In modern navies, an auxiliary ship is primarily designed to operate in support of combatant ships — for replenishment, repair, logistics, survey, or transport. While not primary combat vessels, they may have a limited combat capacity for self-defense.

Armaments on Auxiliary Ships

Their armaments, sometimes referred to as "auxiliary guns," often include:

• ▸Small to medium caliber guns (e.g., 20mm or 40mm cannons) for defense against small, fast surface craft or aircraft
• ▸Close-in Weapon Systems (CIWS) such as the Phalanx — automated, radar-guided systems designed as a last line of defense against anti-ship missiles and aircraft

These ships retain their primary support function, with the guns serving a purely defensive role.

The Armed Merchantman / Auxiliary Cruiser

Historically, especially during the World Wars, the term could refer to a merchant ship armed to function as a warship — known as an armed merchantman or auxiliary cruiser. These vessels were equipped with guns (sometimes concealed) to protect themselves from privateers or, as with German commerce raiders, to attack enemy shipping.

Armed merchantmen were inferior to purpose-built warships due to a lack of armor and poor fire control, but they were cheaper and faster to produce.