Frame Independent Movement with AddRelativeForce

My goal is to create a frame independent vehicle controller script that will turn/rotate and thrust my motor boat forward and backward.

My current code produces movement in rotation super slow and very piddly values (decimals). When the vertical axis is 1 I want the vehicle to increase velocity by 10(acceleration) all the way up to 40(maxVelocity).

Where am I going wrong?

 public float maxVelocity = 40f;
 public float acceleration = 10f;
 
 public float turnAcceleration = 5f;
 
 
 // Update is called once per frame
 void Update () {
     float moveInputAmount = Input.GetAxis ("Vertical");
     float turnInputAmount = Input.GetAxis ("Horizontal");
     
     
     // Rotation/Turning
     // Apply turn
     Vector3 turnForce = Vector3.up * turnInputAmount * this.turnAcceleration * Time.deltaTime;
     rigidbody.AddRelativeTorque(turnForce);
     
     
     // Moving forward backward
     // Throttle the boat to the max velocity in both positive and negative direction
     Vector3 thrustForce = Vector3.forward * moveInputAmount * this.acceleration * Time.deltaTime;
     if(rigidbody.velocity.z < this.maxVelocity && rigidbody.velocity.z > -this.maxVelocity)
     {
         
         rigidbody.AddRelativeForce(thrustForce);
     }
     
     Debug.Log (thrustForce + " ~ " + turnForce);
     Debug.Log ("Boat Velocity: " + rigidbody.velocity);
     Debug.Log ("Boat Rotation: " + rigidbody.rotation);
     
 }