sagging stress
‘Sagging’ is the direct opposite of hogging. When a vessel is supported at bow and stern by wave crests, she will tend to sag in the middle. High buoyancy forces occur at the extremities of the ship. High gravitational forces, from the weight of the ship’s structure, act vertically down about the midship’s point, in opposition to the buoyancy forces. In comparison with the condition of hogging, the vessel has a tendency to bend in the opposite direction .
Incorrect loading of the vessel or design characteristics may accentuate the condition of sagging.Watchkeeping officers should be aware of the frequency of the waves and the likelihood of this condition developing and, if necessary, take action to relieve any sagging or hogging conditions by altering the ship’s course.
Due consideration at the time of loading, with regard to weight distribution may alleviate either hogging or sagging.With shipbuilding producing larger and longer ships either condition is most undesirable, as the prospect of breaking the ship’s back in a heavy seaway or swell becomes a frightening reality. Prudent ballast arrangements, together with increased scantlings at the time of building, coupled with efficient ship and cargo loading, will help minimise any structural damage at a later stage due to hogging or sagging.
