In February 1833, Napier, wishing to have a chance to see combat again, accepted their proposals to become head of the Liberal Fleet and replaced another British officer, Captain George Rose Sartorius, who was already serving as Admiral of Pedro's navy before deciding to rescind his position.

Napier gave himself a nom-de-guerre, 'Carlos de Ponza', in a transparent attempt to disguise his true identity as a British officer when enlisting.

An eccentric but indomitable character, Napier restored order among the fleet, which had been close to mutiny due to lack of pay, and proposed a new strategy to break the siege.

Flying his flag in the frigate Rainha de Portugal, commanded by Captain F.G. MacDonough and with his stepson Charles Elers Napier as Chief of Staff, on 20 June he set sail from Oporto with his small fleet.

After successfully disembarking this force, on the return voyage he encountered the considerably superior fleet of Dom Miguel under the command of Miguelite admiral Manuel Marreiros off Cape St. Vincent on 3 July 1833, and after two days of manoeuvring in calm and very light winds he engaged the Miguelite fleet.

[1][2] As the fighting commenced, it was clear that Napier could not match Marreiros in terms of firepower, and if the battle continued like this his fleet would be defeated in short order.

On 6 July, receiving news of the victory, Dom Pedro named Napier as Viscount Cape St Vincent in the peerage of Portugal.

Napier visited Rear-Admiral Sir William Parker, stationed on the Tagus to protect British interests, and was received according to his Portuguese rank as an Admiral.

On 12 September 1833, he captured the Miguelite The Lord of the Isles steamer at Sao Martinho and in 1836 defended his prize in the Court of Common Pleas.

[1][2] After the final defeat of Miguel and the death of Dom Pedro shortly afterwards, Napier found himself frustrated in his attempts to reform the naval administration of Portugal and returned to England.