Problem with a ship controller script

Hi,I am making a ship game and I have a ship controller I have found online but I have a problem.when i press the up arrow the vessel is developing speed to the right and on the down arrow on the left duno why its happening any ideas?Rudder is working fine.

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here is the code:

 var speed = 1.0;
 
 var acceleration = 1.0;
 
 var maxspeed = 2.0;
 
 var minspeed = -0.25;
 
 var heading = 0.0;
 
 var rudder = 0.0;
 
 var rudderDelta = 2.0;
 
 var maxRudder = 6.0;
 
 var bob = 0.1;
 
 var bobFrequency = 0.2;
 
 
 
 private var elapsed = 0.0;
 
 private var seaLevel = 0.0;
 
 private var rudderControl;
 
 private var rudderAngle = 0.0;
 
 
 
 function signedSqrt( x ){
 
     var r = Mathf.Sqrt(Mathf.Abs( x ));
 
     if( x < 0 ){
 
         return -r;
 
     } else {
 
         return r;
 
     }
 
 }
 
 
 
 function Update () {
 
     // Bobbing
 
     elapsed += Time.deltaTime;
 
     transform.position.y = seaLevel + bob * Mathf.Sin(elapsed * bobFrequency * (Mathf.PI * 2));
 
     
 
     // Steering
 
     rudder += Input.GetAxis("Horizontal") * rudderDelta * Time.deltaTime;
 
     if( rudder > maxRudder ){
 
         rudder = maxRudder;
 
     } else if ( rudder < -maxRudder ){
 
         rudder = -maxRudder;
 
     }



     heading = (heading + rudder * Time.deltaTime * signedSqrt(speed)) % 360;
 
     // transform.Rotate(0, rudder * Time.deltaTime, 0);
 
     transform.eulerAngles.y = heading;
 
     transform.eulerAngles.z = -rudder;
 
     
 
     if( rudderControl ){
 
         rudderAngle = ((-60 * rudder)/maxRudder + heading) % 360;
 
         //rudderControl.transform.localEulerAngles.y = (70 * rudderAngle) % 360;
 
         rudderControl.transform.eulerAngles = Vector3(0, rudderAngle, 0);
 
     }
 
     
 
     // Sail
 
     speed += Input.GetAxis("Vertical") * acceleration * Time.deltaTime;
 
     if( speed > maxspeed ){
 
         speed = maxspeed;
 
     } else if ( speed < minspeed ){
 
         speed = minspeed;
 
     }
 
     
 
     transform.Translate(0, 0, speed * Time.deltaTime);
 
 }
 
 
 
 function Awake (){
 
     seaLevel = transform.position.y;
 
     rudderControl = GameObject.Find("rudderControl");
 
 }