What is a halyard commonly used for on a boat?

A halyard is an essential component of a boat's rigging system specifically designed for raising and lowering sails. By attaching one end of the halyard to the head of a sail and the other end to a winch or block near the mast, sailors can easily hoist the sail into position or bring it down when necessary. This function is critical for navigating since the amount of sail raised can directly affect the performance and speed of the boat.

The other options involve different functions that are not related to the halyard. For instance, tying a boat to the dock pertains to docking lines, stabilizing the mast involves stays and shrouds, and controlling engine RPM relates to throttle mechanisms. Therefore, the correct association of a halyard is specifically with the task of raising and lowering sails.