A truck is a wooden ball, disk, or bun-shaped cap at the top of a
mast, with holes in it through which flag
halyard
In sailing, a halyard or halliard is a line (rope) that is used to hoist a ladder, sail, flag or yard. The term ''halyard'' comes from the phrase "to haul yards". Halyards, like most other parts of the running rigging, were classically made of n ...
s are passed.
Trucks are also used on wooden
flagpole
A flagpole, flagmast, flagstaff, or staff is a pole designed to support a flag. If it is taller than can be easily reached to raise the flag, a cord is used, looping around a pulley at the top of the pole with the ends tied at the bottom. The fla ...
s, to prevent them from splitting.
Without a masthead truck, water could easily seep into the circular growth rings of a wooden mast. However, the grain in the truck is perpendicular to that of the mast, allowing the water to run off it.
References
{{Sail Types
Sailing rigs and rigging
Sailing ship components