I'm still trying to find out more detail about the plan. The date 1811 is clearly marked, as is a name toward the bottom of the upper right corner paragraph: Lord Berkeley. There's not much I can dig up on any Lord Berkeley that existed during the supposed date on the plan. Of those with the title that existed, only one I can discover that served in the Royal Navy before and after the date, and he became a Sea Lord years later.
I'm afraid it's not much to go on right now, but there's clues to be explored.
Edit: I am mistaken, further research has revealed the service of a Sir George Cranfield Berkeley in the Royal Navy during this period, who happened to be stationed in Portugal during the years preceding the date marked on the plan up until the following year. Wikipedia notes this:
Berkeley continued building his political status during the Peace of Amiens and by Berkeley had been appointed inspector of sea fencibles, a job he undertook with vigour, conducting a fourteen-month survey of Britain's coastal defences, which greatly improved the island's defences. In 1806, after a shift in political power, Berkeley fell out of favour somewhat and was dispatched to the North American Station. From there, Berkeley ordered the attack by HMS Leopard on the American frigate USS Chesapeake in retaliation for American recruitment of British deserters. This action, known as the Chesapeake-Leopard Affair, helped precipitate the War of 1812.
Having embarrassed the British government with this action, Berkeley was recalled home. However, public opinion supported his orders, so Berkeley was moved to command in Lisbon in the hope he could organise the chaotic supply system for Wellington's army in the Peninsula War. Berkeley recognised that only a dedicated and organised convoy system could keep the supply of men, food and material regular and consequently set one up. Simultaneously, he reequipped and galvanised the remnants of the Spanish Navy, rescuing several ships from capture by the French as well as used frigates to supply partisan units all along the coast of Portugal and Northern Spain.
By 1810, Wellington could truthfully say of Berkeley that "His activity is unbounded, the whole range of the business of the Country in which he is stationed, civil, military, political, commercial, even ecclesiastical I believe as well as naval are objects of his attention". He was promoted to full admiral and made Lord High Admiral of the Portuguese Navy by the Portuguese Regent in Brazil. By 1810 he had used sailors to man coastal defences all over Spain, freeing soldiers for Wellington and also formed a squadron of river gunboats to harry French units from major rivers like the Tagus.
I cannot say for certain whether it is this man who is mentioned on the plan, but his presence in Portugal and his work on the Iberian Peninsula could only promote my opinion that this is the "Berkeley" mentioned on the plan for this unknown Portuguese warship.
Further evidence (and close to home for me, I may have a look sometime if I can): https://legacy.lib.utexas.edu/taro/ricewrc/00005/rice-00005.html