Many of these boats were designed hydrodynamicly to have an optimum cruise around 10 to 11 knots. This is the speed regime where the drag produced by the hull is at it's minimum, allowing the engines to operate more efficiently. Slower than this, the hull builds up a resistance to movement, due to the increase in drag behind the boat. Above these speeds the hull is fighting both the incoming water, and the water trailing behind the hull.
It is very much like the VG characteristics of an airfoil, or airplane, just one a larger and slower scale. It is the point where lift generated and drag generated are equal. On most aircraft the best glide speed, or the speed at which the minimum altitude lost, for a given distance, is also the optimum cruise speed. Which is suprisingly slow. 70 knots on a cessna 172 at max gross weight.
By that you can see that aircraft are actually operated ineffeciently cause they are operated at speeds way above their best glide.
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