The bolt on hangers are meant for SMALL personal watercraft of a few hundred pounds.
It has angle iron bolt on frames on each side for the springs but the angle iron keeps rusting.
At the front and back of each angle where the U-bolts are, at the top of the frame should be welded metal to prevent the assembly/u-bolts from sliding backwards or forwards. If you are going to use bolt on hangers, with a 1500# load, you really need to do that in front of and behind each U-bolt where it meets the frame. You should not rely entirely on the clamping force of the u-bolts to hold a oscillating 1500# load in place on the road.
Boat trailers tend to have thin wall tubing, the 1/4" or better angle the springs are attached to is meant to distribute the load over a much wider area so you do not pinch or collapse the frame in a single spot. Plus, making it easier to change the center of gravity depending on what boat is being sold.
Many U-bolts are not graded 5 or better and the bolt on hangers shown look like 5/16. If you do decide to buy them, make sure you have at least 4 threads showing beyond the nylon nut. Each hanger is going to be subject to a minimum of 450# on a single ungraded U-bolt (1500# load + 300# trailer) and probably ungraded nut. At the very least I would go to ACE Hardware, get grade 8 alloy washers and grade 8 nyl-nuts.
Just $0.02 from someone that builds and modifies trailers and converted a Rolco (one of the best) tandem axle into an equipment trailer.
