Hovercraft are usually powered by gas turbines or diesel engines driving propellers and impellers. A centrifugal pump containing two or more impellers is called a multistage centrifugal pump. Low-specific speed radial flow impellers develop hydraulic head principally through centrifugal force. Some impellers are similar to small propellers but without the large blades. In man-made objects, it can occur in control valves, pumps, propellers and impellers. The impellers may be mounted on the same shaft or on different shafts. Another application of radial flow impellers are the mixing of very viscous fluids. Top-loading machines in Asia use impellers instead of agitators. For small pumps and those dealing with suspended solids, open impellers are generally used. These fans increase the speed and volume of an air stream with the rotating impellers. Among the most common are impellers and mixers. Most modern high efficiency impellers use "backsweep" in the blade shape. These impellers can't perform effectively with solids and become difficult to clean if clogged. Both Figures 0.1 and 3.1 show open impellers with splitters. Vessels using impellers are normally steered by changing the direction of the water jet.