Crosstrees are horizontal struts on a mast that serve as attachment points for shrouds, lines that stabilize the mast to keep it in position. They are more traditional on older sailing ships. Boats with more modern rigging schemes use devices known as spreaders for the same purpose. The crosstrees are easy to spot, as they tend to be large and very noticeable when viewers trace the shrouds from the deck to their attachments on the mast.
Shrouds provide stabilization from both sides of the mast. Multiple sets of lines run up to the mast at different heights to provide the most stable support. As the mast gets taller, the angle of these lines becomes more acute, and this is not desirable. The solution to this problem is the crosstrees, which provide a wide horizontal bar to make the angle larger. On ships with multiple masts, each mast will have its own set of horizontal beams for this purpose.
Depending on the size of the ship and the rigging, these beams may support a platform known as the top to provide easy access to the rigging for sailors. The tops are not used for observations like the crow's nest. Smaller ships may simply leave the top of the crosstrees open, requiring sailors to balance on the beams as they perform work high up in the rigging. In both cases, working in the rigging can be dangerous, especially in heaving seas, and sailors may wear safety harnesses to decrease the risks of serious injuries.
Like other components of the rigging, the crosstrees need regular inspection to confirm that they are still in good condition. Sailors will check for signs of mold, rot, loosening attachments, and other issues that might pose a safety risk. They can replace portions of the rigging as needed. Shipbuilders in a port can perform more complex repairs, if necessary, or may supply the needed components for repairs if a ship does not have them on board. When new crosstrees are put in place, they are typically treated to resist salt spray and algae formation, with the goal of keeping them in working order.
On ships with traditional rigging, especially ships maintained to reflect the traditions of a historic period like 18th century sailing, it is possible to see crosstrees in the rigging. There are a number of different ways to set sails on a ship, and the sailors who work on the boat can provide more information about the rigging style and common variations.