
Maneuvering propeller means for ships
Hawthorne - May 1959 - 2885990
Ship stabilizer
Laurenti - August 1966 - 3265029
SEA-GOING CARGO TRANSPORTATION FACILITIES
Farrell - November 1970 - 3537413
HIGH-SPEED SHIP WITH SUBMERGED HULLS
Lang - November 1971 - 3623444
Devices for damping the rolling movement of ships
Kollenberger - August 1958 - 2848971
Maneuvering propeller means for ships
Hawthorne - May 1959 - 2885990
Ship stabilizer
Laurenti - August 1966 - 3265029
SEA-GOING CARGO TRANSPORTATION FACILITIES
Farrell - November 1970 - 3537413
HIGH-SPEED SHIP WITH SUBMERGED HULLS
Lang - November 1971 - 3623444
Application Number:
05/434893
Publication Date:
07/29/1975
Filing Date:
01/21/1974
View Patent Images:
Export Citation:
Assignee:
Litton Systems, Inc. (Beverly Hills, CA)
Primary Class:
International Classes:
B63B39/06; B63B39/00; B63B39/06
Field of Search:
114/.5D,61,66.5R,66.5H,43.5,121-122,126,152 115/34
Primary Examiner:
Blix, Trygve M.
Assistant Examiner:
Sotelo, Jesus D.
Attorney, Agent or Firm:
Rose Jr., Alan Turner Thomas C. A.
Claims:
I claim
1. Fin means for stabilizing a ship in water comprising at least one fin disposed connected to said ship in relatively horizontal relationship with the hull of said ship, said fin being selectively operable for rotation beneath water level about a vertical axis relative to said ship, said fin including relatively smooth, flat, horizontally disposed blades balanced about the connection of said fin with said hull of said ship, wherein said balanced fin is so balanced that the sum of the moments of the vertical water forces acting on said horizontally disposed flat blades equal zero at any given moment, when the ship is stationary.
2. Heave stabilization apparatus for a ship comprising:
3. Apparatus as claimed in claim 2 wherein said fin means is selectively operable for placement in orthogonal relationship with said hull.
4. Apparatus as claimed in claim 2 wherein said fin means is selectively and operably rotatable in a horizontal plane relative to said ship.
5. Apparatus as claimed in claim 2, wherein said ship further includes more than one longitudinal hull, each said hull having horizontally rotatable fin means disposed in selective, operable connection centered relative to its respective hull of said ship, each said rotatable fin means further comprising a fin balanced about its respective said connection.
6. Apparatus as claimed in claim 2 wherein said fin means is operably securable completely within the recessed section of said hull.
1. Fin means for stabilizing a ship in water comprising at least one fin disposed connected to said ship in relatively horizontal relationship with the hull of said ship, said fin being selectively operable for rotation beneath water level about a vertical axis relative to said ship, said fin including relatively smooth, flat, horizontally disposed blades balanced about the connection of said fin with said hull of said ship, wherein said balanced fin is so balanced that the sum of the moments of the vertical water forces acting on said horizontally disposed flat blades equal zero at any given moment, when the ship is stationary.
2. Heave stabilization apparatus for a ship comprising:
3. Apparatus as claimed in claim 2 wherein said fin means is selectively operable for placement in orthogonal relationship with said hull.
4. Apparatus as claimed in claim 2 wherein said fin means is selectively and operably rotatable in a horizontal plane relative to said ship.
5. Apparatus as claimed in claim 2, wherein said ship further includes more than one longitudinal hull, each said hull having horizontally rotatable fin means disposed in selective, operable connection centered relative to its respective hull of said ship, each said rotatable fin means further comprising a fin balanced about its respective said connection.
6. Apparatus as claimed in claim 2 wherein said fin means is operably securable completely within the recessed section of said hull.
Description:
BACKGROUND OF THE INVENTION
When running in waves, ships at times develop a heaving motion. Heave is defined, and hereinafter will be used to refer to a vertical translatory oscillation of an entire hull of a ship. Heaving motion is not to be confused with pitching, which is a rotational oscillation of the hull about a transverse horizontal axis, causing the bow of the ship or vessel to rise and dip alternately relative to the stern.
Hereinafter, for the sake of simplicity and explanation a ship will be considered as having a longitudinal direction parallel to the normal direction of forward movement of the ship. This is the direction on which ship length is measured. Moreover, for the sake of explanation, a ship will be considered hereinafter as having an athwartship direction horizontally perpendicular to the longitudinal direction. It is in the athwartship direction that ship beam is measured.
The heave of a ship is in many instances undesirable. It is particularly undesirable for ships attempting to dock with other ships while at sea. Heave is problematical when ships, such as aircraft carriers, attempt to dispatch and receive aircraft, such as helicopters. Heave is also a serious problem for ships attempting to discharge cargo from a deepwater dock, or while at anchor in rough seas. Such cargo, it will be appreciated, could be liquid gases or petroleum oils. Any force which tends to disrupt the connection of pipes and other apparatus connecting the deepwater anchored ship with the shore could pose a threat to the water environment.
Ships having reduced waterplane areas are susceptible to heaving to a greater degree than conventional ships by reason of their lower heave damping coefficients. The waterplane area is defined, and hereinafter will be used to refer to that area of the hull intersected by a plane at the water level. In a reduced waterplane area ship the superstructure of the ship is ordinarily connected with an underwater hull of the ship by one or more surface-piercing hull elements which are substantially thinner, and often shorter than the underwater hull. The heave of such a ship can become inconveniently large in magnitude. Such a reduced waterplane area ship, such as is shown in FIG. 1 of the attached drawings, is not troubled as much as conventional ships by roll and pitch movements.
Such small waterplane area ships, it will be appreciated, can derive advantages by a device which will reduce the heave motion, but will have minimum intentional effects on pitch movements. Moreover, such ships, when receiving or discharging cargo, and when receiving or dispatching aircraft or vessels, will derive advantage from an apparatus which will diminish or damp heaving while the ship is stationary, and not in forward movement through the water. Such ships, furthermore, will derive advantage from heave control devices which will not interfere with the forward movement of the ship when it is undergoing normal operation. Hereinafter, the word "damp" will be used to describe the resistance to motion and its effects and the phenomenon diminishing oscillatory magnitude in oscillations once excitation has stopped.
In the past, it has been well known to attach fins to the hulls of ships. Such fins are normally placed adjacent the hull of the ship so as to control roll and pitch of vessels. See, for example, the fin apparatus of Emery, U.S. Pat. No. 471,212. As in Emery, or in Noll, U.S. Pat. No. 767,827, or in many other similarly described fins, the fins are operable so that they engage the water by virtue of the forward motion of the ship, at selected angles of impingement of the water on the fin. In such a manner, the pitch and roll of a forward moving ship can be controlled.
These fins, as is shown in Noll, in Rebikoff, U.S. Pat. No. 3,093,105, and in Great Britain Pat. No. 19,886 (1890), have various operable apparatus for eliminating the fins' deleterious drag in the water when so desired. In many cases, the fins are completely retracted within the hull of the ship, such as was described by the Denny-Brown Ship Stabilizer, Engineering Magazine, Sept. 25, 1936, page 334.
In none of the prior art teachings heretofore made, has an apparatus for stabilizing the heave of a ship been shown. More interestingly, in none of the prior art teachings has any device been shown for stabilising vertical translatory motion of a relatively large ship, or of any part of such a ship while the ship is attempting to be stationary.
SUMMARY OF THE INVENTION
A novel ship heave stabilization fin arrangement is described. The arrangement includes fins disposed underneath the water level of a ship. In the case of multi-hull vessels, the fins may be stationarily positioned inwardly of the multi-hulls. The preferred embodiment is to have fins rotatably connected to underwater portions of the hull, the rotation being selective so that the fins may be positioned at any angle relative to the hull, or the fins may be rotated. A method of using such a fin arrangement is described.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view of a small waterplane area ship;
FIG. 2 is a front elevation view of the ship shown in FIG. 1;
FIG. 3 is a plan, schematic view of one alternative embodiment of the ship as shown in FIG. 1;
FIG. 4 is a plan, schematic view of a second alternative embodiment of the ship shown in FIG. 1;
FIG. 5 is a plan, schematic view of the preferred embodiment of the present invention;
FIG. 6 is a front elevation view of the preferred embodiment of the invention shown in FIG. 5;
FIG. 7 is a side elevation view of the preferred embodiment taken along line 7--7 of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT
To resolve the foregoing problems and to obtain the foregoing desired advantages, a ship 10 having heave damping or stabilization fins 20 is described, reference being made to FIGS. 1 and 2 in the attached drawings. The ship 10 shown is one having a smaller than conventional waterplane area, and more than one hull 14. The ship 10 has a superstructure 12 and hulls 14. The ship 10 is also constructed having struts 16 connecting the hulls 14 to the superstructure 12. The normal water level is generally indicated at 18.
In FIG. 2 fins 20 are shown. The space 28 between the hulls 14 and struts 16, underneath the superstructure 12 may be viewed as the inward side of the ship 10. The fins 20 are shown positioned on the inward side of the hulls 14. The fins 20 are horizontal in relation to the ship 10.
Normally, a line of action of gravity is considered a vertical line, or as being in the vertical direction. A plane normal to this line is considered horizontal. A ship upon the sea, however, has its keel-to-mast, or bottom-to-top direction aligned with the vertical only momentarily. More likely, its bottom-to-top line is misaligned from true vertical because of the ship's being bobbed about by the waves of the sea. Hereinafter, therefore, the term vertical shall be used to designate bottom-to-top line of the ship, and the term horizontal shall be used to designate directions normal to this vertical line. In brief, the term vertical and horizontal hereinafter will designate directions in relation to a ship as if its bottom-to-top direction were aligned with the true vertical.
The fins 20 are relatively flat blades parallel to the horizontal. The fins 20 should be situated on the hulls below the water level 18. As shown in FIG. 3, these fins 22 may be relatively short as illustrated in FIG. 4. If the relatively short fins 24 are used, the fins 24 should extend further from the hull 14 into the water on the inward side of the ship than the relatively long fins 22. Alternatively, a plurality of inwardly directed short fins 24 may be affixed to each hull 14. Such an alternative arrangement is not illustrated.
Such fin arrangements as are shown in FIGS. 2, 3 and 4 have significant use in affording stability of a ship relative to heave forces, especially while the ship is moving. Such stationary fixed fin arrangements, however, have proved disappointing at times in damping heave when a ship is stationary, or is moving at slow speeds.
To overcome heave forces at slow speeds or at zero speed, a fin arrangement is shown in FIGS. 5 and 6. FIGS. 5 and 6 are schematic views of the multi-hulled ship 10, as seen from the bottom of the ship 10 looking vertically upward. Fins 30 are rotatably connected to the ship 10 by axles 32. The axles 32 are vertical relative to the ship, as explained above. The fins 30, therefore, are capable of being rotated in the horizontal plane. The fin's blades are horizontal, and should be constructed so as to be relatively flat.
Generally, speaking, the fin blades of the fins 22, 24, 30 should be hydrodynamically constructed so that they move through the water smoothly, presenting as little resistance to horizontal water flow through inward space 28 as possible. It is not contemplated that any of the fin arrangements taught herein should affect the position of the ship relative to the sea by virtue of redirecting horizontal water flow about the ship.
In FIGS. 5 and 6, one fin 30 is shown as being longitudinally disposed alongside the bottom of the respective hull to which it is attached. The other fin 30 is shown as rotated some 90° relative to its respective hull. It is contemplated that these fins 30 may be independently controlled, or that they may be controlled by a unitary control. Of course, the fins 30 may be operated, alternatively, independently but in unison with each other.
In either mode of control, it is contemplated that these fins 30 may be rotated to any angle relative to the longitudinal hulls 14, and there rigidly held in position to maintain that angle. Also, in either mode of control it is contemplated that the fins 30 may be capable of operation in a steady state of rotation. Normally, they should be rotated in opposite directions. Either of the possible pair of opposite directions is contemplated; i.e., clockwise to port and counterclockwise to starboard, or their opposites. The fin cross sections may be such that one pair of directions is preferred over the other by reason of less required rotating power.
If the fins 30 are rotated to a stationary position at an angle from the longitudinal hull 14, the fins 30 would operate with comparable effectiveness as the stationary fins 22, or more likely as the fins 24. The fins 30, however, would be capable of a rotation returning them to a position longitudinally alongside the longitudinal hulls 14, and there out of contact from the flow of water caused by the forward motion of a ship. In such a position, the fins 30 would present minimized resistance or drag to the flow of water around them.
To reduce further or to minimize the drag, a recessed section 34 can be constructed in the hulls 14. This recessed section 34, as illustrated best in FIG. 7 of the attached drawings, should be made so as to allow the fins 30 free movement in and out of them, but designed closely to the fins 30 so that minimum effects of the gap between the fins 30 and their respective hulls 14 are realized.
The fins 30 are shown in the drawings as being balanced about the connection of the fins 30 with their respective axles 32. Preferably, the fins should be balanced so that water forces acting on the blades of the fins 30 sum to zero relative to the fulcrum as which each axle 32 serves. In such a manner, bending moments on the axle 32 are minimized. Moreover, should the fin 30 be asymmetrical, the weight of the fin 30 should be evenly balanced about its respective axle 32.
When the fins 30 are so balanced, or are positioned so as to be substantially so balanced, the torque required to rotate them about their respective axis of their respective axles 32 of each fin is made as uniform as possible. As noted above, in either mode of control it is contemplated that the fins 30 should be capable of rotation at steady state or varying rotational velocities. It has been found that at relatively low ship speeds, or when the ship is standing still, stationary blades or fins provide little reaction against heave forces. In this slow speed or stationary ship condition, however, heave stabilization or damping is achieved when the fins 30 are rotated. The slower the ship 10 is traveling relative to the water, the greater the rotational speed that will be required to satisfactorily damp the heave. In calculations, it has been found that the heave-damping force is approximately proportional to the rotational velocity of the axle 32 times heave velocity. Heave velocity is defined as the time rate of heaving motion.
The axle 32 should be positioned with a watertight opening through the hull 14. The power drive for the axle 32 may be constructed within the hull 14, or in any convenient alternate location such as, for example, the superstructure 12. In any location, however, the powering apparatus should be controllable from a ship's control center. Preferably, each fin 30 should be independently so controllable.
It is of importance that the fins 30 be located along the hull 14 longitudinally so that the heave-damping forces upon the ship are balanced longitudinally and athwartship about the ship center of flotation. It has been found that if only one fin 30 for each hull is used, and it is positioned on an axle disposed more toward the bow or stern relative to the hull's center of flotation, a pitch moment on the ship is generated. Similarly, a fin placed on only one of several parallel longitudinal hulls will generate a roll moment when the ship heaves. Thus, it is relatively important that the positioning of the fins 30 be reasonably balanced relative to the ship's center of gravity. Several fins 30 may be positioned longitudinally along the hull, of course, and still have reasonable balance of the heave-damping forces about the center of gravity of the ship.
As can be seen from the foregoing specification of the preferred embodiment, an improvement to fin arrangements on ships has been described which will effectively afford heave-damping forces, and yet will not much increase the drag on the ship while it is in forward motion through the water. While the preferred embodiment and several alternative embodiments have been described in this foregoing specification, it is appreciated that other alternative embodiments of the invention can be perceived. Thus, the scope of the described invention should not be limited except by the claims which follow.
When running in waves, ships at times develop a heaving motion. Heave is defined, and hereinafter will be used to refer to a vertical translatory oscillation of an entire hull of a ship. Heaving motion is not to be confused with pitching, which is a rotational oscillation of the hull about a transverse horizontal axis, causing the bow of the ship or vessel to rise and dip alternately relative to the stern.
Hereinafter, for the sake of simplicity and explanation a ship will be considered as having a longitudinal direction parallel to the normal direction of forward movement of the ship. This is the direction on which ship length is measured. Moreover, for the sake of explanation, a ship will be considered hereinafter as having an athwartship direction horizontally perpendicular to the longitudinal direction. It is in the athwartship direction that ship beam is measured.
The heave of a ship is in many instances undesirable. It is particularly undesirable for ships attempting to dock with other ships while at sea. Heave is problematical when ships, such as aircraft carriers, attempt to dispatch and receive aircraft, such as helicopters. Heave is also a serious problem for ships attempting to discharge cargo from a deepwater dock, or while at anchor in rough seas. Such cargo, it will be appreciated, could be liquid gases or petroleum oils. Any force which tends to disrupt the connection of pipes and other apparatus connecting the deepwater anchored ship with the shore could pose a threat to the water environment.
Ships having reduced waterplane areas are susceptible to heaving to a greater degree than conventional ships by reason of their lower heave damping coefficients. The waterplane area is defined, and hereinafter will be used to refer to that area of the hull intersected by a plane at the water level. In a reduced waterplane area ship the superstructure of the ship is ordinarily connected with an underwater hull of the ship by one or more surface-piercing hull elements which are substantially thinner, and often shorter than the underwater hull. The heave of such a ship can become inconveniently large in magnitude. Such a reduced waterplane area ship, such as is shown in FIG. 1 of the attached drawings, is not troubled as much as conventional ships by roll and pitch movements.
Such small waterplane area ships, it will be appreciated, can derive advantages by a device which will reduce the heave motion, but will have minimum intentional effects on pitch movements. Moreover, such ships, when receiving or discharging cargo, and when receiving or dispatching aircraft or vessels, will derive advantage from an apparatus which will diminish or damp heaving while the ship is stationary, and not in forward movement through the water. Such ships, furthermore, will derive advantage from heave control devices which will not interfere with the forward movement of the ship when it is undergoing normal operation. Hereinafter, the word "damp" will be used to describe the resistance to motion and its effects and the phenomenon diminishing oscillatory magnitude in oscillations once excitation has stopped.
In the past, it has been well known to attach fins to the hulls of ships. Such fins are normally placed adjacent the hull of the ship so as to control roll and pitch of vessels. See, for example, the fin apparatus of Emery, U.S. Pat. No. 471,212. As in Emery, or in Noll, U.S. Pat. No. 767,827, or in many other similarly described fins, the fins are operable so that they engage the water by virtue of the forward motion of the ship, at selected angles of impingement of the water on the fin. In such a manner, the pitch and roll of a forward moving ship can be controlled.
These fins, as is shown in Noll, in Rebikoff, U.S. Pat. No. 3,093,105, and in Great Britain Pat. No. 19,886 (1890), have various operable apparatus for eliminating the fins' deleterious drag in the water when so desired. In many cases, the fins are completely retracted within the hull of the ship, such as was described by the Denny-Brown Ship Stabilizer, Engineering Magazine, Sept. 25, 1936, page 334.
In none of the prior art teachings heretofore made, has an apparatus for stabilizing the heave of a ship been shown. More interestingly, in none of the prior art teachings has any device been shown for stabilising vertical translatory motion of a relatively large ship, or of any part of such a ship while the ship is attempting to be stationary.
SUMMARY OF THE INVENTION
A novel ship heave stabilization fin arrangement is described. The arrangement includes fins disposed underneath the water level of a ship. In the case of multi-hull vessels, the fins may be stationarily positioned inwardly of the multi-hulls. The preferred embodiment is to have fins rotatably connected to underwater portions of the hull, the rotation being selective so that the fins may be positioned at any angle relative to the hull, or the fins may be rotated. A method of using such a fin arrangement is described.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view of a small waterplane area ship;
FIG. 2 is a front elevation view of the ship shown in FIG. 1;
FIG. 3 is a plan, schematic view of one alternative embodiment of the ship as shown in FIG. 1;
FIG. 4 is a plan, schematic view of a second alternative embodiment of the ship shown in FIG. 1;
FIG. 5 is a plan, schematic view of the preferred embodiment of the present invention;
FIG. 6 is a front elevation view of the preferred embodiment of the invention shown in FIG. 5;
FIG. 7 is a side elevation view of the preferred embodiment taken along line 7--7 of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT
To resolve the foregoing problems and to obtain the foregoing desired advantages, a ship 10 having heave damping or stabilization fins 20 is described, reference being made to FIGS. 1 and 2 in the attached drawings. The ship 10 shown is one having a smaller than conventional waterplane area, and more than one hull 14. The ship 10 has a superstructure 12 and hulls 14. The ship 10 is also constructed having struts 16 connecting the hulls 14 to the superstructure 12. The normal water level is generally indicated at 18.
In FIG. 2 fins 20 are shown. The space 28 between the hulls 14 and struts 16, underneath the superstructure 12 may be viewed as the inward side of the ship 10. The fins 20 are shown positioned on the inward side of the hulls 14. The fins 20 are horizontal in relation to the ship 10.
Normally, a line of action of gravity is considered a vertical line, or as being in the vertical direction. A plane normal to this line is considered horizontal. A ship upon the sea, however, has its keel-to-mast, or bottom-to-top direction aligned with the vertical only momentarily. More likely, its bottom-to-top line is misaligned from true vertical because of the ship's being bobbed about by the waves of the sea. Hereinafter, therefore, the term vertical shall be used to designate bottom-to-top line of the ship, and the term horizontal shall be used to designate directions normal to this vertical line. In brief, the term vertical and horizontal hereinafter will designate directions in relation to a ship as if its bottom-to-top direction were aligned with the true vertical.
The fins 20 are relatively flat blades parallel to the horizontal. The fins 20 should be situated on the hulls below the water level 18. As shown in FIG. 3, these fins 22 may be relatively short as illustrated in FIG. 4. If the relatively short fins 24 are used, the fins 24 should extend further from the hull 14 into the water on the inward side of the ship than the relatively long fins 22. Alternatively, a plurality of inwardly directed short fins 24 may be affixed to each hull 14. Such an alternative arrangement is not illustrated.
Such fin arrangements as are shown in FIGS. 2, 3 and 4 have significant use in affording stability of a ship relative to heave forces, especially while the ship is moving. Such stationary fixed fin arrangements, however, have proved disappointing at times in damping heave when a ship is stationary, or is moving at slow speeds.
To overcome heave forces at slow speeds or at zero speed, a fin arrangement is shown in FIGS. 5 and 6. FIGS. 5 and 6 are schematic views of the multi-hulled ship 10, as seen from the bottom of the ship 10 looking vertically upward. Fins 30 are rotatably connected to the ship 10 by axles 32. The axles 32 are vertical relative to the ship, as explained above. The fins 30, therefore, are capable of being rotated in the horizontal plane. The fin's blades are horizontal, and should be constructed so as to be relatively flat.
Generally, speaking, the fin blades of the fins 22, 24, 30 should be hydrodynamically constructed so that they move through the water smoothly, presenting as little resistance to horizontal water flow through inward space 28 as possible. It is not contemplated that any of the fin arrangements taught herein should affect the position of the ship relative to the sea by virtue of redirecting horizontal water flow about the ship.
In FIGS. 5 and 6, one fin 30 is shown as being longitudinally disposed alongside the bottom of the respective hull to which it is attached. The other fin 30 is shown as rotated some 90° relative to its respective hull. It is contemplated that these fins 30 may be independently controlled, or that they may be controlled by a unitary control. Of course, the fins 30 may be operated, alternatively, independently but in unison with each other.
In either mode of control, it is contemplated that these fins 30 may be rotated to any angle relative to the longitudinal hulls 14, and there rigidly held in position to maintain that angle. Also, in either mode of control it is contemplated that the fins 30 may be capable of operation in a steady state of rotation. Normally, they should be rotated in opposite directions. Either of the possible pair of opposite directions is contemplated; i.e., clockwise to port and counterclockwise to starboard, or their opposites. The fin cross sections may be such that one pair of directions is preferred over the other by reason of less required rotating power.
If the fins 30 are rotated to a stationary position at an angle from the longitudinal hull 14, the fins 30 would operate with comparable effectiveness as the stationary fins 22, or more likely as the fins 24. The fins 30, however, would be capable of a rotation returning them to a position longitudinally alongside the longitudinal hulls 14, and there out of contact from the flow of water caused by the forward motion of a ship. In such a position, the fins 30 would present minimized resistance or drag to the flow of water around them.
To reduce further or to minimize the drag, a recessed section 34 can be constructed in the hulls 14. This recessed section 34, as illustrated best in FIG. 7 of the attached drawings, should be made so as to allow the fins 30 free movement in and out of them, but designed closely to the fins 30 so that minimum effects of the gap between the fins 30 and their respective hulls 14 are realized.
The fins 30 are shown in the drawings as being balanced about the connection of the fins 30 with their respective axles 32. Preferably, the fins should be balanced so that water forces acting on the blades of the fins 30 sum to zero relative to the fulcrum as which each axle 32 serves. In such a manner, bending moments on the axle 32 are minimized. Moreover, should the fin 30 be asymmetrical, the weight of the fin 30 should be evenly balanced about its respective axle 32.
When the fins 30 are so balanced, or are positioned so as to be substantially so balanced, the torque required to rotate them about their respective axis of their respective axles 32 of each fin is made as uniform as possible. As noted above, in either mode of control it is contemplated that the fins 30 should be capable of rotation at steady state or varying rotational velocities. It has been found that at relatively low ship speeds, or when the ship is standing still, stationary blades or fins provide little reaction against heave forces. In this slow speed or stationary ship condition, however, heave stabilization or damping is achieved when the fins 30 are rotated. The slower the ship 10 is traveling relative to the water, the greater the rotational speed that will be required to satisfactorily damp the heave. In calculations, it has been found that the heave-damping force is approximately proportional to the rotational velocity of the axle 32 times heave velocity. Heave velocity is defined as the time rate of heaving motion.
The axle 32 should be positioned with a watertight opening through the hull 14. The power drive for the axle 32 may be constructed within the hull 14, or in any convenient alternate location such as, for example, the superstructure 12. In any location, however, the powering apparatus should be controllable from a ship's control center. Preferably, each fin 30 should be independently so controllable.
It is of importance that the fins 30 be located along the hull 14 longitudinally so that the heave-damping forces upon the ship are balanced longitudinally and athwartship about the ship center of flotation. It has been found that if only one fin 30 for each hull is used, and it is positioned on an axle disposed more toward the bow or stern relative to the hull's center of flotation, a pitch moment on the ship is generated. Similarly, a fin placed on only one of several parallel longitudinal hulls will generate a roll moment when the ship heaves. Thus, it is relatively important that the positioning of the fins 30 be reasonably balanced relative to the ship's center of gravity. Several fins 30 may be positioned longitudinally along the hull, of course, and still have reasonable balance of the heave-damping forces about the center of gravity of the ship.
As can be seen from the foregoing specification of the preferred embodiment, an improvement to fin arrangements on ships has been described which will effectively afford heave-damping forces, and yet will not much increase the drag on the ship while it is in forward motion through the water. While the preferred embodiment and several alternative embodiments have been described in this foregoing specification, it is appreciated that other alternative embodiments of the invention can be perceived. Thus, the scope of the described invention should not be limited except by the claims which follow.