Modern commercial vessels are typically powered by a single propeller driven by a diesel or, less usually, gas turbine engine., but until the mid-19th century they were predominantly square sail rigged. The fastest vessels may use pump-jet engines. Most commercial vessels have full hull-forms to maximize cargo capacity. Hulls are usually made of steel, although aluminum can be used on faster craft, and fiberglass on the smallest service vessels. Commercial vessels generally have a crew headed by a sea captain, with deck officers and engine officers on larger vessels. Special-purpose vessels often have specialized crew if necessary, for example scientists aboard research vessels.