Do a search on displacement hull theory to get a detailed answer to your question.
The short answer is boat length and wave interaction. A longer boat has a higher maximum displacement hull speed because the wave interaction is not as bad. The curve of thrust vs. displacement speed is exponential which is why higher thrust only produces a modest (if any) increase in speed as you approach a boat's maximum displacement speed.
Keith
2008 NWR Bash Crappie Champion
2010 NWR Bash Yellow Perch Champion
2010 Buggs Bash Smallest Crappie Award