it isn't. Most recreational sailboats made in the last 40 years will exceed
"hull speed" without any heavy lifting, and Hobie cats will go 3x "hull
speed"
or more.

Hobie "cat". The hull speed equation is intended to apply to monohulls in
displacement mode.


The arguable point is really the definition of displacement mode, not the
veracity of the hull speed theorem. IMO, a Hobie Cat is on plane....(hell,
sometimes airborne)..when it is making better than hull speed.