addition, fluid in a slack tank sloshes around during ship motion, which may lead to excessive loads on the tank/hold bulkheads, frames, or underdeck structure. In severe weather conditions, this could lead to structural failure. Thus, during ballast change at sea, the ballast in a single tank or pair of tanks should be completely changed before proceeding to the next tank or tanks.
As fuel is consumed during a voyage, the draft and trim of the ship will change. During a long voyage, thousands of tons of fuel may be used. Thus, to keep the hull immersed correctly for maximum efficiency, it is often necessary to take on additional ballast as the voyage progresses. Some ship designs place the fuel tanks so that the ship naturally trims by the stern as fuel oil is consumed, but ballasting may still be required. Ballast capacity and location during a given voyage are established by examining the estimate of fuel to be consumed, weather conditions expected, and the required draft and trim for the arrival port(s).
The transverse stability of a ship is defined as its ability to sail upright and to resist capsizing. To attain proper transverse stability requires careful control of the righting moment of the ship (see Appendix C). Ideally the ship should be loaded and/or ballasted in such a way as to give it an easy rolling period that is neither too fast nor too slow. A vessel that rolls too fast has excess stability (a stiff ship) and has a marked tendency to return to its original upright position quickly. This creates an extremely uncomfortable motion that can exert high loads on the ship's structure and cargo lashings and high sloshing loads in slack tanks. A vessel that rolls too slowly has insufficient stability (a tender ship) and may capsize under heavy weather conditions. When ballast is moved, it creates a condition of slack ballast tanks. The associated free surface effect can lead to a weight shift when the vessel heels that adversely affects the transverse stability of the ship. A more detailed discussion of stability issues is provided in Appendix C.
Ballast operations are carried out in port to maintain ship stability, as discussed above. Ballast operations in port also maintain both the clearance under cargo loading or cargo discharge facilities and the under-keel clearance so the vessel remains safely afloat; maintain the hull bending moments and shear forces within safe limits to avoid the catastrophic damage that can result from incorrect loading; and maintain the ship upright by trimming or heeling the ship. In addition, ballast operations in port establish the efficient ballast condition for the pending voyage.