Which term refers to vessels employed for essential maneuvers that restrict their ability to navigate?

The correct term that refers to vessels employed for essential maneuvers that restrict their ability to navigate is "Restricted Ability to Maneuver," commonly abbreviated as RAM. This classification is specifically designed to encompass a variety of situations where a vessel's maneuverability is compromised due to specific operations, such as dredging, underwater work, or towing.

When a vessel is defined under this term, it is recognized under the COLREGs as having limitations that could affect its ability to change course or speed, which necessitates greater caution from other vessels in the vicinity. Understanding this classification is vital for navigators to ensure safe passage and to avoid collisions, as it requires the surrounding vessels to give way under Article 18 of the COLREGs.

In contrast, "Not Under Command," pertains to vessels that cannot maneuver due to circumstances such as equipment failure or crew incapacitation, while "Sailing vessels" and "Fishing vessels only" do not inherently imply restricted ability to maneuver unless they are engaged in specific activities that affect their navigation. Thus, RAM is the most precise term that captures the essence of the question.