On 2 August 1379, on the Swedish Island of Marstrand, he was installed as Earl of Orkney. As first Sinclair to be Earl of Orkney he had the right to various royal privileges, including wearing a crown. Although he held Orkney from the King of Norway, he was also a Scottish noble. 
However, he was in effect all but an independent king. Tradition has it that, in 1398, he set sail from Orkney and Shetland with an armada of twelve ships. After a journey of some months, he arrived in the New World at Prince Edward Island, Nova Scotia, nearly one hundred years before Columbus. 

Source: Leo van de Pas