That's the theory but on my 31 (same as the 30) once you get close to 6 kts you can see that the transom is squatting so much you are just wasting fuel and energy trying to push it much faster.Hull speed on the 30 (LWL 25.4’) should be 6.8 kts.
Hull speed on the 28 (LWL 22.75) should be 6.4 kts.