Hi,

Bill Roberts addresses this very thoroughly in an article in Multihulls mag a long time ago. (I have a copy of it somewhere - maybe CatSailor can get a hold of it (or Bill) and re-print it?). In summary, though the hull speed is proportional to the square root of it's waterline length (as defined by the Froude Number), there is a coefficient or multiplying factor used which takes other things into account, such as fineness ratio, prismatic coefficient, etc. Most monohulls use 1.33. In Bill's article, he shows a graph of varying fineness ratios, and the coefficient to be used for these values.
Another excellent source for multihull theory is a SNAME (Society of Naval Architects and Marine Engineers) paper written by James Antrim (also published in September 1990 Marine Technology). This paper has a polar plot for a given hull design. For his 40 foot trimaran design he states "...at high speed, dynamic lifting effects comparable to planing are significant." Of course his design was capable of 22 knots in 15 knots true wind, so some dynamic lift was possible. I agree that it is less significant in most beach cats in moderate conditions, except possibly while under spinnaker, where planing type forces may develop.
Copies of his paper are available from SNAME. I highly recommend it!


Steve