Rigid Body relative speed


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'))
		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;

Instead of Vector3.forward, use Ship.transform.forward.