JPH0834389A - Rock reducing device for ship - Google Patents

Abstract

PURPOSE:To increase the rock reducing device for ship by determining a control amount according to the relative amount of a rolling to a pitching and controlling the control panels of a fin and trim tab. CONSTITUTION:A roll angular velocity and a pitch angular velocity detected by a roll angular velocity detecting means 16 and a pitch angular velocity detecting means 17, respectively, are input by an abnormal signal judgment means 20 to judge whether they are normal or not. When they are normal, they are input into a control amount distributing means 22. Also the control amount distribution means 22 adjusts, using a proper control method, the roll control amount or pitch control amount distributedly by the relative amounts of roll components and pitch components. The control amount is output to a control panel control means 23 as an actual roll component command value and an actual pitch component command value. Then they are output as control signals to a fin drive device 26 and a trim tab drive device 27 through a control panel operating amount limiting means 24 so as to control a fin and trim tab to specified angles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention according to the present application is mainly suitable for reducing rolling and pitching fluctuations during traveling in waves on a high-displacement ship, and is particularly suitable for combined rolling and pitching fluctuations. The present invention relates to a ship motion reducing device that automatically controls fins and trim tabs in order to generate a large control force to exert a greater vibration suppressing effect.

[0002]

2. Description of the Related Art In recent years, there has been a strong demand for a device for reducing the sway of a hull caused by rolling, pitching, or the like, which affects the riding comfort of passengers, in response to the need for speeding up of ships such as passenger ships. ing.

Conventionally, there has been known an apparatus in which rolling control is performed by a pair of fin stabilizers protruding on both sides from the center of a hull. This fin stabilizer was intended to reduce rolling of the hull, and was not effective in reducing pitching of the hull. But,
Actually, depending on the direction of waves and wind, the hull of the hull due to pitching may be greater than rolling,
There has been a need for a device that can perform not only rolling control but also pitching control.

Therefore, Japanese Patent Laid-Open Publication No. Hei 3-224892 discloses a hull damping device capable of controlling both (hereinafter referred to as "conventional example 1").
Has been proposed. This device includes a pair of first set of fins selected in advance and four other pairs of second fins among four fins disposed on both the right and left sides of the hull and the left and right sides of the hull. A set of fins, a pitching control signal generator for generating a pitching control signal,
A rolling control generator for generating a rolling control signal, and generally actuating a first set of fins based on the pitching control signal, and actuating a second set of fins based on the rolling control signal; When the demand level is high and the rolling control level is low,
The first set of fins and the second set of fins are operated based on the pitching control signal, and when the pitching control demand is small and the rolling control demand is large, the first set of fins are based on the rolling control signal. And a control signal path switching device for switching the signal path of each control signal so as to operate the second set of fins, and a fin operating device for operating each fin in response to each control signal. is there.

[0005] Further, in the same manner as the one which enables pitching and rolling control as well, it is practically used.
8595 (hereinafter referred to as "conventional example 2")
There is). This device is provided with a pair of left and right lift force generating plates called trim tabs at the stern end, and enables rolling damping by alternately reciprocating the pair of lift force generating plates with a phase difference on each side. At the same time, pitching damping can be performed by simultaneously reciprocating the pair of lift generating plates in phase.

[0006]

In the case of Conventional Example 1, for example, when the required degree of the rolling control is large and the required degree of the pitching control is small, only the rolling control is mainly performed. This means that the effect of pitching actually occurring is not reflected in the fin control amount at all. However, in actuality, the hull often makes complex sway combined with rolling and pitching depending on weather conditions and sea routes, so the demand for either control is small (not more than a certain value). However, controlling without considering this at all is a rough control and cannot provide an appropriate control force, so that the anti-oscillating effect has its own limit.

In the case of Conventional Example 2, rolling and pitching can be controlled. However, pitching control cannot be performed while rolling control is being performed, and rolling is possible when pitching control is being performed. It cannot be controlled. Therefore, in this case as well, for example, during rolling control, there is no choice but to perform rough control in which the influence of pitching cannot be reflected in the control amount, so there is a limit to the vibration reduction effect.

In consideration of strict requirements for passenger safety and ride comfort, such as in recent high-speed ferries, fine control that takes rolling and pitching into account at the same time is required. The system cannot adapt to this.

Fins (or trim tabs) provided on the bow and stern are effective for pitching, and effective control force cannot be obtained unless rolling fins (or trim tabs) on the port side work for rolling. . In the case of Conventional Example 1, in a state where both rolling and pitching are simultaneously increased, pitching control is performed by front and rear fins on the starboard side, and rolling is performed by front and rear fins on the port side. Even if there is no problem, an effective control force cannot be obtained with the fins before and after the rolling control, and a sufficient rolling reduction effect may not be obtained.

Further, in the conventional example 2, since only a pair of trim tabs are provided at the end of the stern for control, the device works effectively as a very small sway reducing device such as a motor boat, but it is effective for a larger ship. Is not valid.

It is an object of the invention according to this application to provide a sway mitigation apparatus for a ship which can enhance a swaying effect by performing fine control on complex sway due to rolling and pitching. Another object of the present invention is to provide an apparatus for performing abnormality determination of each sensor in such a case.

[0012]

In order to achieve the above object, the invention device according to claim 1 is provided with fins on the left and right sides of the bow and trim tabs on the left and right sides of the stern end, and these are operated by a predetermined angle. A device for reducing sway of a ship, comprising roll angular velocity detecting means and pitch angular velocity detecting means for detecting at least roll angular velocity and pitch angular velocity, respectively, and roll control by a relative amount of a roll component and a pitch component upon receiving these detection signals. Amount or pitch control amount is distributed and adjusted using an adaptive control method, and fins and trim tabs are operated by a predetermined angle by the actual roll component command value and the actual pitch component command value output from this control amount distribution means. And a machine-side controller that controls the above.

According to a second aspect of the present invention, in the above configuration, the control amount distribution means includes a roll / pitch control amount distribution ratio α.
Is determined by the roll angular velocity / (roll angular velocity + pitch angular velocity), from which the roll component command value × α is set as the actual roll component command value, and the pitch component command value × (1−α) is set as the actual pitch component command value. It is configured to output to the controller.

According to a third aspect of the present invention, in the configuration of the first aspect, the control amount distribution means determines the roll / pitch control amount distribution ratio β by pitch angular velocity / (roll angular velocity + pitch angular velocity). Roll component command value x (1-β)
Is output to the machine-side controller for fin and trim tab control as the actual roll component command value and the pitch component command value × β as the actual pitch component command value.

According to a fourth aspect of the present invention, in any one of the above-mentioned structures, at least the input signals from the roll angular velocity detecting means, the pitch angular velocity detecting means, and the boat speed detecting means continuously have a preset predicted signal level. An abnormal signal judging means is provided which is configured to judge an abnormality when the set number of times is exceeded, and to automatically disconnect the signal source when the abnormality is judged and to output only a normal signal to the control amount distribution means.

According to a fifth aspect of the present invention, in the structure of the fourth aspect, the control surface of the fins and trim tabs is neutralized in response to the sensor abnormality signal from the abnormality signal determination means, depending on the state of the failure or the boat speed. There is provided a failure state determination means configured to output a control surface position control signal to be fixed to the storage position to the machine side controller.

According to a sixth aspect of the present invention, in the structure of the fourth or fifth aspect, the failure state determination means receives the control device abnormality signal from the control amount distribution means to determine the failure state and controls the fins and trim tabs. Is to be stored or fixed to the neutral position to output a control surface position control signal to the machine side controller.

According to a seventh aspect of the present invention, in any one of the above configurations, the apparatus according to the present invention comprises a device capable of estimating the vertical acceleration of the hull from the roll angular velocity signal and the pitch angular velocity signal.

The invention apparatus according to claim 8 is, in the construction of claim 7, provided with a draft meter for detecting the water flow of the ship, and a gravity center correction calculating means for receiving the draft signal and correcting the vertical acceleration at the gravity center position of the hull. It becomes.

[0020]

According to the first to third aspects of the present invention, an appropriate control force is obtained by using an adaptive control method for complex swaying of the hull caused by rolling, pitching, etc.
Control the fins and trim tabs by determining the control amount corresponding to the relative amount of rolling and pitching (for example, if the sway is due to pitching other than rolling, always consider the effect of pitching in the control amount) Since the surface can be controlled, a more effective and large anti-rolling effect is exhibited as compared with the related art.

According to the present invention, it is possible to determine the abnormality of the various sensors and store and fix the abnormality in the appropriate position on the control surface. The same is true when the ship speed is low, at which the anti-sway effect is difficult to be exerted.

Particularly, in the sixth aspect, it is possible to determine the abnormality of the control device, and accordingly, the control surface can be stored or fixed at an appropriate position.

According to the seventh aspect, it is possible to easily and quickly estimate the vertical acceleration that should be used to evaluate the riding comfort without the vertical accelerometer.

According to the present invention, the vertical acceleration at the center of gravity of the ship is corrected, and the ride comfort and the like can be evaluated together with the vertical accelerometer separately provided as needed.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the invention according to this application will be described below with reference to the drawings. FIG. 1 is a diagram showing the device configuration of the invention apparatus according to this application. FIG. 1 is a plan view schematically showing the arrangement of each device when a catamaran having a torpedo-shaped submerged body 2 on both sides of an upper structure 1 is seen through from above. Submerged body 2 on both sides
A pair of fins 3 and 4 are provided on the left and right sides of both sides at the front of the vehicle. As shown in FIGS. 2 (a) (b) (c), each fin 3
(4) is rotatable around the left-right axis 3a (4a) within a fixed fin angle ± θ range, and is controlled so as to have an optimum angle of attack according to external conditions such as wind waves. It has become. θ = 0 ° indicates the neutral position of the fin. The pair of left and right fins 3 (4) arranged on each side are configured to interlock with each other, and the same operation is performed by a drive device such as a hydraulic cylinder. A hydraulic unit 5 for driving the fins 3, 4 and a machine-side controller 6 are provided respectively. On the other hand, trim tabs 7 and 8 are provided on both sides of the stern end. The trim tabs 7 and 8 have a well-known configuration as shown in FIG.
(8a) The hydraulic cylinder 9 is configured to be swingable around
Normally, the trim tab angle δ is set within a certain range by the operation of. Then, when the water jet propulsion device (the jet port is above the trim tab) deployed at the stern end is set to the reverse state, it can be repelled up to the position of the imaginary line (storage position) so that the jet flow does not hit. Has become. A hydraulic unit 10 for driving the trim tabs 7 and 8 and a machine-side controller 11 for controlling the hydraulic unit 10 are provided on the trim tabs 7 and 8, respectively. Further, a ship speedometer (ship speed detecting means) 12, a ride control system (hereinafter abbreviated as "RCS") controller 13, and an operation panel 14 are arranged in the upper structure 1 in the steering room. In addition, a vertical accelerometer 15, a rate gyro (roll angular velocity detecting means) 16 which is a rolling angular velocity meter, and a rate gyro (pitch angular velocity detecting means) 17 which is a pitching angular velocity meter are provided.

FIG. 4 is a block diagram of the RCS system. RC
As described above, the S sensor has two rate gyroscopes 16 and 17 for detecting the roll angular velocity and the pitch angular velocity, one vertical accelerometer 15 mainly used for evaluating the calculated vertical acceleration, and detects the boat speed. Speedometer 12 and so on.
In the wheelhouse, an RCS control unit 13 including an RCS main processing unit 13a and a relay logic unit 13b, an RCS operation panel 14, a hull / engine unit data logger 19 for monitoring an abnormal signal.
On the other hand, the machine side controllers (for the port fins and the port side trim tabs) 6 and 11 which are operated by the command signal from the RCS control device 13, and the predetermined fin angle according to the commands from the machine side controllers 6 and 11. , Hydraulic control valves 6a, 11 controlled to have trim tab angles
a, Hydraulic servos 6b and 11b are arranged on the machine side.
The left and right port fins 3 and 4 and the left and right port side trim tabs 7 and 8 in FIG. 1 are operated by the hydraulic servo 6b so as to form a predetermined angle.

The RCS control device 13 is a control calculation process for automatically controlling the fins and trim tabs, sensors R, etc.
It performs abnormality diagnosis and abnormality processing of CS component devices, and input / output processing of various control signals and monitor signals.
The main functions are a control state transition process (sequence process) by operation from the operation panel 14, a fin and trim tab angle for generating an appropriate control force in order to reduce hull sway, and instruct the machine controller. Outputs as values, outputs the fin or trim tab angle input from the RCS operation panel when manual adjustment is selected to the machine side controller, diagnoses and processes abnormalities of the main processing unit, sensors, etc., and displays LED lamps for device abnormalities.

The RCS operation panel 14 monitors the operating conditions of the fins and trim tabs, monitors equipment abnormalities, remotely starts and stops the hydraulic drive device, and manually inputs / changes the set values for each control. , Remote control of hydraulic drive of fin and trim tab angle, manual setting of fin and trim tab angle, fin and trim tab angle monitor, representative display of equipment malfunction.

The machine-side controllers 6 and 11 control the angles of the fins and trim tabs in accordance with the command value from the RCS controller 13, and also perform remote / machine-side switching of control locations, drive device abnormality processing, and the like. , Manually set the fin or trim tab to the set angle. Return the fins to the neutral position and the trim tab to the retracted position. Performs abnormal processing of the hydraulic drive device of the fin or trim tab.

An abnormality / monitor signal for each drive unit, an abnormality signal for the hydraulic servo, a fin angle / trim tab angle (a hood back signal of an actually taken angle), a fin /
A trim tab limit signal or the like is transmitted to the RCS control device for monitoring / abnormality monitoring. The relay logic unit 13b has a function of isolating from the main processing unit 13a, and plays a role of a safety circuit that allows a control signal or an abnormal signal to be output to the machine side controller even if the main processing unit 13b fails. It has a function as an input / output signal amplifier.

FIG. 5 is a control block diagram of the RCS. The detection signals from the roll angular velocity detecting means 16, the pitch angular velocity detecting means 17 and the boat speed detecting means 12 are detected by the abnormal signal discriminating means 2.
Sent to 0. The abnormal signal determining means 20 has a function of determining whether a signal from each sensor is normal or abnormal. That is, as shown in FIG. 6, the predictive signal level of each sensor is stored in advance, and when the input signal continuously exceeds the predictive signal level the set number of times, it is determined to be “abnormal”. . Then, the state of the sensor signals of the roll angular velocity and the pitch angular velocity is checked, and if it is determined that there is an abnormality, the signal source is automatically disconnected (skip even if an abnormal signal comes).

When it is judged that the sensor is abnormal, the abnormal signal judging means 20 sends the sensor abnormal signal to the failure state judging means 21. On the other hand, the control device abnormality signal output from the control amount distribution means 22 to be described later is also the failure state determination means 2
Input to 1. Then, the failure state determining means 21
The control surface of the fin or the trim tab is fixed to a neutral or storage position or a control surface position control signal to be stored according to a failure state of a device (control device, each sensor, etc.) constituting S. (Device-side controller) 23.

On the other hand, when the abnormal signal discriminating means 20 determines that the signals are normal, the detection signals of the roll angular velocity and the pitch angular velocity are directly input to the control amount distributing means 22. The control amount distributing means 22 has a function of distributing and adjusting the roll control amount or the pitch control amount using an adaptive control method based on the relative amount of the roll component and the pitch component.

As an example of adaptive control, the roll / pitch control amount distribution ratio α is α = roll angular velocity / (roll angular velocity +
The actual roll component command value (this is an angle command value of how much the fin or trim tab should take) to be given to the control surface control means 23
The roll component command value × α and the actual pitch component command value are set as the pitch component command value × (1−α). Instead of the distribution ratio α, the distribution ratio β may be pitch angular velocity / (roll angular velocity + pitch angular velocity). In this case, the actual roll component command value is set as the roll component command value × (1−β) and the actual pitch component command value is set as the pitch component command value × β so as to be given to the control surface control means. The roll component command value and the pitch component command value are calculated by the control amount distribution means 22 based on the roll angular velocity and the pitch angular velocity.

The actual roll component command value and the actual pitch component command value are transmitted to the control surface control means 23, and the fin angle control signal of each fin and the trim tab angle control signal are sent to the fin through the control surface operation amount limiting means 24. Or trim tab drive device 2
6 and 27, whereby each fin and trim tab are controlled to have a predetermined angle. The control surface control means 23 receives the command signal from the control amount distribution means 22 and outputs to the control surface operation amount restriction means 24 a signal indicating how much hydraulic servo should be actually operated. In this case, the control surface operation amount limiting means 2
Reference numeral 4 has a function of determining whether the signal is abnormal and ignoring the signal if the signal is abnormal.

For example, the upper and lower limits of the fin angle are ± 15 °
When it is set to, the function has a function of ignoring a signal sent with a fin angle of 20 °. Since the fins and trim tabs are also usually provided with mechanical limiters, the operating range of the fins and trim tabs is limited both electrically and mechanically.

When the ship speed detected by the ship speed detecting means 12 becomes lower than a predetermined value, a command value for fixing the fins and the trim tabs to the neutral or stowed position is output from the control amount distribution means 22 to the control surface control means. 23 is output. This is because when the boat speed is low, little control force is generated on the fins and the trim tabs.

The roll / pitch angle setting means 25 is shown in FIG.
It is incorporated in the operation panel 14 of the above, and by this, the fin angle and the trim tab angle can be manually set. When setting the fin angle and trim tab angle manually, for example, when the ship is trimmed due to the loading condition of cargo, give the fin or trim tab an initial setting angle to run the ship without trim. For example, when doing so. This eliminates the need for an even keel by adjusting the ballast, which is convenient.

By the function of the control amount distribution means 22 which controls the adaptive control, when the hull makes a complex sway such as rolling or pitching depending on the sea condition or the route, the control amount is always determined in consideration of the influence degree of both. Since the fins and the trim tabs are controlled, fine control is possible, and the vibration reduction can be effectively achieved. For example, when the roll component is large in the hull component, the port fins (and trim tabs) and the starboard fins (and trim tabs) operate with a phase difference to reduce hull motion. In this case, the fins and trim tabs on the port side operate at different angles by the influence of the pitch component. Also, for example, when the hull sway is only the pitch component, the port fins at the bow (and the trim tabs at the stern end) both operate in phase, but the hull includes a roll component in addition to the pitch component. As a result, the fins and the trim tabs on the left and right ports are controlled so as to take a slightly different angle. As a result, the fins on the left and right ports and the trim tabs on the left and right sides operate with a phase difference.

As shown in FIG. 5, the roll angular velocity and the pitch angular velocity are also sent to the acceleration calculating means 28, where the vertical acceleration is calculated. FIG. 7 is a block diagram showing that the roll and pitch angular velocity signals are the transfer function T of the differential.
₂ s / (1 + T ₁ s) is converted into roll and pitch accelerations, and on the other hand, the center of gravity correction calculation means 29 is received in response to the draft signal detected by the stuttering meter provided at the stern of the bow.
By being (incorporated in the acceleration calculation means 28), the vertical acceleration at the center of gravity of the hull can be obtained. Usually, this vertical acceleration is 0.2G
If it is below, it is supposed to provide passengers with a good ride without seasickness.

The abnormal signal discriminating means 2 described in FIG.
0, the acceleration calculation means 28, the control amount distribution means 22, and the failure state determination means 21 are incorporated in the RCS control device 13 of FIG.

[0042]

EFFECTS OF THE INVENTION According to claims 1 to 8, by adopting an adaptive control method, it is possible to prevent complex shaking due to roll and pitch.
Even for control surfaces (fins, trim tabs) with a small area, these control surfaces can be efficiently and finely controlled in order to generate an appropriate control force on these control surfaces, and as a result, wobbling of the ship can be suppressed even more effectively. .

In particular, in claims 4 to 6, by automatically setting the fins and trim tabs to the neutral or retracted position even when the control device or the sensor fails, it is possible to expect the safety of the ship and the effect of vibration reduction.

According to the seventh and eighth aspects, the acceleration sensor can be eliminated by calculating the vertical acceleration by soft-processing the roll angular velocity signal and the pitch angular velocity signal, and it is possible to reduce sensor troubles and maintenance work. In particular, since the acceleration of the position of the center of gravity can be obtained, it is possible to appropriately evaluate the riding comfort and the like.

[Brief description of drawings]

FIG. 1 is a plan view of a ship schematically showing the arrangement of RCS equipment of the invention of this application.

2 (a) is a front view, FIG. 2 (b) is a plan view, and FIG. 2 (c) is a side view of the fin.

FIG. 3 is a perspective view of a trim tab provided at a stern end.

FIG. 4 is an RCS system configuration diagram of the present invention;

FIG. 5 is a control block diagram of the same.

FIG. 6 is a diagram showing a procedure for determining an abnormal signal.

FIG. 7 is a block diagram for calculating vertical acceleration from the roll / pitch angular velocity and converting it to that at the center of gravity of the ship.

[Explanation of symbols]

 3,4 ... fin 5,10 ... hydraulic unit 6,11 ... machine side controller 7,8 ... trim tab 12 ... ship speed detection means 13 ... RCS control device 14 ... RCS operation panel 15 ... vertical accelerometer 16 ... roll angular velocity detection means 17: Pitch angular velocity detecting means 20: Abnormal signal determining means 21: Failure state determining means 22: Control amount distribution means 23: Control surface control means 24: Control surface operation amount limiting means 25: Roll / pitch angle setting means 26: Fin drive Device 27: Trim tab drive device 28: Acceleration calculation means 29: Center of gravity correction calculation means

Front page continuation (72) Inventor Mitsuya Ogawa 3-1, 1-1 Higashikawasaki-cho, Chuo-ku, Kobe-shi, Hyogo Kawasaki Heavy Industries Ltd. Kobe factory (72) Inventor Takashi Takasugi 3-chome, Higashi-kawasaki-cho, Chuo-ku, Kobe-shi, Hyogo No. 1 Kawasaki Heavy Industries Ltd. Kobe factory

Claims (8)

[Claims] 1. A device for reducing sway of a ship by providing fins on the port side of the bow and trim tabs on the port side of the stern end and operating these at a predetermined angle, wherein at least the roll angular velocity and the pitch angular velocity are reduced. Roll angular velocity detecting means and pitch angular velocity detecting means for detecting each,
Receiving these detection signals, the control amount distribution means for distributing and adjusting the roll control amount or the pitch control amount using the adaptive control method according to the relative amounts of the roll component and the pitch component, and the actual roll output from this control amount distribution means. A ship motion reduction apparatus comprising: a machine-side controller that controls a fin and a trim tab to operate at a predetermined angle according to a component command value and an actual pitch component command value. 2. The control amount distribution means determines a roll / pitch control amount distribution ratio α by roll angular velocity / (roll angular velocity + pitch angular velocity), and from this, roll component command value × α is taken as an actual roll component command value, 2. The ship motion reducing apparatus according to claim 1, wherein the pitch component command value × (1−α) is output to the machine side controller as an actual pitch component command value. 3. The control amount distribution means determines a roll / pitch control amount distribution ratio β by a pitch angular velocity / (roll angular velocity + pitch angular velocity), from which a roll component command value × (1−1)
β) as the actual roll component command value, and the pitch component command value × β
2. The apparatus for reducing sway of a ship according to claim 1, wherein the control signal is output to the machine-side controller for controlling the fins and the trim tabs as an actual pitch component command value. 4. When the input signals from at least the roll angular velocity detecting means, the pitch angular velocity detecting means, and the ship speed detecting means continuously exceed a preset predicted signal level for a preset number of times, it is determined to be abnormal, and it is determined as abnormal. 4. The ship motion reducing apparatus according to claim 1, further comprising: an abnormal signal discriminating unit configured to automatically disconnect the signal source and output only a normal signal to the control amount distribution unit when the determination is made. 5. A control surface position control signal for receiving the sensor abnormal signal from the abnormal signal discriminating means and fixing the control surfaces of the fins and trim tabs to a neutral or retracted position in accordance with the state of the failure or the boat speed. The vessel motion reducing apparatus according to claim 4, further comprising failure state determination means configured to output to a controller. 6. A control surface position control signal for receiving the control device abnormality signal from the control amount distribution means, determining the failure state, and storing the control surfaces of the fins and trim tabs or fixing the control surfaces to a neutral position. The ship motion reducing apparatus according to claim 4 or 5, which is configured to output to a machine-side controller. 7. The shaking suppression device for a ship according to claim 1, further comprising a device capable of estimating vertical acceleration of the hull from a roll angular velocity signal and a pitch angular velocity signal. 8. A shaking motion reducing apparatus according to claim 7, further comprising a draft meter for detecting the water of the boat and a center-of-gravity correction calculation means for receiving the draft signal and correcting the vertical acceleration at the center-of-gravity position of the hull.