[MHml] Multihulls Digest, Vol 55, Issue 11 (

[Timothy Dunn]

Mid-size cruising catamarans (in the 40-50 range) typically have hull beam ratios of 8 to 1.  Their performance is superior to sailing monohulls, but a far cry from optimum.  Large cruising catamarans and fast cruisers typically have hull beam ratios of 12 to 1 or thereabouts.  Anything higher than 13 to 1 is definitely a racer.  The Stars and Stripes (the America's cup catamaran) had a hull beam ratio of 16 to 1, and it was basically a huge, high tech beach cat.  Somebody should get right aft or forward of the offshore racing cats and take a photo, so we can calculate their hull length beam ratios.

Planing is dynamic lift, and it requires a huge amount of power to achieve for a given weight.  There are planing catamarans.  They are always power catamarans.  You can see profile drawings online of planing power cats-they have a chine that rises well above the waterline at the bow, which dives down under water well forward, and which is pretty much parallel to the waterline from midships aft, with a well immersed transom.

Sailing catamarans are always displacement, except for those few which have hydrofoils, which induce planing.  The old rules of thumb about speeds of over hull speed (traditionally 1.35 x the square root of the waterline) were for monohulls of typical beam and weight (hull beam ratios of about 3 to 1.)  

Here are two links relevant to the discussion regarding resistance (speed) versus hull waterline beam, and concerning power and expected speeds.

Hull beam and speed-

http://www.southwindssailing.com/articles/multihulls/CruiseMulti.shtml (Information about Bruce's research.)

Rules of thumb for sailing and powering speeds of multihulls, presumably for 'average' cruising multihulls, from Derek Kelsall-

http://www.kelsall.com/images/articles/A11_Kelsall%20Formulas_KC.pdf

I have never seen Mr. Bruce's work itself, just the synopsis shown in Mr. Kanter's article.  I would love to read the original source.  Does anyone know where to find a copy?






      
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