In the transatlantic slave trade, it is known that captains of the slave ships used to fly false US flags. This was due to the USA not allowing the Royal Navy to search their ships.

"Knowing that many slavers would fly false US flags to avoid being boarded, some slavers were even registered in southern US states." [1]

I believe this was due to the USA not having an good relationship with the British at that time after the American revolutionary war. The slave trade continued until later in the 19th century with the American Civil war.

The British used what they called "squadrons" to patrol the coastlines of Africa in order to help end the transatlantic slave trade. This is known as the Blockade of Africa. Many other countries were involved in this including the US.


How long were these ship captains able to fly false US flags before the US permitted the Royal Navy to search their ships? Was there a way found to somehow identify that a ship was flying a false flag?


  1. Seizure of American Vessels- Slave Trade in Eltis, D. Abolition of the Slave Trade: Suppression
  • 5
    The US had troubles long after the Revolution with the British Navy boarding their ships and impressing their citizens, I have trouble believing a US flag would have afforded any protection until after the War of 1812.
    – T.E.D.
    Commented Sep 4, 2020 at 19:30
  • 2
    Note that if the ships were registered in a US state, then they were not flying a false US flag.
    – jamesqf
    Commented Sep 5, 2020 at 18:16
  • There is a difference between "flying a false US flag", and "falsely flying a US flag"...
    – DJohnM
    Commented Sep 6, 2020 at 2:16
  • @DJohnM: Either way you care to phrase it, if the ships were registered in the US, they are entitled (required?) to fly the US flag. Just as a lot of ships today are registered in Liberia, Panama, and the Marshall Islands: en.wikipedia.org/wiki/Flag_of_convenience
    – jamesqf
    Commented Sep 6, 2020 at 5:53
  • @jamesqf I was picturing flags with maybe 14 stripes, or smiley faces instead of stars...
    – DJohnM
    Commented Sep 6, 2020 at 6:28

1 Answer 1


You have asked two questions here. Part of the answer would appear to be contained in the pdf copy of documents relating to the seizure of American vessels from the New York Public Library site on the Abolition of the Slave Trade (titled 'Doc No 34') that you linked in your question (the first link, currently labelled simply '1').

For the question in your title:

How were false US flags identified on slave ships in the transatlantic slave trade?

The answer is stated simply in the extract of the communication between Mr Stevenson and Mr Webster, dated May 14th, 1841, which appears on pp 11-12 of that document:

"His lordship [Lord Palmerston] then expressed an opinion that the right existed of ascertaining in some way or another, the character of the vessel; and that by her papers, and not the colors or flag which might be displayed."

  • (emphasis appears in the original)

If the ship's papers did not match the flag being displayed, then the vessel was flying a false flag.

The US reluctance to permit such checks is explained in the same document:

"I at once assured him that under no circumstances would the Government of the United States consent to the exercise of the right on the part of any foreign nation, to interrupt, board, or search their vessels on the high seas. That to admit the right of a foreign naval officer to decide upon the genuineness of the papers of American vessels by boarding them, or bringing their captains on board of British cruisers, was in effect allowing the right of search, and therefore utterly indefensible."

It seems that Lord Palmerston disagreed, and had replied that, in the view of Her Majesty's Government:

"... it could not be regarded as a right of search".

So, the US position was that by boarding their vessels to determine whether the ship's papers matched the flag she was flying, the Royal Navy was violating their sovereignty. The British position was that they were merely confirming the nationality by checking the vessel's papers.

That remained the US position until 1862.

Which brings us to your second question:

How long were these ship captains able to fly false US flags before the US permitted the Royal Navy to search their ships?

The answer is until a treaty was agreed between the US and the UK in 1862, fully two decades after the correspondence discussed above.

Permission was granted to the Royal Navy by the United States (and to the the US Navy by the United Kingdom) to visit and search vessels suspected of being involved in the illegal slave trade under the terms of the Treaty between the United States and Great Britain for the Suppression of the Slave Trade, also known as the Lyons–Seward Treaty.

Article 1 of the Lyons–Seward Treaty stated that:

The two high contracting parties mutually consent that those ships of their respective navies which shall be provided with special instructions for that purpose, as hereinafter mentioned, may visit such merchant vessels of the two nations as may, upon reasonable grounds, be suspected of being engaged in the African slave trade, or of having been fitted out for that purpose; or of having, during the voyage on which they are met by the said cruisers, been engaged in the African slave trade, contrary to the provisions of this treaty; and that such cruisers may detain, and send or carry away, such vessels, in order that they may be brought to trial in the manner hereinafter agreed upon.

  • (my emphasis)

From that date, Royal Navy ships had the right to stop and search ships flying the flag of the United States which were suspected of involvement in the illegal slave trade. US Navy ships had similar rights for vessels carrying the UK flag, if they were suspected of involvement in the illegal slave trade..

  • Thank you @sempaiscuba very helpful and detailed answer!
    – Stochastic
    Commented Sep 5, 2020 at 11:46
  • I can post the second part of the question as another question if that is preferred? You are then welcome to give the second part of your answer as the answer there... @sempaiscuba
    – Stochastic
    Commented Sep 5, 2020 at 11:53
  • 1
    @Stochastic No, that's fine. In future, it may be better to post questions separately, perhaps posting links within them to the other question(s) for context, since that's what the SE format prefers, but I think the questions & answer work fine in this case. Commented Sep 5, 2020 at 12:55
  • I would think there was one other case - which would be prior knowledge of the particular ship and getting close enough for a positive identification. An outlier of course, but still there. Commented Sep 5, 2020 at 16:20
  • There is some curiosity about Palmerston's position, when looking at the 1856 Arrow incident leading to the Second Opium War, where the Chinese had arrested the Arrow while it was flying a British flag but with an expired British registration.
    – Henry
    Commented Feb 24, 2023 at 15:41

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