When you increase the velocity of an object going through a fluid, the DRAG increases, too.
Drag doesn’t increase in a straight-line relationship with the velocity.
The resistance due to drag has a “squared” component in it. In other words, in the rather nasty equation describing drag, “velocity squared” is working against us. So, doubling the velocity would add a multiplier of “4” somewhere in the drag equation.
This means that when we increase the difference in speed between the boat and the water, the increase in resistance due to drag goes up more than the increase in speed. This is one of the reasons it takes full time training to compete internationally when domestic competitiveness can usually be attained with only once daily training in the “off season” and twice daily in the “on season”.
The faster you go, the harder it is to go faster. (Why can a good novice men’s 8+ improve from 10 minutes to just about 6 minutes in 1 season, a top-level university 8+ might get to 5:30 with tailwind, and a world champion 8+ might get below 5:25 if everything “works”, and the new “best time” is now below 5:20.)
The increase in resistance is out of proportion to the increase in speed.
Thanks to Walter Martindale.