El Primero was a steam yacht built in 1893.
 This vessel was once considered one of the most luxurious yachts on the West Coast of the United States, and was one of the few steam yachts to be operated on Puget Sound.
 The yacht has since been converted to diesel, but it remained operational as of 2010.
Azzam (Arab.
 "resolute") is a private yacht built by Lürssen Yachts.
 "Azzam" was launched on 5 April 2013 at 180 m in length as the largest private motor yacht in the world.
 It has a beam of 20.8 m (68'4") and an unusually shallow draft of 4.3 m (14'1").
Heidi Horten (née Jelinek; born 13 February 1941) is an Austrian billionaire, and the widow of businessman Helmut Horten.
 In November 2015 "Forbes" estimated her net worth at US $2.6 billion.
MY "Vava II" is the 97-metre superyacht commissioned by Swiss entrepreneur Ernesto Bertarelli.
 Built by Devonport Yachts at Plymouth, England, the hull was built by sister shipyard Appledore Shipbuilders then taken to Devonport for the accommodation section to be lifted to ship and then launched in 2009.
 When constructed she was claimed to be the largest British-built superyacht.
 She was the last yacht built by Devonport Yachts following its acquisition by Pendennis Shipyard.
Skat is a luxury yacht built by Lürssen of Bremen, Germany as project 9906, a number prominently displayed on the hull in a typeface matching that of military vessels.
 The project started in November 1999 and the yacht launched in 2001.
 The owner is Charles Simonyi, a former Software Engineer from Microsoft and the fifth space tourist.
 The yacht is 71 m long.
SV Nilaya is a 16-metre long steel yacht built in Auckland, New Zealand in 1997.
 The vessel is a custom steel pilothouse yacht built to superyacht standards.
 "SV Nilaya's" systems are 'works of art' and were designed for ease of maintenance and operation.
 In 2012, it was believed to be sailing towards Antarctica crewed by Jarle Andhøy and Samuel Massie, among others.
 Their intention appears to be to investigate the disappearance of their yacht "Berserk II", which sank in February 2011 killing three people.
 New Zealand authorities are searching for the yacht, as there is no authority issued for the journey to Antarctica.
 The vessel was reregisterd under a Russian flag.
 It left Auckland New Zealand on 23 January 2012.
 new Zealand Customs attempted a search.
An armed yacht was a yacht that was armed with weapons and was typically in the service of a navy.
 Their speed and maneuverability made them useful as patrol vessels.
 In the United States Navy armed yachts were typically private yachts expropriated for government use in times of war.
 Armed yachts served as patrol vessels during the Spanish–American War and the World Wars.
 In the latter conflicts, armed yachts were used as patrol vessels, convoy escorts, and in anti-submarine duties.
 In the United States, yachts were purchased from their owners with the owners given an option to repurchase their yacht at the close of hostilities.
The yacht Carinthia VII was built in the Lürssen Yachts yard in 2002 and refurbished in the same yard three years later.
 One of the largest motor yachts in the world, it is owned by Heidi Horten, widow of the German entrepreneur Helmut Horten.
Ace is a 87 m yacht built by Lürssen.
 She was delivered to her owner in July 2012.
 The yacht accommodates 10 guests and 28 crew members.
 Her exterior and interior were both created by Andrew Winch Designs.
 One of ACE’s interior features is the spa.
 The spa includes a hammam, Russian Banyan, massage room, plunge pool and a Jacuzzi.
 Other features are a beach club, gym, movie theater, toys & tenders and helicopter landing capabilities.
 The yacht Ace is available for charter.
Ona, (formerly "Dilbar"), is a motor yacht built in 2008 by Lürssen.
 She is owned by Russian oligarch Alisher Usmanov and is named after his mother.
 With an overall length of 110.00 m and a beam of 16.00 m she is the 28th largest yacht in the world.

Question: What Australian billionaire owns a yacht built in the Lürssen Yachts yard?
Answer: Heidi Horten