What is a requirement for all vessels when navigating in restricted visibility according to Rule 19b?

The requirement for all vessels when navigating in restricted visibility, as outlined in Rule 19b of the COLREGs, specifies that vessels must have their engines ready for immediate maneuvering. This provision is crucial because restricted visibility can prevent a vessel from seeing other vessels, navigational aids, and potential hazards. Being ready to maneuver allows a vessel to react promptly to avoid collisions, which is essential for safety in these conditions.

In restricted visibility, the ability to quickly change course or speed can make the difference between a safe passage and a dangerous incident. The other options do not align with the principles of safe navigation in such conditions. For instance, proceeding at maximum speed can increase the risk of collision since it limits the time available to react. Constantly signaling other vessels does not replace the necessity of being able to maneuver effectively. Similarly, maintaining a straight course may not be safe or practical in a situation where adjustments are frequently needed to avoid potential dangers. Thus, having engines ready for immediate maneuver is a fundamental safety measure for all vessels in restricted visibility.