(except that stay is usually "hauban")

Mega-multis have crossbeams linking the floats. Different architects have different views on what forlm these shoudl take. They are referred to as beams or cross beams.

Anything to the back (stern) of a bnoat is referred to as being "aft". That which is to the front is generally said to be "for'ard". There are nuances, but that's a starting point.

I checked out the link to the Atlantic Arc site where the term "aft barce" has been used in a text about Primgaz, a trimaran whose skipper was Laurent Bourgnon, Yvan Bourgnon(Bayer)'s elder brother. This strikes me as an odd exception. Elsewhere, "aft brace" is used to refer to struts used duringthe construction stage to apply pressure to either side.