This is a Solid aluminum 80 inch Cross Member that is used to mount between the main frame of boat trailers. While this is considered an 80 inch cross member, its end to end measurement is 79 1/2 inches. In most, if not all cases, they are mounted to trailers with I Beam frames. Using Stainless Steel U-Bolt 1/2 inch x 3 inch x 4 5/16 inch U Bolts, they are mounted up through the I Beam.
You are most likely replacing this Aluminum Cross Member because your U-Bolts are eaten through your cross beam. This Galvanic Corrosion is very common when Galvanized U-Bolts are used to mount your trailers Bunk Brackets onto the frame. It sounds crazy, but the Aluminum will actually become corroded while the U-Bolt remains in tact. To prevent this issue from ever recurring, use Stainless Steel U-Bolts and bolts when remounting your cross member.
This component is made of 3" x 3" x 1/4" thick aluminum square tubing that has a bend in the middle that drops 3". This drop is specifically intended to prevent impact from the V-Hull on your boat when loading and unloading at the ramp. This component is extremely important on your boat trailer as it is responsible for supporting the weight of your boat, and keeping the frame square.