A tugboat is a boat designed to push or tow. It has a great deal of power and is versatile, able to push or tow something much bigger than itself and to go backward or sideways almost as well as forward. Its screw propellers are designed for thrust, and the propellers are often contained in shrouds to increase thrust and improve the tug's ability to change direction. The bottom of the hull slopes up for good flow of water to the propellers. Tugboats are often fitted with power winches, fire control equipment, and salvage and rescue equipment. Some tugboats working the Coos Bay waterways since the 1930s are still in service, while new tugs continue to be designed and built.