When I bought zzZippety Doo Dah, my Spirit 23 sailboat, it came with a double axle trailer. Unfortunately the previous owner did not take care of the trailer and it was in such pitiful condition when I picked it up that it could not even support the weight of a person. The frame was made of steel and incredibly rotten. The hitch took a 1 7/16^th inch ball, which seemed odd to me. Turns out the hitch was actually 2 inch but had so much rust and junk in it that the 2 inch ball did not fit. A friend does an amazing job welding and replaced all the bad steel. It was a big job. Check out these before and after pictures.

Original Trailer

Rebuilt Trailer

And as an example of how much steel required replacement, in the background of the above picture you can see some of the original steel. Here is a closer image.

Replaced Steel