Rigid Body relative speed

Hello.

I am working on a game with ships. I modify the ships speed using addRelativeForce. I am limiting the speed to say 100. After the ship accelerates to the max speed I change it's direction (while still traveling in the original direction at max speed), now I need to get the speed it's traveling at in the new direction (forward) but I keep getting the global speed in the forward direction, and since the ship is already at full speed I can't accelerate in new direction. How do I read the ships speed in the local forward direction? I have added the code below.

 if (Input.GetButton("w") && transform.InverseTransformDirection(Ship.rigidbody.velocity).z < 100)
     {
         if (Input.GetButtonDown('w'))
         {
             //thruster_audio(1);
         }
         Ship.rigidbody.AddRelativeForce(Vector3.forward*1000*Time.deltaTime);
         if (Thruster1.particleSystem.startSpeed < 20)
         {
             Thruster1.particleSystem.startSpeed += 10 * Time.deltaTime;
             Thruster2.particleSystem.startSpeed += 10 * Time.deltaTime;
             Thruster3.particleSystem.startSpeed += 10 * Time.deltaTime;
             Thruster4.particleSystem.startSpeed += 10 * Time.deltaTime;
             Thruster5.particleSystem.startSpeed += 10 * Time.deltaTime;
         }
     }