What has happened is that the boat has dug itself a hole. If the boat maintains hull speed, its bow and stern are well supported by their respective waves and it can continue moving forward efficiently. But if it tries to go faster and the stern wave is pulled further aft by the lengthening trough of the bow wave, the back of the boat falls into the hole, and the boat is left trying to climb up the hill presented by its own bow wave, which by now is relatively large. From this point forward, disproportionately larger increases in power are needed to achieve ever smaller increases in speed.