Some vessels, particularly containerships, need to control trim and heel carefully during cargo loading and discharging so that the cargo operations can proceed both safely and efficiently. Some vessels have computerized heel control systems that move ballast between heeling tanks to maintain the vessel within a set tolerance of vertical. Roll-on, roll-off vessels have restrictive draft limits for the use of their ramps. Both the cargo weight distribution at the various intermediate stages of loading and wind heel effects on a light vessel can heel the vessel to the extent that containers will not move in their cell guides.
When ballasting a ship for a voyage, the crew, under the direction of the master, defines the amount of water required taking into account the loaded condition, route, predicted weather conditions, and the need to complete the voyage in a safe and efficient manner. In the future there may be some possibility of modifying ballast systems in new ship designs for facilitate cleaning and improve the safety of changing ballast at sea, but the complete elimination of ballast is not currently practicable (see Appendix D).
Ballast water is taken on board using sea chests with ballast pumps or by gravity feed. Sea chests can be located under the ship, on the turn of the bilge, or on the ship's side and are usually replicated on both sides of the vessel (see Figure 2-2). The ballast system usually works in reverse during deballasting, with the water passing through an overboard discharge valve located on the side of the ship's hull. Ballast water loading and discharging operations are usually controlled from a central ballast monitoring/control station and ballast water can be gravitated in or out of a particular tank or hold, pumped in or out, or a combination of these methods can be used. Ballast pumps remove most of the ballast water. In some cases, separate stripping pumps further reduce the amount of water remaining in the tanks. Trimming of the ship by the stern may also be used to aid ballast removal. Despite these efforts, some ballast water and sediment will always remain on board.
The ballast intake is covered with a grate or a strainer plate with small holes, and inboard of the sea chest there is usually a suction strainer. The primary purpose of grates and strainers is to protect the pumping system from foreign objects being drawn in. In some poorly maintained vessels, the integrity of grates and strainers can be compromised, allowing larger organisms to enter the ballast system. The additional use of portable screens to prevent intake of unwanted