Hey guys. I wanted to pass along my experience with my 3x axle trailer(rated at 15,000#) I noticed it was bending along the I beam towards the rear and the trailer manufacturer used 7-1/2" I beam but placed the brackets beyond the rear axle. One of the brackets was 5' past the axle and the other was about 2'. The front bracket was placed about 2' from the front axle. The boat weighs about 10,000#. This is what i did to remove all the deflection. My local marina did not allow me to work on the trailer so i had to do this with the boat on the trailer at my house.
1. Adjusted the boat so it would sit flat on the main bunks. Flat 100%. used blocks,wood and carpet along the front keel and raised and lowered trailer jack as i adjusted the front supports. That distributed the weight and removed some of the deflection.
2. Added an additional cross member and support brackets between the rear and center axle. Then jacked up the wood bunk to align the bracket holes (same as the other brackets)
End result deflection removed. Why ? because a 7-1/2" Aluminum I beam in cantilever has very little load capacity. When you add the additional cross member & brackets between the axles you distribute the load at it's strongest point i.e. axles
Best of luck !!