This is an Aluminum Tandem Axle Boat Trailer Fender support. This is solid 1/4 inch thick aluminum, meaning it will not rust! This Aluminum Fender Mount is used to support the trailer fender in the middle, between the trailer tires, typically mounting from the I-Beam of the trailer frame to the under side of the fender. These Fender supports are very commonly seen mounting Tandem Axle Aluminum Diamond Plate Trailer Fenders. There are a couple different variations of this fender support. Check the measurements on the attached images to ensure you have the correct one. This fender bracket has pre-drilled 3/8 inch mounting holes, which would commonly use stainless steel 3/8 inch Trailer Bolts. The bends you see in this bracket are each roughly 45 Degrees, making the mounting angle of this bracket at 90 Degrees, which would support the middle of a tandem axle trailer fender perfectly. Typically on a tandem axle you would only need (1) per fender, so replacing two fenders would mean needing two of these supports.
Occasionally, you will also see these brackets used to help support and stabilize the large bunks on boat trailers that have been designed with the bunks mounted on top of the aluminum I-Beam frame.