Hogging - is the straining of a ship lightly loaded amidships and more heavily forward and aft, causing the bow and stern to be lower than the middle section.
Sagging - is the straining of a ship heavily loaded amidships and more lightly forward and aft, causing the bow and stern to be higher than the middle section.
Hogging and sagging can be tolerated, but only within limits.
When they exceed acceptable limits, hogging or sagging will cause the hull to crack in the area of greatest stress.
In a improper loading of a vessel, a hogged or sagged condition can be caused by a trochoidal wave action, a action of a wave whose distance between crests is equal to the length of the ship and whose height is 1/20 of its length. When a vessel encounters a wave of that size, it will be hogged when the midsection of the vessel passes over a crest and sagged when the bow and stern sections are supported by crests.
In determining whether a vessel is hogged or sagged it is helpful to clear mental uncertainties if look at draft as a "theoretical" draft and load (midships) draft as an "actual" draft.
