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BOX 2-2Safety Is Paramount
The numerous ballasting requirements applicable to ship safety result in very complex and time-consuming operations for crew members. Ballast operations are performed in a dynamic environment, either at sea or in port, and safety is paramount. The ultimate responsibility for the safety of the ship and its crew always rests with the master of the ship.
(the area under the bow) will emerge periodically from the water surface. This leads to slamming—or heavy impact—of the hull when the bow hits the water with a high velocity on re-entry. Excessive slamming can lead to hull structural damage or even to hull failure and ship loss in extreme conditions. In heavy weather conditions, the ship's master usually chooses to decrease speed, which reduces the rate of occurrence and severity of slamming. Deeper drafts forward will generally reduce the tendency for the ship to slam. Typically, ships ballast to a light-ballast draft in normal weather, then ballast to a deep-ballast draft in heavy weather.
Efficient propeller operation usually requires the propeller to be immersed, even in calm water conditions. Thus, if the stern is not deep enough, ballast may be needed to trim the vessel. Further, if the stern draft is not sufficient in rougher sea conditions, the ship's propeller will race (i.e., increase its revolutions per minute) when it emerges from the water and will slow down when it re-enters the water. This causes engine control problems and increased loading on the propeller shafting and machinery. Increasing stern drafts reduces the tendency for the propeller to emerge and, thus, reduces racing. Designs typically seek to achieve a stern draft in heavy ballast of about 80 percent of the load draft.
Accordingly, safe ship operation in heavy weather requires the addition of ballast to designated cargo holds, ballast holds, or tanks to achieve a heavy-ballast load condition (storm ballast).
Sailing with Full Tanks
Ballast tanks used for controlling trim or heel, some fuel oil tanks, and tanks containing fresh water for domestic use, may be partially full at sea, depending on the stability and strength requirements of the ship. It is usually necessary to sail with as many as possible of the tanks on board either completely full or entirely empty. When a tank is not completely full (i.e., "slack"), and the ship heels, the free surface effect of the liquid moves the center of gravity of the liquid in the tank, thus reducing the transverse stability of the ship (see Appendix C). In