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Few Catamarans are actually planning vessels. Most are semi-displacement boats (like most modern IMS/IRC derived monohulls). Both forms of semi-displacement boats achieve speeds that are higher than normal hullspeed (i.e. 1.34 x the square root of the waterline length) by minimizing wave production. In the case of multihulls, this is done through a very narrow waterline beam to length ratio, and minimal interference between the waves produced by the other hull(s). Semi-displacement mono-hulls cheat a bit by using very fine entries to minimize the size of the bow wave that they produce. In both cases, clean hull forms and minimal drag is critical to overall performance, but properly designed, semi-displacement boats can achieve sustained speeds that can literally exceed twice their theoretical hull speed without planning.
Of course as Alex points out, and as the article that he linked to explains, few cruising cats achieve passage speeds that exceed or even match those of modern performance mono-hull cruising boats.
Respectfully,
Jeff
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