Heavy substances loaded by a vessel to improve stability, trim, sea-keeping and to increase the immersion at the propeller. In the days of sail rocks and sand were used. Modern ships use seawater loaded in ballast tanks placed at the bottom of the ship, or in some cases on the sides called wing tanks. Tankers admit ballast water into the cargo tanks to submerge the vessel to a proper trim. When ballast tanks are not connected with the cargo system they are referred to as SEGREGATED BALLAST TANKS.