Stringer question

[Steplift 72']

I was on an old 21' Cobia that broke both main stringers offshore in big seas, the boat began flexing terribly. The floor of the boat was visibly humping up several inches as we crossed waves. Both stringers broke about 5' from the transom, an area that apparently gets more stress than I'd imagined. If you choose to replace only part of a stringer, be sure to properly grind, fasten and bond an adequate length "sister" stringer to join the new and old without creating any hard points to the bottom.