You call that after On Collision so it will always be after the impact has already occurred.
If you want the ship's velocity just before impact, you need to have a separate event that tracks the distance of your ship to the floor.
Use the test two value,
distance(Ship.X, Ship.Y, Floor.X, Floor.Y)
Less than 100 (or whatever pixel distance you need for it to work).
Then set the variable HitVelocity then. Before impact.
That should work, good luck!