Armed merchantman is a term that has come to mean a merchant ship equipped with guns, usually for defensive purposes, either by design or after the fact. In the days of sail, piracy and privateers, many merchantmen would be routinely armed, especially those engaging in long distance and high value trade. The most famous of this type were the East Indiamen able to defeat regular warships in battle (see Battle of Pulo Aura).
In more modern times, auxiliary cruisers were used offensively to disrupt trade chiefly during both World War I and World War II, particularly by Germany.
East Indiamen of various European countries were heavily armed for their long journeys to the Far East. In particularly dangerous times, such as when the home countries were at war, a convoy system would be used whereby the ships were escorted by a warship. However, many East Indiamen also travelled on their own, and therefore were heavily armed in order to defend themselves against pirates and privateers. They also defended themselves against warships, scoring signal victories at the Battle of Pulo Aura and the Action of 4 August 1800. The British Royal Navy purchased several that it converted to ships of the line.
In 1856, privateering (or seizure of a belligerent country's merchant ships as a private enterprise) lost international sanction under the Declaration of Paris. From 1861-65 European countries built high speed ships to run the Union Blockade during the American Civil War. Some of these were armed and served as Confederate States Navy raiders.
Russia purchased three ships in 1877 of 6,000 long tons (6,100 t) armed with 6-inch (150 mm) guns for use as auxiliary cruisers for a Russian Volunteer Fleet. Germany and the United Kingdom responded to the precedent by asking their shipping companies to design fast steamers with provision for mounting guns in time of war.
- 1890 - German and British shipyards built new civilian ships designed for wartime conversion, and France, Italy, Japan, Austria-Hungary, and the United States made similar agreements with their shipyards.
Russia built two more auxiliary cruisers in 1892.
In 1895 the Imperial German Navy mobilized the provisional auxiliary cruiser Normannia for a 15-day trial armed with eight 6-inch guns, two 3.5-inch (89 mm) guns, six 37-millimetre (1.46 in) guns, and two torpedo boats.
In both World Wars both Germany and the United Kingdom used auxiliary cruisers. Whilst the British used armed passenger liners defensively for protecting their shipping, the German approach was to use them offensively to attack enemy shipping.
Armed merchant cruisers
The Armed Merchant Cruisers (AMC) of the British Royal Navy were employed for convoy protection against enemy warships. They ultimately proved to have limited value and many (particularly liners) were later converted into troopships, a role for which they were more suited. Documentary evidence quoted by the BBC researched from the early stages of the First World War suggests that the express liners had greater speed than most warships- few warships of the period could exceed 21 knots -which made them suitable as AMC's. The downside proved to be their high fuel consumption, and that using them in a purely AMC role would have burned through the Admiralty reserve supplies of steam coal in less than three months. The ships were vulnerable to enemy fire because they lacked warship armour and they used local control of guns rather than director fire-control systems, which reduced their effective fire power.
One famous AMC of World War I was the British RMS Carmania which, after a heated battle that caused heavy damage on both sides, succeeded in sinking the German auxiliary cruiser SMS Cap Trafalgar near the Brazilian island of Trinidade in 1914. By coincidence, Cap Trafalgar was disguised as Carmania.
In World War II, HMS Jervis Bay—the sole escort for convoy HX 84 in November 1940— stood off the pocket battleship Admiral Scheer, when the German ship attacked the convoy. Though she was sunk, this enabled the convoy to escape. Her master, Acting Captain Edward Fegen was awarded the Victoria Cross (posthumous) for his actions. Another famous action involving an armed merchant cruiser was the November 1939 battle between HMS Rawalpindi and the German battlecruisers Scharnhorst and Gneisenau. Fighting against hopeless odds, the Rawalpindi was quickly sunk.
Both the Spanish and United States Navy used auxiliary cruisers during the Spanish-American War of 1898. In World War I too, American auxiliary cruisers fought several engagements with German U-boats.
The German practice was to arm merchantmen with hidden weapons and use them as commerce raiders. An auxiliary cruiser—Hilfskreuzer or Handels-Stör-Kreuzer (HSK)—usually approached her target under a false flag with guns concealed, and sometimes with her appearance altered with fake funnels and masts and often a fake paint job. The victim was thus engaged at point-blank range and had no chance to escape. In World War I, the Imperial German Navy initially used fast passenger ships (such as past holders of the Blue Riband for fastest North Atlantic crossings), but they made obvious and easy targets because of their very familiar silhouettes. The Germans therefore soon moved on to using captured and refitted Allied vessels, but principally only modified transport ships. These were slower, but less recognizable. In both world wars, these ships were vulnerable to attack, and were withdrawn before the war ended. Many were sunk after being caught by regular warships—an unequal battle since auxiliary cruisers had poor fire control and no armor. There were, however, a few success stories. Kaiser Wilhelm der Grosse was a former passenger liner that sank two freighters in 1914 before being caught by HMS Highflyer. Her sister ship, Kronprinz Wilhelm, had a legendary journey, sinking or capturing a total of 15 ships in 1914 and 1915, before finally running out of supplies and having to put into port in Virginia, where the Americans interned her and eventually converted her into the United States Navy troop transport USS Von Steuben. The most famous German commerce raider of World War I probably was Seeadler, a sailing ship under the command of the legendary Count Felix von Luckner. However, both Wolf and Möwe were each much more successful than Seeadler.
In World War II, Nazi Germany's Kriegsmarine operated ten very successful auxiliary cruisers which ranged in tonnage from 3,860-9,400; typically these vessels were equipped with:
- 6 in (15 cm) guns
- Smaller armaments (typically hidden away behind specially designed and hinged bulwarks, or beneath fake deckhouses and/or skylights)
To preserve their cover, these ships flew the flags of neutral or occasionally Allied nations. They re-fuelled and provisioned themselves from special supply ships, from Japanese island bases, or simply from prizes they had taken.
In one incident, the German Kormoran (ex-merchantman Steiermark) managed to surprise and sink the Australian light cruiser HMAS Sydney, which approached too close, though Kormoran herself was also sunk in the engagement. This was the only occasion in history when an armed merchantman managed to sink a modern warship; in most cases, auxiliary cruiser raiders tried to avoid confrontation with warships. It should be noted that Kormoran's attack upon Sydney was motivated by desperation. However, she was not the most successful German raider of World War II (both Atlantis and Pinguin scored higher kill tonnages). Another, Stier, was also sunk in a mutually destructive engagement with the American Liberty ship SS Stephen Hopkins.
The only encounters between Allied and Axis auxiliary cruisers in World War II were all with the raider Thor. This small vessel, which captured or sank 22 merchantmen, encountered three British AMCs in her career, defeating HMS Alcantara and HMS Carnarvon Castle and later sinking HMS Voltaire.
During World War II, German auxiliary cruisers are believed to have either sunk or captured some 800,000 long tons (810,000 t) of Allied shipping.
Compare to the Q-ship, which was a disguised merchantman for anti-submarine operations.
The CAM Ship (from catapult armed merchantman) was a British merchantman fitted with a catapult that could launch, but not recover, a single fighter aircraft.
The Merchant Aircraft Carrier or MAC was a British or Dutch cargo ship with a flight deck that could carry a small number of aircraft.
Despite a rise in modern piracy, it is very unusual for modern merchant ships to be armed, save for maybe a number of small arms and the use of the ship's fire hoses to repel boarders. One exception to this are the ships of Pacific Nuclear Transport Limited, which are used to transport spent nuclear fuel and reprocessed uranium on behalf of BNFL. Transporting enough fissile material between them to produce 50-60 nuclear weapons, these ships—beginning with the Pacific Pintail and Pacific Teal—became armed in 1999 to avoid the cost of a Royal Navy escort. Travelling together in convoy during these ships intermittent voyages, they have an onboard escort of armed police from the UKAEAC and its successors  and are equipped with two or three 30 mm (1.18 in) autocannons.
In April 2010, it was reported that a Russian company is offering a version of the 3M-54 Klub missile that can be disguised and launched from a shipping container, in theory enabling any cargo ship to be armed with an anti-ship missile. This type of missile is allegedly capable of disabling or even sinking an aircraft carrier, but "...it's not known how many of them would have to hit a carrier to knock it out of action, much less sink it. "
During the 2011 Libyan civil war, forces loyal to Gaddafi armed several merchant vessels and attempted to use them to blockade the port of Misrata.
Since late XIX century various navies have used armed merchant ships in the role of auxiliary cruisers, also called armed merchant cruisers. Significant use of this type of ship was made by Britain and Germany in both World Wars.
Some of the ships used in this role include:
- Saint Paul - Spanish-American War (United States)
- Shinano Maru - Russo-Japanese War (Japan)
- Ural - Russo-Japanese War (Russia)
- Berrima – World War I (Australia)
- Cap Trafalgar – World War I (Germany)
- Olympic – World War I (Great Britain)
- Mar Negro – Spanish Civil War (Nationalist Spain)
- Kormoran - World War II (Germany)
- Thor - World War II (Germany)
- Jervis Bay - World War II (Great Britain)
- Rawalpindi - World War II (Great Britain)
- Ramb I - World War II (Italy)
- Aikoku Maru - World War II (Japan)
- Defensively Equipped Merchant Ships
- Hired armed vessels
- False flag operation
- Merchant raider
- List of auxiliary and merchant cruisers
- Schmalenbach, Paul (1977). German Raiders. Annapolis, Maryland: Naval Institute Press. ISBN 0-85059-351-4.
- Carmania I
- PNTL Fleet
- "Nuclear fuel ship docks in Japan". BBC. 27 September 1999. Retrieved 2008-08-27.
- Brown, Paul (20 January 1999). "Nuclear fuel ships to be armed with heavy guns". The Guardian. Retrieved 2008-08-27.
- "UK British nuclear fuel ships armed". BBC. 8 July 1999. Retrieved 2008-08-27.
- Arming Container Ships With Anti-Ship Missiles
- strategypage.com - Arms to be permitted to protect UK ships that have to pass through certain areas
- Duffy, James P., Hitler's Secret Pirate Fleet, 2001, Praeger, Westport (Connecticut) and London, ISBN 0-275-96685-2
- The Oxford Companion to World War II (2005)
- Alfred von Niezychowski, The Cruise of the Kronprinz Wilhelm, 1928, published by Doubleday
- Hilfskreuzer page
- Maritimequest Kronprinz Wilhelm fact sheet and photo gallery for the German liner
- Personal account of Lt. Surgeon J. Robart who served on the Armed Merchant Cruiser HMS Ranpura in World War II
- Convoy escort movements for Royal Navy AMCs, World War 2