Hi fellows,
1) Wave induced distortion of a hull is called "flexing". Such flexing is not only along the ships for and aft axis up and down, but also twisting to either side at the same time.
2) Hogs and sags are by definition either induced by damage to hulls (ie more/less permanent, since not economic to repair) OR only present for the duration of the voyage or leg of the voyage due to stowage mistakes or neccessitated for cargo separation in separate tanks or holds.
3) Next to a circular sphere a pipe is about the strongest "box girder" man can build pound for pound. Thus a relatively narrow sub hull is like pipe with various appendicies welded to the outside. Further they did not have much hull flare and buoyancy forward (good for quick diving). Thus they were quite literally slicing through head seas - resulting in rather wet rides for the watch. Therefore the pitching motion on them was not really that bad compared to the lateral rolling, which was a bitch due to their low center of gravity, narrow beam etc. Wave action as often as DC caused outer skin panels to dish between frames.
Due to above I seriously doubt that there was much longitutinal flexing in heavy seas.
4) Hogs or sags induced by incorrectly arranged drydocking are extremely rare. Lead for ballast is being used practically on an exclusive basis on yachts nowadays, where improper lifting and blocking is causing damage to boats more often than owners realize till it is to late. Regards, RB