A yacht is a sail or power vessel used for pleasure, cruising, or racing. There is no standard definition, so the term applies to such vessels that have a cabin with amenities that accommodate overnight use. To be termed a yacht, as opposed to a boat, such a pleasure vessel is likely to be at least 33 feet (10 m) in length and may have been judged to have good aesthetic qualities.
The Commercial Yacht Code classifies yachts 79 ft (24 m) and over as large. Such yachts typically require a hired crew and have higher construction standards. Further classifications for large yachts are: commercial--carrying no more than 12 passengers, private--solely for the pleasure of the owner and guests, or by flag, the country under which it is registered. A superyacht (sometimes megayacht) generally refers to any yacht (sail or power) longer than 131 ft (40 m).
Racing yachts are designed to emphasize performance over comfort. Charter yachts are run as a business for profit. As of 2020 there were more than 15,000 yachts of sufficient size to require a professional crew.
The term, yacht, originates from the Dutch word jacht (pl. jachten, which means "hunt"), and originally referred to light, fast sailing vessels that the Dutch Republic navy used to pursue pirates and other transgressors around and into the shallow waters of the Low Countries.
The history of pleasure boats begins with rowed craft in Pharaonic Egyptian times. Pleasure vessels in the waters of Burma, Egypt, India, Mindanao and Japan. It described 8th-century Anglo-Saxon royal pleg-scips (play ships) that featured ornamented bows and sterns and which had the capability of cooking on board.
The history of sailing yachts begins in Europe in the beginning of the 1600s with the building of a pleasure vessel for the son of King James I of England. While other monarchs used naval ships for transportation and conquest, James I was the first English monarch to commission the construction of a yacht--for his son Henry, Prince of Wales in the early 1600s. Pleasure vessels acquired the name yacht after the time of Charles II, who spent time exiled in Europe and visited the Netherlands, where a variety of jachten were already well developed as pleasure boats for the elite classes since the beginning of the 17th century. Upon his restoration to the English crown, Charles commissioned a series of royal yachts, which included at least one experimental catamaran. The first recorded yacht race between two vessels occurred in 1661, followed by the first open sailing competition in 1663 in English waters.
Starting in 1739, England found itself in a series of wars--a period that saw a decline in yachting. In Ireland, however, the gentry enjoyed yachting and founded the first yacht club in Cork as the Cork Harbor Water Club in 1720. English yacht racing continued among the English gentry who founded England's oldest yacht club in 1775 to support a fleet at Cumberland. With maritime peace, starting in 1815, came a resurgence of interest in yachting. Boatbuilders, who had been making fast vessels both for smugglers and the government revenue cutters, turned their skills again to yachts.
The fast yachts of the early 19th century were fore-and-aft luggers, schooners, and sloops. By the 1850s, yachts featured large sail areas, a narrow beam, and a deeper draft than was customary until then. Racing between yachts owned by wealthy patrons was common, with large wagers at stake. The America's Cup arose out of a contest between the yacht, America, and its English competitors. Both countries had rules by which to rate yachts, the English by tonnage and the American by length.
In the late 19th century, a yacht owner would base his choice of vessels upon preferred lifestyle and budget, which would determine the size and type of vessel, which would most likely be a fore-and aft, two-masted sailing vessel. A treatise on the subject, A Manual of Yacht and Boat Sailing, provided detailed information on selecting, equipping, sailing, seamanship, management of the paid crew, and racing such vessels. It included a brief section on steam yachts, the recirculating coil steam engine just having made such yachts efficient enough for leisure travel on the water.
While sailing yachts continued to exist, the development of reliable power plants created a new category of pleasure craft. The power plants started with the steam engine and transitioned to the internal combustion engine. Whereas sailing yachts continued to be steered from the after portion of the vessel, power yachts adopted the bridge in a forward cabin structure that afforded better forward and sideways visibility.
The history of steam yachts starts with large sailing yachts with a steam auxiliary engine. Early examples, driven with paddle wheels, had a railed platform from which the person conning the vessel could walk across the vessel above the main deck, the origin of the bridge. In the late 18th century, steam engines became more efficient, spars were removed and a screw propellors 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 as 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 propellors 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.
Nicolaus Otto and Gottlieb Daimler developed practical four-stroke gasoline engines, starting 1876. Beginning in 1898 engines increased in horsepower from 25 horsepower (19 kW) to 500 brake horsepower (370 kW) 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.
The Large Commercial Yacht Code (LY2) of Great Britain and its dominions defines a large yacht as one that is 24 metres (79 ft) or more at the waterline and is in commercial use for sport or pleasure, while not carrying cargo or more than 12 passengers and carrying a professional crew. The code regulates the equipping of such vessels, both at sea and in port--including such matters as crew duty times and the presence of a helicopter on board. The code has different levels of standard for vessels above and below 500 gross tons. Such yachts may be considered superyachts and are more commonly at 40 metres (130 ft) or more in length. Other countries have standards similar to LY2.
Whereas commercial large yachts may carry no more than 12 passengers, private yachts are solely for the pleasure of the owner and guests do not carry the passenger restriction. Yachts may be identified by flag--the country under which a yacht is registered. An industry publication categorizes superyachts by size, by speed, as "explorer" yachts, as sailing yachts, and classic yachts.
Originally, all yachts were made of wood, using a wooden keel and ribs, clad with planks. These materials were supplanted with iron or steel in steam yachts. In the 1960s fiberglass became a prevalent material. These materials and others continue in use. Whereas yachts of 79 feet (24 m) and below may be constructed of fiberglass, larger yachts are more likely to be constructed of steel, aluminum or composite fiber-reinforced plastic.
Depending on size, a cruising yacht is likely to have at least two cabins, a main salon and a forward stateroom. In smaller yachts, the salon is likely to have convertible berths for its crew or passengers. Typically the salon includes a dining area, which may have a folding, built-in table. The salon is typically contiguous to the galley. A cruising yacht is likely to have a head (bathroom) with a marine toilet that discharges waste into a holding tank. Larger yachts may have additional staterooms and heads. There is typically a navigation station that allows planning the route. 
Onboard systems include:
Modern yachts employ a suite of electronics for communication, measurement of surroundings, and navigation.
Sailing yachts for cruising versus racing embody different tradeoffs between comfort and performance. Cruising yachts emphasize comfort over performance. Racing yachts are designed to compete against others in their class, while providing adequate comfort to their crews.
Cruising yachts may be designed for near-shore use or for passage-making. They may also be raced, but they are designed and built with the comfort and amenities necessary for overnight voyages. Qualities considered in cruising yachts include: performance, comfort under way, ease of handling, stability, living comfort, durability, ease of maintenance, affordability of ownership.
Cruising sailboats share the common attribute of providing overnight accommodations. They may be classified as small (easy to haul behind a trailer),near-shore and off-shore.Multihull sailing yachts are a category, apart.
Design considerations for a cruising yacht include seaworthiness, performance, sea kindliness, and cost of construction, as follows:
Multihulls offer tradeoffs as cruising sailboats, compared with monohulls. They may be catamarans or trimarans. They rely on form stability--having separate hulls far apart--for their resistance to capsize. Their advantages include greater: stability, speed, (for catamarans) living space, and shallower draft. Their drawbacks include: greater expenses, greater windage, more difficult tacking under sail, less load capacity, and more maneuvering room required because of their broad beam. They come with a variety of sleeping accommodations and (for catamarans) bridge-deck configurations. 
Gaff rigs have been uncommon in the construction of cruising boats, since the mid 20th century. More common rigs are Bermuda, fractional, cutter, and ketch. Occasionally employed rigs since then have been the yawl, schooner, wishbone, catboat.
Sailboats employ standing rigging to support the rig, running rigging to raise and adjust sails, cleats to secure lines, winches to work the sheets, and more than one anchor to secure the boat in harbor. A cruising yacht's deck usually has safety line to protect the crew from falling overboard and a bow pulpit to facilitate handling the jib and the anchor. In temperate climates, the cockpit may have a canvas windshield with see-through panels, called a "dodger". Steering may be either by tiller or wheel.
Cruising yachts have an auxiliary propulsion power unit to supplement the use of sails. Such power is inboard on the vessel and diesel, except for the smallest cruising boats, which may have an outboard gasoline motor. A 31-foot (9.4 m) sailboat might have a 13-horsepower (9.7 kW) engine, whereas a 55-foot (17 m) sailboat might have a 110-horsepower (82 kW) engine.
High-performance rigs provide aerodynamic efficiency and hydrodynamically efficient hulls minimize drag through the water and sideways motion.
Racing yachts have a wide selection of weights and shapes of sail to accommodate different wind strengths and points of sail. A suite of sails on a racing yachts would include several weights of jib and spinnaker, plus a specialized storm jib and trysail (in place of the mainsail). Performance yachts are likely to have full-battened kevlar or carbon-fiber mainsails.
Underwater foils can become more specialized, starting with a higher-aspect ratio fin keel with hydrodynamically efficient bulbs for ballast. On some racing yachts, a canting keel shifts angle from side to side to promote sailing with less heeling angle (sideway tilt), while other underwater foils take care of leeway (sideways motion).
Motor yachts range in length from 33-130 feet (10-40 m) before they are considered super-yachts or mega-yachts, which are 130 feet (40 m) and longer. They also vary by use, by style, and by hull type. As of April 2020 a 600-foot (180 m) yacht, REV Ocean, was under construction, which when launched would replace the 590-foot (180 m) Azzam as the longest superyacht. As superyachts have grown size, the distinction between a yacht and a ship (perhaps converted for personal use) has become unclear. A proposed definition for calling a vessel a yacht rather than a ship would if it was constructed solely for personal use and has a combined occupancy of less than 100, including crew.
A motor yacht's style can both be functional and evoke an aesthetic--trending towards the modern or the traditional. Among the styles, mentioned in the literature, are:
There are three basic types of motor yacht hull: full-displacement, semi-displacement, and planing, which have progressively higher cruise speeds and hourly fuel consumption with increased engine power:
A typical semi-displacement yacht has a wedge-shaped bow, which promotes penetrating waves, that transitions to flatter, wider surfaces aft, which promotes lifting the vessel out of the water--the "deep vee" hull, designed by Ray Hunt, found in approximately 75% of modern power boats.
Cruising motor yachts are available in a range of styles as two-engine catamarans, ranging in length from 40-150 feet (12-46 m) with top speeds ranging from 20-60 knots (37-111 km/h).
Motor yachts typically have one or more diesel engines. Gasoline-powered motors and engines are the provenance of outboard motors and racing boats, due to their power-to-weight ratios. Two engines adds expense, but provides reliability and maneuverability over a single engine.
Motor yachts in the 42-foot (13 m) range might have the following hull, horsepower, cruise speed, and hourly fuel consumption characteristics:
any of various recreational watercraft: such as a) a sailboat used for racing b) a large usually motor-driven craft used for pleasure cruising
a boat with sails and sometimes an engine, used for either racing or traveling on for pleasure
a large boat with sails or a motor, used for racing or pleasure trips