Crosstrees are the two horizontal
spars at the upper ends of the
topmasts of
sailing ship
A sailing ship is a sea-going vessel that uses sails mounted on Mast (sailing), masts to harness the power of wind and propel the vessel. There is a variety of sail plans that propel sailing ships, employing Square rig, square-rigged or Fore-an ...
s that are used to anchor the
shrouds from the
topgallant mast. They may also be mounted at the upper end of the topgallant to anchor the shrouds from the
royal mast
The mast of a sailing vessel is a tall spar, or arrangement of spars, erected more or less vertically on the median line of a ship or boat. Its purposes include carrying sails, spars, and derricks, giving necessary height to a navigation light ...
(if fitted). Similar transverse spars remain on
steamship
A steamship, often referred to as a steamer, is a type of steam-powered vessel, typically ocean-faring and seaworthy, that is propelled by one or more steam engines that typically move (turn) propellers or paddlewheels. The first steamships ...
and motor vessel masts to secure wire
antennae or signal flag
halyards.
In modern sailing vessels, spreaders serve the same purpose.
Explanation
Any vertical structure like a mast is subject to dynamic swaying stress from wind, which levers immense force at the base of the mast. Such stress is countered through
guy ropes which are diagonally supporting ropes from mast top to its base. These ropes share the load on the mast tops and communicate that force to the base structure.
The taller the mast, the wider a base is required for the
guy-wire
A guy-wire, guy-line, guy-rope, down guy, or stay, also called simply a guy, is a tensioned cable designed to add stability to a freestanding structure. They are used commonly for ship masts, radio masts, wind turbines, utility poles, and ten ...
s so as to form an appropriate angular support against the sway the mast is exposed to, yet ships are fixed in their
beam (width) and hence only a limited angle is possible for the guy-ropes to support very high masts. Thereby, the taller the ship's mast, the more narrow and unfeasible would be the angle between its support wires and its top. This is where a simple innovation like the crosstree helps to overcome such limitation.
The crosstree serves as a fresh base to spread the next level of supporting guy ropes, thereby providing a stable height extension to the masts. Without the crosstree, the ship's mast would have been severely limited in height, in relation to the beam of the ship.
Each crosstree serves to spread another level of holding ropes on a fresh wider spar so as to provide support to the next mast top section. Effectively, the crosstree allows to extend the height, mount yet another layer of sail shrouds and option more wind power to the ships. The crosstree also serves to spread the shroud tops.
Sources
External links
* {{commonscat-inline, Crosstrees
Sailing rigs and rigging