Patent Publication Number: US-2021188406-A1

Title: Posture control system for hull, control method therefor, and marine vessel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2019-229001 filed on Dec. 19, 2019. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a posture control system for a hull, a control method therefor, and a marine vessel. 
     2. Description of the Related Art 
     Conventionally, a planing boat has posture control plates such as trim tabs on a port side and a starboard side of a stern (see, for example, Japanese Laid-open Patent Publication (Kokai) No. 2001-294197 and Zipwake “Dynamic Trim-Control System” (URL: http://www.zipwake.com; hereafter referred to as Zipwake)). The planing boat moves the posture control plates up and down to control the posture of a hull, in particular, the pitch angle of the hull. For example, according to a technique described in Japanese Laid-open Patent Publication (Kokai) No. 2001-152898, the posture of the hull is controlled by moving the trim tabs based on information on an engine rpm, speed, acceleration, steering angle, and so forth. 
     As the planing boat accelerates, it shifts into a hump state due to upward movement of a bow (bow-up). In the hump state, resistance is increased, and thus the planing boat cannot smoothly accelerate, causing fuel efficiency to decrease. To resolve the hump state, the bow is lowered by moving down the posture control plates. Conventionally, a time when the posture control plates are lowered is the time when the rpm of an engine reaches an rpm corresponding to a speed at which the planing boat shifts into the hump state. 
     However, it takes a certain period of time to lower the posture control plates, and thus, if the posture control plates are lowered at the time when the engine has reached the rpm corresponding to the speed at which the planing boat shifts into the hump state, the hump state may last longer than expected. For this reason, there is room for improvement of fuel efficiency. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide posture control systems that each improve fuel efficiency. 
     According to a preferred embodiment of the present invention, a posture control system for a hull includes a posture control plate attachable to a stern of the hull to control a posture of the hull, a driver to drive the posture control plate, an engine to generate a propulsive force on the hull, and a controller configured or programmed to control the driver, wherein, based on a throttle opening angle of the engine, the controller is configured or programmed to determine a time when the posture control plate is to be lowered by the driver. 
     According to a preferred embodiment of the present invention, a posture control system for a hull includes a posture control plate attachable to a stern of the hull to control a posture of the hull, a driver to drive the posture control plate, an engine to generate a propulsive force on the hull, and a controller configured or programmed to control the driver, wherein, based on an operated amount of a throttle operator, the controller is configured or programmed to determine a time when the posture control plate is to be lowered by the driver. 
     According to the above preferred embodiments, the time when the posture control plate is lowered is determined based on the throttle opening angle of the engine or the operated amount of the throttle operator. Thus, lowering of the posture control plate is started without waiting for the engine to increase to an rpm corresponding to the predetermined speed, and thus if it takes time to lower the posture control plate, the posture control plate is lowered before the hull reaches the predetermined speed. As a result, the hump state is prevented from lasting longer than expected, and fuel efficiency is improved. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a marine vessel to which a posture control system for a hull according to a preferred embodiment of the present invention is applied. 
         FIG. 2  is a side view of a trim tab attached to the hull. 
         FIG. 3  is a block diagram of a maneuvering system. 
         FIGS. 4A to 4C  are views useful in explaining changes in the posture of the marine vessel during acceleration. 
         FIG. 5  is a view useful in explaining a method for decreasing the pitch angle of the hull in a hump state. 
         FIG. 6  is a graph showing the relationship between speed and fuel efficiency of the marine vessel. 
         FIG. 7  is a graph showing the relationship between speed that is reached by the marine vessel and throttle opening angle. 
         FIG. 8  is a view useful in explaining how an engine rpm follows the throttle opening angle. 
         FIG. 9  is a flowchart showing a posture control process during acceleration of the marine vessel according to a preferred embodiment of the present invention. 
         FIG. 10  is a flowchart showing a variation of the posture control process during acceleration of the marine vessel according to a preferred embodiment of the present invention. 
         FIG. 11  is a view useful in explaining a first variation to which the posture control system for the hull according to a preferred embodiment of the present invention is applied. 
         FIGS. 12A and 12B  are views useful in explaining a second variation to which the posture control system for the hull according to a preferred embodiment of the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
       FIG. 1  is a top view of a marine vessel to which a posture control system for a hull according to a preferred embodiment of the present invention is applied. The marine vessel  11  is a planing boat and includes a hull  13 , a plurality of (for example, two) outboard motors (outboard motors  15 A,  15 B in  FIG. 1 ) as marine propulsion devices mounted on the hull  13 , and a plurality of (for example, a pair of) trim tab units (trim tab units  20 A,  20 B in  FIG. 1 ). A central unit  10 , a steering wheel  18 , and a throttle lever  12  (throttle operator) are provided in the vicinity of a cockpit in the hull  13 . 
     In the following description, a fore-and-aft direction, a crosswise direction, and a vertical direction mean a fore-and-aft direction, a crosswise direction, and a vertical direction, respectively, of the hull  13 . For example, as shown in  FIG. 1 , a centerline C 1  extending in the fore-and-aft direction of the hull  13  passes through the center of gravity G of the marine vessel  11 . The fore-and-aft direction is a direction along the centerline C 1 . Fore means a direction toward the upper side of the view along the centerline C 1 . Aft means a direction toward the lower side of the view along the centerline C 1 . The crosswise direction is based on a case in which the hull  13  is seen from behind. The vertical direction is vertical to the fore-and-aft direction and the crosswise direction. 
     The two outboard motors  15 A and  15 B are attached to a stern of the hull  13  side by side. To distinguish the two outboard motors  15 A and  15 B, the one located on the port side is referred to as the “outboard motor  15 A”, and the one located on the starboard side is referred to as the “outboard motor  15 B”. The outboard motors  15 A and  15 B are mounted on the hull  13  via mounting units  14 A an  14 B, respectively. The outboard motors  15 A and  15 B have respective engines  16 A and  16 B, which are preferably internal combustion engines. The outboard motors  15 A and  15 B obtain propulsive forces from propellers (not illustrated) which are rotated by driving forces of the corresponding engines  16 A and  16 B. 
     The mounting units  14 A and  14 B each include a swivel bracket, a cramp bracket, a steering shaft, and a tilt shaft (none of them is illustrated). The mounting units  14 A and  14 B also include power trim and tilt mechanisms (PTT mechanisms)  23 A and  23 B, respectively ( FIG. 3 ). The PTT mechanisms  23 A and  23 B rotate the corresponding outboard motors  15 A and  15 B about the tilt shaft. This makes it possible to change the tilt angle of the outboard motors  15 A and  15 B with respect to the hull  13 , and thus a trim adjustment is able to be made, and the outboard motors  15 A and  15 B are tilted up and down. Moreover, the outboard motors  15 A and  15 B are able to rotate about a center of rotation C 2  (about the steering shaft) with respect to the swivel bracket. By operating the steering wheel  18 , the outboard motors  15 A and  15 B are rotated about the center of rotation C 2  in the crosswise direction (direction R 1 ). Thus, the marine vessel  11  is steered. 
     The pair of trim tab units  20 A and  20 B are attached to the stern on the port side and the starboard side such that they are able to swing about a swing axis C 3 . To distinguish the two trim tabs  20 A and  20 B, the one located on the port side is referred to as the “trim tab unit  20 A”, and the one located on the starboard side is referred to as the “trim tab unit  20 B”. 
       FIG. 2  is a side view of the trim tab unit  20 A attached to the hull  13 . The trim tab units  20 A and  20 B have the same construction, and thus a construction of only the trim tab unit  20 A will be described. The trim tab unit  20 A includes a trim tab actuator  22 A (driver) and a tab main body  21 A. The tab main body  21 A is attached to the rear of the hull  13  such that it is able to swing about the swing axis C 3 . For example, a base end portion of the tab main body  21 A is attached to the rear of the hull  13 , and a free end portion of the tab main body  21 A swings up and down (in a swinging direction R 2 ) about the swing axis C 3 . The tab main body  21 A is an example of a posture control plate that controls the posture of the hull  13 . 
     The trim tab actuator  22 A is located between the tab main body  21 A and the hull  13  such that it connects the tab main body  21 A and the hull  13  together. The trim tab actuator  22 A drives the tab main body  21 A to swing it with respect to the hull  13 . It should be noted that the tab main body  21 A indicated by a chain double-dashed line in  FIG. 2  is at a position where its free end portion is at the highest level (a position at which the amount of descent is 0%), and this position corresponds to a retracted position. The tab main body  21 A indicated by a solid line in  FIG. 2  is at a position where its free end portion is at a lower level than a keel at the bottom of the marine vessel  11 . It should be noted that a range where the tab main body  21 A is able to swing is not limited to the one illustrated in  FIG. 2 . The swinging direction R 2  is defined with reference to the swing axis C 3 . The swing axis C 3  is perpendicular or substantially perpendicular to the centerline C 1  and parallel or substantially parallel to, for example, the crosswise direction. It should be noted that the swing axis C 3  may extend diagonally so as to cross the center of rotation C 2 . 
       FIG. 3  is a block diagram of a maneuvering system. The maneuvering system includes the posture control system according to a preferred embodiment of the present invention. The marine vessel  11  includes a controller  30 , a throttle position sensor  34 , a steering angle sensor  35 , a hull speed sensor  36 , a hull acceleration sensor  37 , a posture sensor  38 , a receiving unit  39 , a display unit  9 , and a setting operating unit  19 . The marine vessel  11  also includes engine rpm detecting units  17 A and  17 B, turning actuators  24 A and  24 B, the PTT mechanisms  23 A and  23 B, the trim tab actuators  22 A and  22 B (see  FIG. 2  as well). 
     The controller  30 , the throttle position sensor  34 , the steering angle sensor  35 , the hull speed sensor  36 , the hull acceleration sensor  37 , the posture sensor  38 , the receiving unit  39 , the display unit  9 , and the setting operating unit  19  are included in the central unit  10  or disposed in the vicinity of the central unit  10 . The turning actuators  24 A and  24 B and the PTT mechanisms  23 A and  23 B are provided for the corresponding outboard motors  15 A and  15 B. The engine rpm detecting units  17 A and  17 B are provided in the corresponding outboard motors  15 A and  15 B. The trim tab actuators  22 A and  22 B are included in the trim tabs  20 A and  20 B, respectively. 
     The controller  30  includes a CPU  31 , a ROM  32 , a RAM  33 , and a timer, which is not illustrated. The ROM  32  stores control programs. The CPU  31  expands the control programs stored in the ROM  32  into the RAM  33  and executes them to implement various types of control processes. The RAM  33  provides a work area for the CPU  31  to execute the control program. 
     Results of detection by the sensors  34  to  39  and the engine rpm detecting units  17 A and  17 B are provided to the controller  30 . The throttle position sensor  34  detects the opening angle of a throttle valve, which is not illustrated. It should be noted that the opening angle of the throttle valve varies according to the operated amount of the throttle lever  12 . The steering angle sensor  35  detects the rotational angle of the steering wheel  18  that has been rotated. The hull speed sensor  36  and the hull acceleration sensor  37  detect the speed and acceleration, respectively, of the marine vessel  11  (the hull  13 ) while it is sailing. 
     The posture sensor  38  includes, for example, a gyro sensor, a magnetic direction sensor, and so forth. Based on a signal output from the posture sensor  38 , the controller  30  calculates a roll angle, a pitch angle, and a yaw angle. It should be noted that the controller  30  may calculate the roll angle and the pitch angle based on a signal output from the hull acceleration sensor  37 . The receiving unit  39  includes a GNSS (Global Navigation Satellite Systems) receiver such as a GPS and has a function of receiving GPS signals and various types of signals as positional information. From a speed restriction zone or land in its vicinity, an identification signal for providing notification that an area is a speed restriction zone is transmitted. The speed restriction zone means an area in a harbor or the like where marine vessels are required to limit their speed to a predetermined speed or lower. The receiving unit  39  also has a function of receiving the identification signal. It should be noted that the acceleration of the hull  13  may also be obtained from a GPS signal received by the receiving unit  39 . 
     The engine rpm detecting units  17 A and  17 B detect the number of revolutions of the corresponding engines  16 A and  16 B per unit time (hereafter referred to as “the engine rpm”). The display unit  9  displays various types of information. The setting operating unit  19  includes an operator to perform operations relating to maneuvering, a PTT operating switch, a setting operator to make various settings, and an input operator to input various types of instructions (none of them is illustrated). 
     The turning actuators  24 A and  24 B rotate the corresponding outboard motors  15 A and  15 B about the center of rotation C 2  with respect to the hull  13 . The rotation of the outboard motors  15 A and  15 B about the center of rotation C 2  changes a direction in which a propulsion force acts with respect to the centerline C 1  of the hull  13 . The PTT mechanisms  23 A and  23 B tilt the corresponding outboard motors  15 A and  15 B with respect to the clamp bracket by rotating the corresponding outboard motors  15 A and  15 B about the tilt shaft. The PTT mechanisms  23 A and  23 B are activated by, for example, operating the PTT operating switch. As a result, the tilt angle of the outboard motors  15 A and  15 B with respect to the hull  13  is changed. 
     The trim tab actuators  22 A and  22 B are controlled by the controller  30 . For example, the controller  30  operates the trim tab actuators  22 A and  22 B by outputting control signals to them. The operation of the trim tab actuators  22 A and  22 B, which correspond to the drivers, swings the corresponding tabs  21 A and  21 B. It should be noted that actuators used for the PTT mechanisms  23 A and  23 B and the trim tab actuators  22 A and  22 B may be either a hydraulic type or an electric type. 
     It should be noted that the controller  30  may obtain results of detection by the engine rpm detecting units  17 A and  17 B via a remote control ECU, which is not illustrated. The controller  30  may also control each of the engines  16 A and  16 B via outboard motor ECUs (not illustrated) provided in the respective outboard motors  15 A and  15 B. 
     When the marine vessel  11 , which is a planing boat, is sailing at low speed, for example, at several km/h, a large lift is not generated at the bottom of the hull  13 , and as with displacement-type marine vessels, buoyant force mainly acts on the entire hull  13 . Thus, when the marine vessel  11  is sailing at low speed, the hull  13  is kept substantially horizontal, and the pitch angle is kept at substantially 0 degrees, as shown in  FIG. 4A . 
     After that, when the marine vessel  11  accelerates and reaches, for example, a speed of about 10 to about 20 km/h, the stern of the hull  13  sinks into a valley of waves generated by the bow of the hull  13 , resulting in the bow being raised to bring the marine vessel  11  into a hump state ( FIG. 4B ). Since the stern of the hull  13  sinks in the hump state, the pitch angle increases to, for example, about 7 degrees to about 8 degrees. 
     When the marine vessel  11  further accelerates and reaches, for example, a speed of more than about 30 km/h, lift generated at the bottom of the hull  13  significantly increases, and as shown in  FIG. 4A , the hull  13  shifts into a planing state. In the planing state, waves generated by the bow of the hull  13  have a long wavelength due to the high speed, the stern never sinks into a valley of the waves, and lift acts on the entire bottom surface of the hull  13 . As a result, not only the bow but also the stern rises, making the pitch angle of the hull  13  smaller than in the hump state. 
     As described above, in the hump state, the pitch angle of the hull  13  is large, and the angle which the bottom of the hull  13  defines with respect to a water surface H (indicated by a chain double-dashed line) while sailing increases, making the bottom of the hull  13  more likely to contact the water. For this reason, a resistance acting on the bottom of the hull  13  becomes very large. 
     To cope with this, the pitch angle of the hull  13  in the hump state is decreased by lowering the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B. As shown in  FIG. 5 , when the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B are lowered, lift L is generated by the tab main bodies  21 A and  21 B, and a bow-down moment  25  around the center of gravity G is generated in the hull  13 . This causes the bow to move down and reduces the pitch angle of the hull  13 . When the pitch angle has decreased, the angle with which the bottom of the hull  13  defines with respect to with the water surface H decreases, and thus during sailing, the bottom of the hull  13  is less likely to receive water. As a result, the resistance acting on the bottom of the hull  13  is able to be decreased. 
       FIG. 6  is a graph showing the relationship between the speed and fuel efficiency of the marine vessel  11 . In  FIG. 6 , a broken line indicates changes in fuel efficiency at a descent rate of 0% at which free end portions of the tab main bodies  21 A and  21 B lie at the highest position, and a solid line indicates changes in fuel efficiency at a descent rate of 100% at which the free end portions of the tab main bodies  21 A and  21 B lie at the lowest position. 
     As shown in the graph of  FIG. 6 , the fuel efficiency at the descent rate of 0% is higher than the fuel efficiency at the descent rate of 100% until the speed of the marine vessel  11  reaches, for example, approximately 17 km/h. This is because while the marine vessel  11  is sailing at low speed, the hull  13  is kept substantially horizontal, and thus if the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B are lowered, the tab main bodies  21 A and  21 B act as resistance plates, causing the fuel efficiency of the marine vessel  11  to decrease. 
     The fuel efficiency at the descent rate of 100% is higher than the fuel efficiency at the descent rate of 0% until the speed of the marine vessel  11  reaches, for example, approximately 43 km/h after reaching, for example, approximately 17 km/h. This is because when the speed of the marine vessel  11  has become higher than approximately 17 km/h, the marine vessel  11  shifts into the hump state, in which if the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B are lowered, a bow-down moment  25  is generated to reduce the pitch angle of the hull  13 , resulting in the resistance acting on the bottom of the hull  13  to be decreased. 
     When the marine vessel  11  further accelerates after reaching, for example, approximately 43 km/h, the fuel efficiency at the descent rate of 0% becomes higher again than the fuel efficiency at the descent rate of 100%. This is because when the speed of the marine vessel  11  has become higher than approximately 43 km/h, the marine vessel  11  shifts into the planing state, in which not only the bow but also the stern rises, resulting in the pitch angle of the hull  13  being decreased. Thus, if the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B are lowered, the tab main bodies  21 A and  21 B act as resistance plates again, causing the fuel efficiency of the marine vessel  11  to decrease. 
     More specifically, when the marine vessel  11  accelerates, first, it reaches a speed (approximately 17 km/h in  FIG. 6 ) at which the fuel efficiency is higher in the case in which the tab main bodies  21 A and  21 B are lowered than in the case in which the tab main bodies  21 A and  21 B are not lowered (hereafter referred to as a “first fuel efficiency reversing speed”) (a predetermined speed). Further, when the marine vessel  11  continues to be accelerated even after it reaches the first fuel efficiency reversing speed, the marine vessel  11  reaches a speed (approximately 43 km/h in  FIG. 6 ) at which the fuel efficiency is higher in the case in which the tab main bodies  21 A and  21 B are not lowered than in the case in which the tab main bodies  21 A and  21 B are lowered (hereafter referred to as a “second fuel efficiency reversing speed”). 
     Accordingly, in the present preferred embodiment, before the marine vessel  11  reaches the first fuel efficiency reversing speed, the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B are lowered so that the descent rate is, for example, 100%. Also, before the marine vessel  11  reaches the second fuel efficiency reversing speed, the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B are raised so that the descent rate is, for example, 0%. It should be noted that the graph of  FIG. 6  is merely one example, and the first fuel efficiency reversing speed and the second fuel efficiency reversing speed vary according to the shape and weight of the hull  13  of the marine vessel  11 . For this reason, the relationship between the speed and fuel efficiency of the marine vessel  11  needs to be obtained whenever the shape and weight of the hull  13  of the marine vessel  13  change. 
     Moreover, in the present preferred embodiment, swinging of the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B is controlled according to the speed of the marine vessel  11 , and it takes a certain period of time, for example, about four seconds to lower the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B from the position at which the descent rate is 0% to the position at which the descent rate is 100%. Thus, in the case in which lowering of the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B is started at a time when the marine vessel  11  reaches the first fuel efficiency reversing speed, it takes a certain period of time to generate the bow-down moment  25 , and hence the hump state of the marine vessel  11  lasts longer than expected. 
     Furthermore, a speed that can be achieved by the marine vessel  11  (hereafter referred to merely as an “achievable speed”) depends on the throttle opening angle, and for example, as shown in  FIG. 7 , the achievable speed is uniquely determined with respect to the throttle opening angle. In the present preferred embodiment, a throttle opening angle at which the first fuel efficiency reversing speed (approximately 17 km/h in  FIG. 6 ) is achievable is a first throttle opening angle α (for example, about 24%), and a throttle opening angle at which the second fuel efficiency reversing speed (approximately 43 km/h in  FIG. 6 ) is achievable is a second throttle opening angle β. In other words, the first throttle opening angle α is a request to accelerate the marine vessel  11  to the first fuel efficiency reversing speed, and the second throttle opening angle β is a request to accelerate the marine vessel  11  to the second fuel efficiency reversing speed. 
       FIG. 8  is a view useful in explaining how the engine rpm follows the throttle opening angle. In  FIG. 8 , a solid line indicates the throttle opening angle, and a broken line indicates the engine rpm. In the actual engines  16 A and  16 B, even if air and fuel supplied to the engines  16 A and  16 B are increased by further opening the throttle, the engine rpm does not immediately increase because of inertial mass of a piston or the like, and as shown in  FIG. 8 , the engine rpm lags behind the throttle opening angle while following the throttle opening angle. The amount by which the engine rpm lags behind the throttle opening angle is large particularly in a region where the throttle opening angle is small. 
     It should be noted that the engine rpm is substantially proportional to the speed of the marine vessel  11 , and thus the speed of the marine vessel  11  also lags behind the throttle opening angle while following the throttle opening angle. For example, even when a user operates the throttle lever  12  to make the throttle opening angle correspond to the first throttle opening angle α, it takes a predetermined period of time for the marine vessel  11  to achieve the first fuel efficiency reversing speed. 
       FIG. 9  is a flowchart showing a posture control process during acceleration of the marine vessel  11  according to a preferred embodiment of the present invention. The process in  FIG. 9  is implemented by the CPU  31  of the controller  30  executing a control program expanded in the RAM  33 . 
     Referring to  FIG. 9 , when the marine vessel  11  is accelerating, first, whether or not the throttle opening angle has reached the first throttle opening angle α is judged in response to an operation on the throttle lever  12  by the user (step S 91 ). Here, the throttle opening angle is determined according to the amount of the operation on the throttle lever  12 , but the actual opening angles of the throttle valves in the respective engines  16 A and  16 B have be measured, and the judgment in step S 91  may be made using the measured values. 
     In step S 91 , when the throttle opening angle has not reached the first throttle opening angle α, the process returns to step S 91 . When the throttle opening angle has reached the first throttle opening angle α, the controller  30  lowers the tab main bodies  21 A and  21 B using the respective trim tab actuators  22 A and  22 B so that the descent rate is, for example, 100% (step S 92 ). Lowering the tab main bodies  21 A and  21 B causes the bow-down moment  25  around the center of gravity G to be generated in the hull  13  and lowers the raised bow to reduce the pitch angle of the hull  13 . 
     Next, the marine vessel  11  maintains its accelerating state, and in response to an operation on the throttle lever  12  by the user, the controller  30  judges whether or not the throttle opening angle has reached the second throttle opening angle β (step S 93 ). 
     In step S 93 , when the throttle opening angle has not reached the second throttle opening angle β, the process returns to step S 93 . When the throttle opening angle has reached the second throttle opening angle β (predetermined time), the controller  30  raises the tab main bodies  21 A and  21 B using the respective trim tab actuators  22 A and  22 B so that the descent rate is, for example, 0% (step S 94 ). After that, the present process is ended. 
     According to the process in  FIG. 9 , when the throttle opening angle has reached the first throttle opening angle α, the tab main bodies  21 A and  21 B are lowered. More specifically, the time when the tab main bodies  21 A and  21 B should be lowered is determined based on the throttle opening angle. As a result, lowering of the tab main bodies  21 A and  21 B is started without waiting for the speed of the marine vessel  11  to accelerate to the first fuel efficiency reversing speed. Thus, even if it takes a certain period of time to lower the tab main bodies  21 A and  21 B, the tab main bodies  21 A and  21 B are lowered before the speed of the marine vessel  11  reaches the first fuel efficiency reversing speed. As a result, the hump state is prevented from lasting longer than expected, and the fuel efficiency of the marine vessel  11  is improved. 
     In other words, in the process in  FIG. 9 , the tab main bodies  21 A and  21 B are lowered without waiting for the engine rpm to increase to the rpm corresponding to the first fuel efficiency reversing speed. Here, assume that the rpm corresponding to the first fuel efficiency reversing speed is an rpm that is about 1500 rpm higher than an rpm in an idling state ( FIG. 8 ). In this case, if the tab main bodies  21 A and  21 B are lowered in a case in which the throttle opening angle reaches the first throttle opening angle α, the tab main bodies  21 A and  21 B are lowered, for example, t seconds earlier than in a case in which the tab main bodies  21 A and  21 B are lowered after the engine rpm reaches the rpm corresponding to the first fuel efficiency reversing speed. Thus, the tab main bodies  21 A and  21 B are lowered before the engine rpm reaches the rpm corresponding to the first fuel efficiency reversing speed. Here, t seconds mentioned above corresponds to the predetermined period of time required for the speed of the marine vessel  11  to reach the first fuel efficiency reversing speed after the throttle opening angle reaches the first throttle opening angle α. More specifically, in the present preferred embodiment, it can also be said that by using the predetermined period of time (t seconds), the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B are lowered before the engine rpm reaches the rpm corresponding to the first fuel efficiency reversing speed. 
     Furthermore, in the process in  FIG. 9 , when the throttle opening angle has reached the second throttle opening angle β, the tab main bodies  21 A and  21 B are raised. As a result, the tab main bodies  21 A and  21 B are raised before the speed of the marine vessel  11  reaches the second fuel efficiency reversing speed. Thus, even if the marine vessel  11  shifts into the planing state, the tab main bodies  21 A and  21 B are prevented from acting as resistance plates while being kept down, and the fuel efficiency of the marine vessel  11  is improved. 
       FIG. 10  is a flowchart showing a variation of the posture control process during acceleration of the marine vessel  11  according to a preferred embodiment of the present invention. The process in  FIG. 10  is also implemented by the CPU  31  of the controller  30  executing a control program expanded in the RAM  33 . Here, a throttle opening angle at which a speed at which the marine vessel  11  shifts into the hump state is achievable is referred to as a “hump shifting opening angle”, and a throttle opening angle at which a speed at which the marine vessel  11  shifts into the planing state is achievable is referred to as a “planing shifting opening angle”. In other words, the hump shifting opening angle is a request to accelerate the marine vessel  11  to a speed at which the marine vessel  11  shifts into the hump state, and the planing shifting opening angle is a request to accelerate the marine vessel  11  to a speed at which the marine vessel  11  shifts into the planing state. 
     Referring to  FIG. 10 , when the marine vessel  11  is accelerating, first, whether or not the throttle opening angle has reached the hump shifting opening angle is judged in response to an operation on the throttle lever  12  by the user (step S 101 ). 
     In step S 101 , when the throttle opening angle has not reached the hump shifting opening angle, the process returns to step S 101 . When the throttle opening angle has reached the hump shifting opening angle, the controller  30  lowers the tab main bodies  21 A and  21 B using the respective trim tab actuators  22 A and  22 B so that the descent rate is, for example, 100% (step S 102 ). 
     Next, the marine vessel  11  maintains its accelerating state, and in response to an operation on the throttle lever  12  by the user, the controller  30  judges whether or not the throttle opening angle has reached the planing shifting opening angle (step S 103 ). 
     In step S 103 , when the throttle opening angle has not reached the planing shifting opening angle, the process returns to step S 103 . When the throttle opening angle has reached the planing shifting opening angle, the controller  30  raises the tab main bodies  21 A and  21 B using the respective trim tab actuators  22 A and  22 B so that the descent rate is, for example, 0% (step S 104 ). After that, the present process is ended. 
     According to the process in  FIG. 10 , when the throttle opening angle has reached the hump shifting opening angle, the tab main bodies  21 A and  21 B are lowered. Thus, the tab main bodies  21 A and  21 B are lowered before the speed of the marine vessel  11  has reached the speed at which it shifts into the hump state. When the throttle opening angle has reached the planing shifting opening angle, the tab main bodies  21 A and  21 B are raised. Thus, the tab main bodies  21 A and  21 B are raised before the speed of the marine vessel  11  has reached the speed at which it shifts into the planing state. As a result, the fuel efficiency of the marine vessel  11  is improved. 
     Although in a preferred embodiment of the present invention, swinging (lowering and raising) of the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B is controlled according to the throttle opening angle, swinging of the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B may be controlled according to a fuel injection quantity because the fuel injection quantity varies depending on the throttle opening angle. For example, the tab main bodies  21 A and  21 B may be lowered when the fuel injection quantity has reached a fuel injection quantity at which the first fuel efficiency reversing speed or the speed at which the marine vessel  11  shifts into the hump state is achievable. Also, the tab main bodies  21 A and  21 B may be raised when the fuel injection quantity has reached a fuel injection quantity at which the second fuel efficiency reversing speed or the speed at which the marine vessel  11  shifts into the planing state is achievable. 
     Moreover, although in a preferred embodiment of the present invention, the throttle opening angle is determined based on the operated amount of the throttle lever  12 , the throttle opening angle may be determined based on the operated amount of a stick-type operator of an auxiliary operating unit, for example, a joystick. 
     Furthermore, although in a preferred embodiment of the present invention, the marine vessel  11  is equipped with the outboard motors  15 A and  15 B, the marine vessel  11  may be equipped with other types of marine propulsion devices such as inboard/outboard motors (stern drive, inboard motor/outboard drive) and inboard motors. In this case, the marine vessel  11  shifts into the hump state when accelerating, and thus preferred embodiments of the present invention may be applied to this marine vessel  11 . 
     It should be noted that as the posture control plates, interceptor tabs described in Zipwake mentioned above may be used as alternatives to the tab main bodies  21 A and  21 B. The interceptor tabs are attached to both sides of the stern of the hull  13  and shift their position in substantially the vertical direction. Specifically, in the water, each of the interceptor tabs changes its position from a position at which it projects from a lower surface (bottom) of the hull  13  to a position above the lower surface of the hull  13 . The interceptor tabs change the direction of water current by projecting from the lower surface of the hull  13 , and thus they generate a larger lift than the lift L generated by the tab main bodies  21 A and  21 B and consequently generate the bow-down moment  25  as with the tab main bodies  21 A and  21 B. Thus, if the interceptor tabs are used, it is preferred that the amount of displacement of the interceptor tabs is controlled according to the throttle opening angle. 
     In addition, at the start of the maneuvering system, whether or not to execute the posture control method according to a preferred embodiment of the present invention (the process in  FIG. 9  or the process in  FIG. 10 ) may be set using the setting operating unit  19 . 
     Although in a preferred embodiment of the present invention swinging of the tab main bodies  21 A and  21 B of the respective trim tab units  20 A and  20 B is controlled according to the throttle opening angle, that is, the request to accelerate the marine vessel  11  to the predetermined speed, operation of other equipment on the marine vessel  11  may be controlled according to the request to accelerate the marine vessel  11  to the predetermined speed. For example, when the throttle opening angle corresponding to the operated amount of the throttle lever  12  has become equal to the throttle opening angle at which the predetermined speed is achievable, deflation of air cushions  26  ( FIG. 11 ), which are provided on sides of the marine vessel  11  and used to bring the marine vessel  11  alongside a pier, may be started. It takes a predetermined period of time to deflate the air cushions  26 . Accordingly, by starting to deflate the air cushions  26  when the throttle opening angle has become equal to the throttle opening angle at which the predetermined speed is achievable, the air cushion  26  are able to be shrunk before the speed of the marine vessel  11  reaches the predetermined speed. As a result, air resistance on the hull  13  is decreased, and the fuel efficiency of the marine vessel  11  is improved. 
     Moreover, when the throttle opening angle corresponding to the operated amount of the throttle lever  12  has become equal to the throttle opening angle at which the predetermined speed is achievable, closure of a movable roof  27  of a collapsible-type such as a canvas top or a sun shade may be started ( FIG. 12A ). It takes a certain period of time to flatten the top of a cabin  28  as shown in  FIG. 12B  by closing the movable roof  27 . On the other hand, by starting to close the movable roof  27  when the throttle opening angle has become equal to the one at which the predetermined speed is achievable, the top of the cabin  28  is flattened when the marine vessel  11  has reached the predetermined speed. As a result, air resistance on the hull  13  is decreased, and the fuel efficiency of the marine vessel  11  is improved. 
     It should be noted that when the marine vessel  11  moves backward, the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B are raised to the position at which the descent rate is 0%, and even if the marine vessel  11  is accelerated, the process in  FIG. 9  or  FIG. 10  is not carried out, and swinging of the tab main bodies  21 A and  21 B of the trim tab units  20 A and  20 B is not controlled. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.