How can I modify the thrust direction to match the image orientation in SceneKit?

I have a spaceship image asset which is designed to be "facing" in the +Z direction, with +X on the left and +Y up. I want a SCNVector3() to push in the direction the boat is facing, so that if I apply a force in the direction of the thrust, the boat will move forward. I found an article telling me that I can shipNode.worldFrontuse to get the desired vector, but it doesn't work as I expected. I created shipNode and rotated it as below.

shipNode.rotation = SCNVector4(0,1,0,0)

when i position the camera like this

cameraNode.position = SCNVector3(x: 0, y: 0, z: 10)

I saw the front of the boat aimed at me. So far so good. In touchesBegan, I save the thrust direction and print some values. In touchesEnd I reset the position and thrust direction and rotate the boat π/2 radians in the XZ plane

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    thrustDirection = shipNode.worldFront * Float(-1)
    print("touchesBegan.rotation         \(shipNode.rotation)")
    print("touchesBegan.worldFront       \(shipNode.worldFront)")
    print("touchesBegan.thrustDirection: \(thrustDirection)")
}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    print("touchesEnded")
    thrustDirection = SCNVector3()
    shipNode.position = SCNVector3()
    shipNode.rotation.x = 0
    shipNode.rotation.y = 1
    shipNode.rotation.z = 0
    shipNode.rotation.w += Float.pi / 2
}

Even though SceneKit is showing the spaceship correctly (show me first, then right, then back, then left, then again,), the data being printed has (for me) some uninterpretable but consistent values .

touchesBegan.rotation         SCNVector4(x: 0.0, y: 1.0, z: 0.0, w: 0.0)
touchesBegan.worldFront       SCNVector3(x: 0.0, y: 0.0, z: -1.0)
touchesBegan.thrustDirection: SCNVector3(x: -0.0, y: -0.0, z: 1.0)
touchesEnded
touchesBegan.rotation         SCNVector4(x: 0.0, y: 1.0, z: 0.0, w: 1.57079625)
touchesBegan.worldFront       SCNVector3(x: -0.25, y: 0.0, z: -0.899999976)
touchesBegan.thrustDirection: SCNVector3(x: 0.25, y: -0.0, z: 0.899999976)
touchesEnded
touchesBegan.rotation         SCNVector4(x: 0.0, y: 1.0, z: 0.0, w: 3.1415925)
touchesBegan.worldFront       SCNVector3(x: -3.77489542e-08, y: 0.0, z: -0.124999881)
touchesBegan.thrustDirection: SCNVector3(x: 3.77489542e-08, y: -0.0, z: 0.124999881)
touchesEnded
touchesBegan.rotation         SCNVector4(x: 0.0, y: 1.0, z: 0.0, w: 4.71238899)
touchesBegan.worldFront       SCNVector3(x: 0.25, y: 0.0, z: -0.899999976)
touchesBegan.thrustDirection: SCNVector3(x: -0.25, y: -0.0, z: 0.899999976)
touchesEnded
touchesBegan.rotation         SCNVector4(x: 0.0, y: 1.0, z: 0.0, w: 6.28318501)
touchesBegan.worldFront       SCNVector3(x: 7.54979084e-08, y: 0.0, z: -1.0)
touchesBegan.thrustDirection: SCNVector3(x: -7.54979084e-08, y: -0.0, z: 1.0)
touchesEnded

The first data section looks correct - the boat is heading in the desired direction, but the second, third and fourth data sections have strange values ​​for .worldFrontthe given number of reported rotations . In case 2, the boat went towards me, but slipped slightly to the right. In the 3rd case, the spaceship backed up towards me. In case 4, the ship is heading towards me, but swiping a little to the left. When I rotate 2π radians, the spaceship is facing and moving forward correctly again.

As of this writing, I've read all the articles suggested by SO, and read the Apple SceneKit documentation, but can't explain the behavior I'm seeing. What am I missing? thank you for your help!