This is the standard term. The "corss" does not mean that the arms are in the form of a cross, although a couple of boat did have these back in the late 1980's and 1990s. The "cross" means from one side to the other. They aer often referred to as a "beam" alone.



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Note added at 4 hrs (2015-10-27 16:35:30 GMT)
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http://www.unihedron.com/tek-composites/articles/seaworth/NE...

http://www.sailingcatamarans.com/index.php/faqs/15-general-q...

http://www.boatdesign.net/forums/multihulls/designing-constr...

http://fr.slideshare.net/JackDale/asa-114-catamaran




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Note added at 4 hrs (2015-10-27 16:36:16 GMT)
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On the last reference entry, see slide 8.

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Note added at 4 hrs (2015-10-27 16:36:55 GMT)
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(FYI, the corssbeams which are in an X form are referred to a "X-beams").

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Note added at 4 hrs (2015-10-27 16:40:18 GMT)
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Also thus named (crossbeams), as the "arms" run across the boat's beam.

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Note added at 4 hrs (2015-10-27 16:42:34 GMT)
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http://www.thedailysail.com/offshore/14/67311/0/sodebo-ultim...

Sodebo suffers a collision

Disaster for Thomas Coville's new maxi-tri in the Route du Rhum

Monday November 3rd 2014, Author: James Boyd, Location: France
At 23:30 French time the French rescue services informed the Route du Rhum - Destination Guadeloupe race management that a collision had occurred between the maxi-trimaran Sodebo Ultim and a cargo ship. Skipper Thomas Coville was unharmed.

Lying in second place in the Ultime class, Sodebo Ultim was sailing under three reefs and ORC (storm jib) at the time, making 15-18 knots in winds gusting up to 30 knots from the southwest.

In the collision, the trimaran has lost the front of its starboard float back to the cross beam. The middle hull has also been damaged at the bow.

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Note added at 4 hrs (2015-10-27 16:44:48 GMT)
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http://www.seahorsemagazine.com/17-content/march-2014/178-a-...

Legendary C-Class designer Steve Killing was paying close attention as the C-Class catamaran fleet was rolled over by the first successful generation of full foiling designs


You won’t find many sailors who will dispute that the 72ft catamarans used in the most recent America’s Cup were inspired by the C-Class catamarans. In fact, if you change the units from feet to metres you are not far off a direct match. The 25ft C-Class has a bit more length in relation to its 14ft beam than the 22m by 14m AC72, but the parallel is there. And if you happened to have been at the 2010 International C-Class Catamaran Champ - ionships in Newport the recruiting of C-Class talent by potential America’s Cup syndicates was somewhat frantic. It is then fitting perhaps that the transfer of knowledge has come full circle. For this latest C-Class championship in Falmouth, England the C-Class boats have taken their cue from the successful flying-boat AC72 devel opment by Team New Zealand and Oracle.

C-Class design has been, for most campaigns, a backyard build-it-and-test-it process, but it would be unfair to ignore the parts of each programme that have seen some serious science. In our own Canadian programme Fred Eaton, owner, helmsman and experimentation enthusiast, has always pushed the technical side and we have incorporated some Finite Element Analysis (FEA) of crossbeam attachments and Computational Fluid Dynamics (CFD) on hulls and multi-element foils.

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Note added at 4 hrs (2015-10-27 16:49:02 GMT)
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https://www.youtube.com/watch?v=TFjxEfg_mEw

joutée le 17 août 2013
Emirates Team New Zealand rounded the windward mark to starboard and dug the bows in to the main crossbeam in a violent maneuver. Two crew, Rob Waddell and Chris ward went overboard and were recovered by the team's chase boat apparently unharmed. But Aotearoa suffered damage to the fairing on the aft side of the main crossbeam.
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Note added at 4 hrs (2015-10-27 16:59:51 GMT)
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Well -established term, see back in the 1980's, British yacht Paragon :

http://naturepl.fr/photocaptions25.html

08116915
Paragon-BCA moored in Millbay Docks, Plymouth, UK, before the start of the 1985 Round Britain Race. Paragon was designed by Adrian Thompson. At 18.6m long, she had stream-lined compound-curve crossbeams, arched to prevent them dragging in the water.

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Note added at 4 hrs (2015-10-27 17:04:46 GMT)
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The term "arm" sometimes used, but only very occasionally compared to "cross beam/crossbeam/beam" and generally nowadays for spindly affairs.