This square tube V-Bend aluminum cross-member is common on aluminum I Beam boat trailers.  The tube measures 3" tall x 3" wide x 80" long.  The tubing has a 1/4" wall thickness.  Boat trailer cross members are normally attached using U-bolts and bolts.  If you have an aluminum framed boat trailer that has galvanized cross-members this is a great upgrade to switch the whole trailer to aluminum.