A cross-tree is a structural support on a sailing craft, used to hold the lines, known as shrouds, that support the mast. It is part of the rigging known as the top, and is also referred to as a crosstree, depending on regional spelling preferences. Other ships may use devices known as spreaders for a similar purpose. More generally, a cross-tree is any kind of supportive horizontal beam, such as a beam put in place for tethering horses in a stable.
Each mast has two cross-trees, located on either side of the mast. The shrouds attach to them and run down to the deck. Providing support from both sides, they keep the mast fixed in place, even in adverse conditions. Sometimes, other anchoring lines will be placed down the length of the mast as well. Rigging can be positioned above the cross-trees of the mast for various purposes and the entire rigging is carefully designed to avoid tangling or impinging on lines, as this could pose a safety issue for sailors and people on board.
These structural supports are installed with care, as they need to be able to withstand pressure and endure hash conditions. Salt spray and ultraviolet radiation can both be found in abundance around a ship and both can be very damaging. Durable woods are used and may be tarred or otherwise treated to make them more resistant, and the cross-tree is carefully fixed in place to make sure it will not give under stress. It is also regularly inspected by sailors for signs of wear, along with other aspects of the mast and rigging. Failure of a cross-tree could be catastrophic and this section of the ship is very important.
When ships are designed, the designer thinks about the kinds of stresses on board and in the conditions where the ship will be used and addresses these concerns. This can include moving rigging to accommodate special needs, as well as varying rigging designs for different kinds of boats. Most ships today primarily use masts for communications apparatus rather than sailing rigging, as other modes of power are available, and the cross-tree can serve a dual purpose by holding up equipment.
Replacement of worn and damaged cross-trees ideally takes place in the controlled environment of a shipyard, where the ship can be stripped of rigging to perform maintenance. The mast will need to be carefully lowered and is often serviced at the same time, checking for cracks, rot, and other signs of wear to determine if it also needs to be replaced.