Henry I Sinclair, Earl of Orkney

Monument to the alleged landing site of the Sinclair Expedition, Guysborough, Nova Scotia

One of the most common theories about Sinclair is that he was one of the first Europeans to visit North America in a voyage pre-dating Columbus. In 1784, he was identified by Johann Reinhold Forster[14] as possibly being the Prince Zichmni described in letters allegedly written around the year 1400 by the Zeno brothers of Venice, in which they describe a voyage throughout the North Atlantic under the command of Zichmni.[15]

The letters and the accompanying map, allegedly rediscovered and published in the early 16th century, are regarded by most historians as a hoax by the Zenos or their publishers.[15] Moreover, the identification of Zichmni as Henry Sinclair has not been accepted by most historians, although this identification and their overall authenticity are taken for granted by the supporters of the theory.

The claim that Henry Sinclair explored North America has been popularised by several other authors, notably by Frederick J. Pohl,[16] Andrew Sinclair,[17] Michael Bradley,[18] William S. Crooker (who claimed to have discovered Henry Sinclair's castle in Nova Scotia),[19] Steven Sora,[20] and more recently by David Goudsward.[21]

The claim requires the acceptance not only that the letters and map ascribed to the Zeno brothers and published in 1558 are authentic, but that the voyage described in the letters as taken by Zichmni around the year 1398 to Greenland actually reached North America and that Zichmni is Henry Sinclair. It is also bolstered by claims that carvings in Rosslyn Chapel represent American plants.

The name "Zichmni" is either totally fictitious, or quite possibly a transliteration error when converting from handwritten materials to type. Forster tried to relate this to the name "Sinclair",[14] while to Pohl "Prince Zichmni" was a misread rendering of the title "Prince of Orkney", which he attributed to Sinclair.[16]

One primary criticism of this theory is that if either a Sinclair or a Templar voyage reached the Americas, they did not, unlike Columbus, return with a historical record of their findings. In fact, there is no known published documentation from that era to support the theory that such a voyage took place. The physical evidence relies on speculative reasoning to support the theory, and all of it can be interpreted in other ways. For example, according to one historian, the carvings in Rosslyn Chapel may not be of American plants at all but are nothing more than stylized carvings of wheat and strawberries.[22]

Intertwined with the Sinclair voyage story is the claim that Henry Sinclair was a Knight Templar and that the voyage either was sponsored by or conducted on the behalf of the Templars, though the order was suppressed almost half a century before Henry's lifetime.

Knight and Lomas speculate that the Knights Templar discovered under the Temple Mount in Jerusalem a royal archive dating from King Solomon's times that stated that Phoenicians from Tyre voyaged to a westerly continent following a star called "La Merika" named after the Nasoraean Mandaean morning star.[23]^: 76–77 According to Knight and Lomas, the Templars learned that to sail to that continent, they had to follow a star by the same name. Sinclair supposedly followed this route.[24]

The theory also makes use of the supposed Templar connection to explain the name Nova Scotia ("New Scotland" in Latin). It is based on the 18th-century tale that some Templars escaped the suppression of their order by fleeing to Scotland during the reign of Robert the Bruce[25] and fought in the Battle of Bannockburn.[26]

Claims persist that Rosslyn Chapel contains Templar imagery. Andrew Sinclair speculates that the grave slab now in the crypt is that of a Templar knight:[27] According to author Robert Lomas, the chapel also has an engraving depicting a knight templar holding the sword over a head of an initiate, supposedly to protect the secrets of the templars.[28] Rosslyn Chapel was built by Sir William St Clair, last St Clair Earl of Orkney, who was the grandson of Henry. According to Lomas, Sir William, the chapel builder, is also the direct ancestor of the first Grand Master of Masons of Scotland, also named William St Clair (Sinclair).[28]

According to Lomas, the Sinclairs and their French relatives, the St. Clairs, were instrumental in creating the Knights Templar. He claims that the founder of Templars Hugues de Payens was married to a sister of the Duke of Champaine (Henri de St. Clair),[29] who was a powerful broker of the first Crusade and had the political power to nominate the Pope, and to suggest the idea and empower it to the Pope.

However, a biography of Hughes de Payens by Thierry Leroy identifies his wife and the mother of his children as Elizabeth de Chappes. The book draws its information on the marriage from local church cartularies dealing chiefly with the disposition of the Grand Master's properties, the earliest alluding to Elizabeth as his wife in 1113, and others spanning de Payens’ lifetime, the period following his death and lastly her own death in 1170.[30]

Historians Mark Oxbrow, Ian Robertson,[31] Karen Ralls and Louise Yeoman[32] have each made it clear that the Sinclair family had no connection with the mediaeval Knights Templar. Karen Ralls has shown that among those testifying against the Templars at their 1309 trial were Henry and William Sinclair – an act inconsistent with any alleged support or membership.[33][34]