HMS Polyphemus 

https://en.wikipedia.org/wiki/HMS_Polyphemus_%281782%29#/media/File:Gezicht_van_de_zeeslag_bij_Copenhagen,_1801,_RP-P-OB-73.109.jpg

‘My dear Sir, Since leaving England, this is my first opportunity to send you a letter. It will sail with HMS Polyphemus, a 64-gun warship sent by the government to protect us from the French. After convoying us safely to this latitude the Polyphemus will return to England.’ This is a quote from The Claudians: gardens, landscapes, reason and faith: John Claudius Loudon and Claudius Buchanan, Tom Turner (Kindle, 2024).

More about HMS Polyphemus

 Launched in 1782, HMS Polyphemus was a 64-gun third-rate ship of the line. She participated in several notable battles, including the Battle of Copenhagen (1801), the Battle of Trafalgar (1805), and the Siege of Santo Domingo (1809). Ships of the line were designed for line-of-battle tactics. This involved two opposing fleets of ships forming lines and exchanging broadsides, firing all guns on one side at close range. 

Passengers or crew members could entrust letters to the captain or ship's officers, who would deliver them upon reaching port. East India Company ships often followed established routes and had designated meeting points along their journeys. For example, the Cape of Good Hope was a common stopping point for ships travelling between Europe and India, allowing the exchange of news and letters between passing vessels.

Though Buchanan had little contact with HMS Polyphemus, the history of this 18th-century warship is helpful in understanding the military context of Buchanan's voyage to India. His ship, the Busbridge, was at risk of attack by French warships. The Polyphemus was involved in the Battle of Copenhagen and the Battle of Trafalgar. Though also involved in convoy protection it was a powerful ship of the line.