Under what condition does Rule 19a apply?

Rule 19 of the COLREGs, specifically Rule 19(a), is applicable when vessels are navigating in or near an area of restricted visibility. This ruling is essential for ensuring safe navigation in conditions where visibility is compromised, such as fog, heavy rain, or other environmental factors that limit a mariner's sight. Under these circumstances, the Rule dictates that vessels must navigate with additional caution, maintaining a lookout and proceeding at a safe speed to avoid collisions.

The emphasis on restricted visibility distinguishes this situation from scenarios where vessels can see one another, which would involve different rules under the COLREGs. Therefore, the context of navigating in or near areas of restricted visibility aligns precisely with the conditions under which Rule 19(a) must be applied, enhancing safety and preventing collisions during challenging navigation situations.