As far as the hull speed formula, we have to remember that it does a very good job of predicting the speed of a "ship of the line" which had a relative bluff bow.
From Wikipedia, we learn that: "Wave making resistance depends dramatically on the general proportions and shape of the hull: modern displacement designs that can easily exceed their 'hull speed' without planing include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. These benefits are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.
"Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.
"Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing. Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases above hull speed. For example, world-class racing kayaks can exceed hull speed by more than 100%,[2] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes."
So, it's possible that America could have exceeded her theoretical hull speed, but probably not by much, considering her fine ends as compared to a ship of the line. 26mph is probably achievable only by a planing hull.
Catamarans and trimarans are well exceeding the speeds predicted by the waterline length equation - they are exceedingly narrow for length, and can carry considerably more sail area due to the wide platform with much increased heeling stability...
Brian
