Over time, international trade has moved from sailing vessels to much larger engine powered cargo vessels. Each ship is able to carry a greater amount of cargo in a shorter period of time than its predecessors. Safety and efficiency are key for a large vessel moving at significant speed. To obtain the best performance from a boat in terms of steering and propulsion, the vessel's trim must be level. In other words, the ship should not list, or tip, to either side. Ballast water is carried in tanks within the hull of the boat to enhance stability and handling. Water is often taken on in one coastal harbor where a vessel picks up cargo, and emptied in the arrival harbor where the cargo is delivered. Imagine that you are the captain of the vessel pictured. In the next few screens, you will be asked to adjust the trim of the boat by filling or emptying tanks to make it level in the water.