Which term is NOT defined in the Inland Navigation Rules?

The term that is NOT defined in the Inland Navigation Rules is "vessel constrained by her draft." While the other terms listed have clear definitions within the context of the Inland Navigation Rules, "vessel constrained by her draft" is a term more specifically associated with the international navigational rules—often referred to as COLREGs (Convention on the International Regulations for Preventing Collisions at Sea)—rather than being defined in the inland rules.

Understanding this distinction is essential for mariners, as knowing which terms are explicitly defined in the rules helps in interpreting situations on the water accurately. "Restricted visibility," "underway," and "vessel aground" are clearly defined terms within the Inland Rules, providing crucial context for navigation and safety practices. This clarity is vital for effective communication and compliance with the rules during navigation. Knowing which terms are specific to inland rules versus international rules ensures mariners operate safely and adhere to the appropriate guidelines for the waters in which they are operating.