Which of the following is used to securely tie a vessel to a piling?

The round turn with two half hitches is the ideal knot for securely tying a vessel to a piling because it effectively distributes the load and provides a strong, stable connection. This knot is designed to wrap around the object, in this case, the piling, and the two half hitches create additional friction, enhancing the security against slipping or loosening.

By using a round turn, the knot utilizes both a significant amount of the line’s length and the properties of tension, which is critical when mooring a boat. The two half hitches added to the round turn ensure that the knot remains tight and secure, even in changing conditions such as waves or wind that may apply additional stress to the tie.

Other knots listed may serve different purposes. For instance, a square knot is typically used for joining two ends of rope but does not provide the secure hold needed for tying a vessel. A sheet bend is great for joining two ropes of different diameters but is not suitable for securing a vessel to a piling. Lastly, the figure eight knot is primarily utilized as a stopper or to create a loop at the end of a rope; it lacks the adaptability needed for securing against movement caused by water conditions.