hereinafter , an embodiment of a jet - propulsion small watercraft of the present invention will be described with reference to the drawings . here , a personal watercraft will be described . referring now to fig7 and 8 , reference numeral a denotes a body of the personal watercraft . the body a comprises a hull h and a deck d covering the hull h from above . a line at which the hull h and the deck d are connected over the entire perimeter thereof is called a gunnel line g . in this embodiment , the gunnel line g is located above a waterline l of the personal watercraft . as shown in fig8 an opening 16 , which has a substantially rectangular shape seen from above , is formed at a relatively rear section of the deck d such that it extends in the longitudinal direction of the body a , and a straddle - type seat s is mounted above the opening 16 such that it covers the opening 16 from above as shown in fig7 and 8 . an engine e is contained in a chamber 20 surrounded by the hull h and the deck d below the seat s and having a convex shape in a cross section of the body a . the engine e is a multiple - cylinder ( e . g ., three cylinder ) engine equipped with a carburetor - type fuel supply device . as shown in fig7 the engine e is mounted such that a crankshaft 10 b extends along the longitudinal direction of the body a . an output end of the crankshaft 10 b is rotatably coupled integrally with a pump shaft 21 s of the water jet pump p through a propeller shaft 15 . the impeller 21 is covered with a pump casing 21 c on the outer periphery thereof . a water intake 17 is provided on the bottom of the hull h . the water is sucked from the water intake 17 and fed to the water jet pump p through a water intake passage 28 . the water jet pump p pressurizes and accelerates the water . the pressurized and accelerated water is discharged through a pump nozzle 21 r having a cross - sectional area of flow that gradually reduces rearward , and from an outlet port 21 k provided on the rear end of the pump nozzle 21 r , thereby obtaining the propulsion force . in fig7 reference numeral 21 v denotes fairing vanes for fairing water flow inside the water jet pump p . in fig7 and 8 , reference numeral 10 denotes a bar - type steering handle . by operating the steering handle 10 to the right or to the left , a steering nozzle 18 provided behind the pump nozzle 21 r swings to the right or to the left the watercraft can be turned to any desired direction while the water jet pump p is generating the propulsion force . as shown in fig7 a bowl - shaped reverse deflector 19 is provided above the rear side of the steering nozzle 18 such that it can swing downward around a horizontally mounted swinging shaft 19 a . the deflector 19 is swung downward toward a lower position behind the steering nozzle 18 to deflect the water ejected from the steering nozzle 18 forward , and as the resulting reaction , the personal watercraft moves rearward . in fig7 and 8 , reference numeral 12 denotes a rear deck . the rear deck 12 is provided with an operable hatch cover 29 . a rear compartment ( not shown ) with a small capacity is provided under the hatch cover 29 . in fig7 or 8 , reference numeral 23 denotes a front hatch cover . a front compartment ( not shown ) is provided under the front hatch cover 23 for storing equipment and the like . the personal watercraft according to this embodiment of the present invention , as shown in fig7 and 8 , may further include a steering position sensor sp comprised of a proximity switch having parts which are provided on a rotational side and a fixed side of a portion of a rotational shaft 10 a of the steering handle 10 . a throttle position sensor ss is provided on a carburetor cb ( see fig1 ) of the engine e . as represented by a solid line in fig1 the steering position sensor sp is connected to an electric control unit ecu provided for the engine e through a signal line ( e . g ., electric wire ) and the throttle position sensor ss is connected to the electric control unit ecu through the signal line . when a steering operation is performed , that is , the steering handle 10 ( see fig7 and 8 ) is steered a predetermined angle to the right or to the left , the steering position sensor sp sends a signal indicating that the steering operation has been performed , to the electric control unit ecu . when the throttle lever lt attached on the steering handle 10 ( see fig8 ) is operated to be closed to cause the throttle valve of the carburetor cb to be moved to a closed position , the throttle position sensor ss is configured to send a signal indicating this to the electric control unit ecu . as shown in fig1 the electric control unit ecu is connected to the actuator 31 , more precisely to a control board 31 c ( see fig4 ) of the actuator 31 through the signal line and is configured to control an operation of the actuator 31 . in this embodiment , upon reception of an operation signal from the electric control unit ecu , the control board 31 c of the actuator 31 controls power to motor 31 m of the actuator 31 . as shown in fig2 or 3 , a reel member 31 r is rotatable integrally with an output shaft 31 s of the actuator 31 . the reel member 31 r is connected to a steering throttle arm ( steering throttle drive means ) 33 a of the carburetor cb through a wire ( or rod ) 32 . the steering throttle arm 33 a is provided to be rotatable integrally with a throttle operation shaft 33 of the carburetor cb only in one direction ( direction in which the throttle arm 33 a causes the engine speed to increase , clockwise in fig2 and 3 ). the carburetor cb is provided with a stopper 38 as an upper limiter configured to stop further rotation of the steering throttle arm 33 a , i . e ., to inhibit the engine speed from exceeding approximately 2000 rpm , in this embodiment . the carburetor cb is further provided with a stopper 40 as a lower limiter so as to inhibit the engine speed from decreasing to a value not more than an idling speed . the throttle operation shaft 33 of the carburetor cb is provided with a helical torsion spring 39 configured to cause the operation shaft 33 to return to an idling position ( i . e ., rotate counterclockwise in fig2 and 3 to be brought into contact with the stopper 40 ), when an external force from the steering throttle arm 33 a and an external force from a propulsion throttle arm 33 b of the carburetor cb are not exerted on the operation shaft 33 any more , i . e ., the steering throttle arm 33 a and the propulsion throttle arm 33 b become non - operating states . as in a general carburetor , a throttle valve ( not shown ) provided within an air - intake passage of the carburetor cb to be openable and closable , is attached on the throttle operation shaft 33 . by rotating the throttle operation shaft 33 , the throttle valve within the air - intake passage is opened and closed , thereby causing the amount of taken - in air flowing within the carburetor cb to be increased and decreased . the propulsion throttle arm 33 b as the propulsion throttle drive means is provided on the throttle operation shaft 33 of the carburetor cb to be rotatable integrally with the throttle operation shaft 33 only in one direction ( direction in which the throttle arm 33 b rotates to cause the engine speed to increase , i . e ., clockwise in fig2 and 3 ). as represented by a dashed line in fig1 the throttle arm 33 b is connected to the throttle lever lt attached on the handle 10 ( see fig8 ) through a wire 34 . upon the rider operating the throttle lever lt , the throttle operation shaft 33 of the carburetor cb rotates by rotation of the throttle arm 33 b in fig2 thereby causing the throttle valve within the carburetor cb to open , so that the amount of taken - in air flowing within the carburetor cb is increased . the steering throttle arm 33 a and the propulsion throttle arm 33 b respectively engage with the throttle operation shaft 33 to cause the throttle operation shaft 33 to rotate to open the throttle valve ( clockwise in fig2 and 3 ). on the other hand , the steering throttle arm 33 a and the propulsion throttle arm 33 b respectively disengage from the throttle operation shaft 33 to cause the throttle operation shaft 33 to rotate to close the throttle valve ( counterclockwise in fig2 and 3 ). that is , when the steering throttle arm 33 a and the propulsion throttle arm 33 b rotate to close the throttle valve , they rotate independently of the throttle operation shaft 33 . the configuration for transmitting the rotation from the throttle arm 33 a or 33 b to the operation shaft 33 only in one direction is accomplished by using a ratchet mechanism . further , the steering throttle arm 33 a and the propulsion throttle arm 33 b operate independently of each other so that an operation of one of the steering throttle arm 33 a and the propulsion throttle arm 33 b does not affect an operation of the other . in this structure , upon the rider operating the throttle lever lt in the vicinity of the steering handle 10 , the propulsion throttle arm 33 b connected to the throttle lever lt through the wire 34 rotates to cause the throttle operation shaft 33 of the carburetor cb to rotate , but the steering throttle arm 33 a remains unmoved . conversely , when the steering throttle arm 33 a is rotated by the actuator 31 to cause the throttle operation shaft 33 to rotate , another ratchet mechanism having a similar configuration allows the propulsion throttle arm 33 b to remain unmoved . in fig2 reference numerals 51 a and 51 b denote air - intake pipes . it should be appreciated that in fig2 and 3 , the positional relationship between the actuator 31 and the carburetor cb is not illustrated correctly , but rotational relationship between the output shaft 31 s and the steering throttle arm 33 a , and connection between them through the wire 32 , are illustrated . as shown in fig7 and 8 , the actuator 31 is secured to an inside of the deck d below a front end of the seat s through a mounting plate member 37 ( see fig2 and 3 ) by means of bolts . the carburetor cb is provided on the left side of the engine e . referring to fig4 a detailed structure of the actuator 31 will be described . the above actuator 31 comprises a motor ( e . g ., servo motor ) 31 m , the control board 31 c configured to control energizing the motor 31 m and its current value , a spur gear train 31 r comprised of gears having different number of teeth as reduction gear means ( device ), the output shaft 31 s for outputting rotation from the motor 31 m to outside , and a water - proof cover 31 w that covers these components . the cover 31 w is comprised of three parts , i . e ., a motor casing portion 31 w 1 , a concave base portion 31 w 2 , and a lid portion 31 w 3 . at joint portions of these portions , rubber packings are provided to function as seal . a pinion gear ( gear with smaller number of teeth ) 31 ra is attached on a rotational shaft 31 mb of the motor 31 m and a large gear ( gear with larger number of teeth ) 31 rb is attached on the output shaft 31 s to form a spur geat train 31 r . therefore , in this embodiment , the rotational shaft 31 mb of the motor 31 m and the output shaft 31 s are arranged in parallel . the output shaft 31 s is rotatably supported by two ball bearings 31 g and 31 h to reduce rotational resistance . a base end portion 31 s b of the output shaft 31 s is accommodated within the cover 31 w , and a tip end portion 31 s a of the output shaft 31 s is exposed outside so as to protrude from the lid portion 31 w 3 of the cover 31 w . an oil seal 31 p is provided outside the ball bearing 31 g in the lid portion 31 w 3 , to inhibit entry of water or the like . the control board 31 c is accommodated within the base portion 31 w 2 to be covered by resin ( e . g ., polyurethane resin ) for waterproofing . turning now to a timing chart in fig5 the relationship between the control state of the power of the actuator and the operational state of the actuator is shown . in fig5 “( a )” represents an ampere as a unit of a current value and an arrow in the form of a curved line represents the relationship among “ motor power value ”, “ motor current ”, and “ motor smoothing current .” the control board 31 c is configured to control energizing the motor 31 m ( including the current value ) in the manner as shown in fig5 . specifically , as can be seen from an operation signal located on the uppermost side in fig5 upon the operation signal from the electric control unit ecu being input to the control board 31 c , the control board 31 c executes control to allow the motor 31 m to be energized at a power value ( current value ) of substantially 100 %. as a result , as can be seen from “ output shaft operation ” in fig5 the motor 31 m operates at the power value of substantially 100 % ( the output shaft 31 s rotates clockwise in fig2 ). as can be seen from the “ motor current ” in fig5 the current flowing within the motor 31 m has a maximum value ( max ( 1 ) in fig5 ) at the activation of the motor 31 m , and thereafter , decreases gradually with an elapse of time . thereby , by the wire 32 with a base end connected to the rotational shaft of the motor 31 m , the steering throttle arm 33 a rotates clockwise in fig2 to be brought into contact with the stopper 38 . under this condition , as can be seen from the “ motor current ” in fig5 the current value of the motor 31 m rapidly increases to the maximum value ( see max ( 2 ) in fig5 ) again . when the current value becomes the maximum value in this manner , the control board 31 c detects fluctuation ( increase ) in the current value and executes control so that the value of the current flowing within the motor 31 m is reduced to substantially 30 % ( see “ motor power value ” in fig5 ). such an energized state continues until the operation signal from the electric control unit ecu to the control board 31 c is stopped as shown in fig5 . upon the operation signal from the electric control unit ecu to the control board 31 c being stopped , that is , as soon as the operation signal is not output any more from the electric control unit ecu , the control board 31 c stops energizing the motor 31 m . thereby , a spring force of the helical torsion spring 9 for return causes the throttle operation shaft 33 and the steering throttle arm 33 a of the carburetor cb to return to their respective initial states ( idling states ). correspondingly , the reel member 31 r returns to its initial state . the actuator 31 and the small watercraft comprising the actuator 31 function as described below , during cruising . hereinbelow , with reference to the flowchart in fig6 this will be described . here , it is assumed that the rider operates the throttle lever lt ( see fig8 ) to cause the throttle valve of the engine e to be closed , and thereby , the engine speed of the engine e decreases to an idling state . under this condition , upon the rider operating the handle 10 a predetermined angle or more to the right or to the left , the throttle position sensor ss and the steering position sensor sp detect the corresponding operations ( steps 1 and 2 ) and send detection signals to the electric control units ecu . upon detecting these signals , the electric control unit ecu sends the operation signal to the control board 31 c of the actuator 31 , and upon reception of the operation signal , the actuator 31 operates ( step 3 ). specifically , the control board 31 c starts energizing the motor 31 m at the power value of substantially 100 %. as a result , the steering throttle arm 33 a rotates until it makes contact with the stopper 38 . as can be seen from fig3 when the steering throttle arm 33 a makes contact with the stopper 38 , the control board 31 c executes control so that the power value of the motor decreases from substantially 100 % to substantially 30 %, as can be seen from the “ motor power value ” in fig5 . thereby , the steering throttle arm 33 a continues the condition of contact with the stopper 38 and the engine e achieves an operating state at approximately 2000 rpm in this embodiment . now , the small watercraft can be steered . and , when the small watercraft arrives at a shore or pier , for example , and the rider turns off a switch of the engine e , energizing the electric control unit ecu and the actuator 31 are stopped . as a result , the motor 31 m stops , and the spring force of the helical torsion spring 39 causes the throttle operation shaft 33 to return to cause the engine e to enter the idling state . the steering throttle arm 33 a moves away from the stopper 38 and returns to its initial state ( see the steering throttle arm 33 a in fig2 ). thereby , by the spring force applied through the wire 32 , the output shaft 31 s of the actuator 31 returns to its initial state . upon the rider turning on the switch of the engine e again , the engine e starts in the idling state . that is , it is not necessary to reset the actuator 31 so that the engine e re - starts in the idling state . the water - proof actuator 31 can be placed within the engine room 20 without a sealed space for placing the actuator 31 . in this state , installation of electric components such as the control board 31 c , the motor 31 m , etc , is not affected by water or the like . instead of using the actuator 31 for operating the throttle valve of the carburetor cb , this actuator 31 may be used for operating another component , for example , a valve provided within an exhaust passage of the engine e . in an alternative configuration , the actuator 31 may be used for operating a valve within a leading passage for taking in ambient air into the engine room . as this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof , the present embodiment is therefore illustrative and not restrictive , since the scope of the invention is defined by the appended claims rather than by the description preceding them , and all changes that fall within metes and bounds of the claims , or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims .