A royal yacht is a ship used by a monarch or a royal family. If the monarch is an emperor the proper term is imperial yacht. Most of them are financed by the government of the country of which the monarch is head. The royal yacht is most often manned by personnel from the navy and used by the monarch and his/her family on both private and official travels.
Some royal yachts have been/are small vessels only used for short trips on rivers or in calm waters, but others have been/are large seaworthy ships.
Depending on how the term is defined royal yachts date back to the days of antiquity with royal barges on the Nile in ancient Egypt.
Later the Vikings produced royal vessels. They followed the pattern of longships although highly decorated and fitted with purple sails (purple sails remained standard for royal vessels the next 400 years).
In England, Henry V sold off the royal yachts to clear the Crown's debts. The next royal vessels in England were built in the Tudor period with Henry VIII using a vessel in 1520 that was depicted as having cloth of gold sails.James I had the Disdain, a ship in miniature (she was later recorded as being able to carry about 30 tons), built for his son Prince Henry. The Disdain was significant in that she allowed for pleasure cruising and as a result can be seen as an early move away from royal ships as warships.
The first ships to unquestionably qualify as royal yachts were those owned by Charles II of England Scotland and Ireland. The first was gift from the Dutch but later yachts were commissioned and built in England. This established a tradition of royal yachts in Britain that was later copied by other royal families of Europe. Through the 19th century royal yachts got larger as they became a symbol of national wealth. World War I brought this trend to an end and the royal families that survived found it harder to justify the cost with the result that there are only three royal yachts left in use in Europe. For the most part royal yachts have been superseded by the use of warships in this role, as royal yachts are often seen as a hard-to-justify expenditure. In addition most monarchies with a railway system employ a special set of royal carriages. Most monarchies also employ aircraft as a luxurious (and much more speedy and timely) mode of transportation.