it ain't any theory whatsoever. It was merely a "scientific" explanation given
to 19th century British naval brass to "help" them understand why putting 2x
the power in a boat didn't make the boat go 2x the speed.

The "theory" sounded scientific and it had numbers in it and it was called a
theory so the brass accepted it.

most sailboats built in the last several decades will easily exceed
"theoretical" hull speed. in fact, a deep vee hullled Hobie cat (displacement
hull by any standard) at 18 feet will under many points of of sail easily pass
a 45 foot sailboat (also a displacement hull for most every 45 sailboat out
there).

but please use the term, for it lends credibility and pananche when talking to
young lovelies at the yacht club bar.