the bridge. In the late 18th century, steam engines became more efficient, spars were removed and screw propellers became standard. Large steam yachts were luxurious; their staff included a captain, engineer, and stewards, as well as deck hands. The development of the steam engine figured in yacht design. In England, the practice was to use fire-tube boilers wherein the hot gases of combustion ran through parallel tubes surrounded by water; a design that required a slow process of heating the water to avoid damaging the boiler before making steam. In the United States, the practice was to use water-tube boilers, wherein the water is surrounded by the hot gases of combustion, allowing for making steam more rapidly. While the boiler was the source of steam, the marine steam engine is where the steam was converted to mechanical power. With smaller, more rapidly spinning propellers engines comprised cylinders with pistons, connected to a crank shaft, driven by steam. Near the end of the 19th century, compound engines came into widespread use. Compound engines exhausted steam into successively larger cylinders to accommodate the higher volumes at reduced pressures, giving improved efficiency. These stages were called expansions, with double- and triple-expansion engines being common, especially in shipping where efficiency was important to reduce the weight of coal carried. Steam engines remained the dominant source of power until the early 20th century, when advances in the design of the steam turbine, electric motors and internal combustion engines gradually resulted in the replacement of reciprocating (piston) steam engines. Internal combustion Nicolaus Otto and Gottlieb Daimler developed practical four-stroke gasoline engines, starting in 1876. Beginning in 1898 engines increased in horsepower from to by 1906. Some were destined for speedboats, other for motor yachts. Diesel power plants for boats were demonstrated in 1903. Diesels became a more prevalent type of power plant in the 20th century thanks to their low cost of operation and reliability. Classification The Recreational Craft Directive requires that all vessels sold in the European Union and United Kingdom satisfy one of four design categories, based on the wind force and seas that they are designed to encounter: yachts are fit for conditions that exceed wind force 8——and maximum wave heights, encountered in ocean passages and extended voyages. yachts are fit for conditions that are less than wind force 8——and maximum wave heights, encountered in ocean passages and extended voyages. yachts are fit