A mainbrace is part of the rigging on a traditional square-rigged ship. Together, the collection of rigging known as the braces was used to move a yard, a spar used for the purpose of setting sails. The mainbrace would be the largest and heaviest of the braces, making it easy to identify. Historically, ship's rigging was made from hemp rope, while other materials may be used on modern ships which use sails and this form of rigging.
The mainbrace is a particularly notable part of a ship's rigging because, without it, the ship would be unable to change direction. The braces are used to manipulate the yard to take advantage of prevailing winds and to control the movement of a ship. When the braces are missing or compromised, the ship is at the mercy of the sea, although sailors can furl the sails.
Regular inspections of the braces would be conducted to confirm that they were in working order, with special care being taken with the mainbrace. In addition, reserves of material for repairs would be kept in stock on a ship so that if a problem emerged, action could be taken. These reserves would also need to be regularly inspected for signs of damage such as rot or damage incurred by animals like rats.
As a result of the vulnerability of this part of the rigging, in historic naval battles, the mainbrace was a very popular target. Enemy ships knew that if they could hit the mainbrace, they could severely damage a ship and impede its progress. The brace would need to be repaired immediately for the ship to be functional, and the task of splicing the mainbrace was a bit complex, requiring a skilled sailor. While the mainbrace was under repairs, the ship would also be at the mercy of the enemy in addition to being at risk because it could not be controlled.
The slang term “splicing the mainbrace” in a reference to dispensing alcohol to all members of a group is a reference to the historic practice of issuing rations of grog to the crew after the mainbrace was repaired, as a reward. The practice of issuing alcohol to sailors persisted through the 20th century in some navies before succumbing to concerns about impaired sailors handling delicate and expensive equipment. Today, rations of alcohol are sometimes dispensed on ceremonial occasions by order of a high-ranking military official or the head of state.