That big strong ship is perfectly rigid, right? WRONG! Check out this video. It shows that ships do indeed flex under wave loads. Up close, you can’t see it. But this video shows the deflection over the whole length of the ship. Definitely noticeable.
Ship owners: think about this flexing. Remember that your ship does this in small amounts every single day. Ever wonder if that slowly wears down your hull? If your ship is designed to class society rules, the answer is most likely no. Class rules typically account for the lifetime fatigue of the ship when they size your scantlings. But always look out for problems to develop in some trouble areas:
Most of these areas are fine. But anytime you see problems develop, especially if it is a weld crack, ask yourself if fatigue might be a factor. If you have doubts, call your favorite naval architect.