What signal indicates a vessel that is not able to be maneuvered?

The correct signal indicating a vessel that is not able to be maneuvered is three prolonged blasts. This signal is part of the International Regulations for Preventing Collisions at Sea (COLREGs), which establish guidelines for maritime communication.

When a vessel emits three prolonged blasts, it is a clear and universal indication to other vessels in the area that it is unable to change its course or speed, usually due to a mechanical failure or severe operational limitations. This alert is crucial for ensuring the safety of navigation, as it allows other vessels to be aware of the situation and take appropriate actions to avoid collisions.

The other signals mentioned serve different purposes in maritime communication: short blasts often indicate intentions related to navigation or maneuvers, while prolonged blasts may signify the presence of vessels in restricted visibility or other navigational situations. Therefore, understanding the specific meanings of these signals is essential for maritime safety and compliance with the COLREGs.