What device is used to attach a towline to a trailer eyebolt on boats?

The device used to attach a towline to a trailer eyebolt on boats is a shackle. Shackles are commonly used in marine applications due to their strength and reliability in securing connections. They typically consist of a U-shaped metal bar with a pin or screw that can be fastened. This design allows for quick and easy attachment and detachment while providing the necessary security to handle the stresses encountered during towing.

In the context of towing, a shackle is particularly beneficial because it can accommodate various sizes of towlines and ensures a stable connection to the trailer's eyebolt. This stability is vital for safe towing operations, as it minimizes the risk of disconnection during transit.

Other devices like carabiners might be used in climbing or other outdoor activities for quick connections but may not offer the same level of strength and stability required in marine towing situations. Pulleys, while useful for redirecting lines and reducing load, are not designed for securing towlines directly. Swivels could facilitate rotation but do not provide the robust connection needed for towing. Therefore, the shackle is the most appropriate choice for attaching a towline to a trailer eyebolt.