Re: Pad eyes for tethers and jacklines
There are really two major problems I see with what you are suggesting. That really make U bolts unsutable for this.
The first is that a proper pad eye should have at least three, but better four fastening bolts. This allows the loads to be spread over a larger area than is possible with just two.
Secondly a pad eye not only helps spread the force over a larger area in the back, but also helps spread angular loads. As a though experament consider pulling at 90 degrees to the u-bolt, when you do this it is easy to see that the only contact area of the U-bolt is the very small side of the bolt that penetrates the fiberglass regardless of the tensile strength of the bolt itself. Even a large backing plate here doesn't help, since the force is trying to pull the bolt sideways through the glass.
Now if we assume that the backing plate is fixed, and won't pull sideways, we create torque on the bolt. A pad eyes solid front plate helps to distribute this load evenly over the frontal area, while a u-bolt cant.
You suggested adding a plate to the front side, but the problem is unless it is welded to the u-bolt there is too much movement between the bolt and the plate to absorb this side loading.