A displacement hull does not plane... that is a sailboat, It can slide down a wave or surf at more than hull speed.
The notion is that the displacement hull creates a bow wave as it pushes the water aside. The wave gets steeper and longer as it moves faster. At some point the vessel is in a trough and it would be trying to go uphill. I don't think a powered sailboat will create a longer and longer wave... steeper and steeper as more power is applied to drive it up the back of the wave.
I think the hull speed is 1.34 x square root of the LWL.. which does change a bit depending on the hull shape. So a 36' LWL would have a hull speed of 8 knots... 25' would be 6.7.