Roger:

Thank you for a very lucid explanation.
From this, is it correct to think that "hull Speed" is not some sort of
value at which mathematics goes crazy and produces singularities but
simply represents a speed range in which necesary power to produce a
speed increase seriously increases?
Is Hull Speed defined in some way relating to the slope of the power vs
speed curve?

Now, for the bizarre theory question. Consider a small boat that has a
very long rigid extension on its stern that does not touch the water
except far from the boat where it has a rigid float. Would this have a
higher hull speed than the small boat alone?
Could you arrange for this float at the end to gain back energy from
the trough behind it?
Could you arrange floats on this rigid extension at certain places to
extract energy from the shorter period waves the boat produces?

David