Restricted Visibility

Every so often we find ourselves out on the water when we can't see where we are going due to a change in atmospheric conditions. Such changes could be the result of rain, snow or temperature variation, just to name a few. When visibility conditions begin to deteriorate, we refer to it as "restricted visibility." 

Navigation Rule 3 defines "Restricted Visibility" as "any condition in which visibility is restricted by fog, mist, falling snow, heavy rainstorms, sandstorms or any other similar causes." Wait a minute, did that say "sandstorms?" That's right, a sandstorm along the coast could have an impact on your visibility. Restricted visibility could even be the result of smoke from a land-based fire.

In short, anything that interferes with your ability to see where you are going is a state of restricted visibility. Restricted visibility is one of the strongest arguments for having a current chart onboard. And, no . . . . an artist's rendering of the area on a restaurant place-mat is not a substitute for a good chart.

A current chart will show the location of the buoys, dayboards and other aids to navigation that you need to find your way to your destination. Your chart also provides a few other things that can be useful, especially in restricted visibility. One thing is the water depth, which is represented by the "soundings" printed on the chart.

When you are following a compass course to your next "way point," you can monitor your position by checking the charted depths against your depth-finder readings. Knowing the charted depths can help you to make a course correction, keeping you on course and preventing you from running aground.

Your chart also provides distances from which you can calculate your "estimated time of arrival" to your next waypoint. If you know that you are making 6 knots and that your next way point is 3 nautical miles away, you can expect to be there in approximately a 1/2 an hour. If you don't see the buoy or dayboard that you are heading for in 30 minutes, it's time to stop and regroup.

There are times when the visibility is restricted, but not to the extent that it would be in dense fog or heavy rain. In those cases, your visibility is hampered, but you can still see for several miles. Your ability to see aids to navigation will be somewhat less than it is on a on a clear day. Your chart can help you there too.

There is information on your chart that indicates the number of nautical miles from which you should be able to see a lighted aid to navigation. The standard for that mileage is "10 nautical mile meteorological visibility." Visibility is the measure of your ability to see objects through the atmosphere.

You may see the notation "5 M" on your chart in the description of a light. What that means is when the visibility is 10 miles, the light in question can be seen from 5 nautical miles away. In restricted visibility, you would not expect to see that light from the same distance.

Your chart has many features that can help you in varying conditions of visibility. It pays to keep it current and onboard!

Until next time, I wish you clear skies, fair winds, and calm seas!