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post #1 of 12 Old 07-18-2011 Thread Starter
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Aids to Navigation - Light List Question

Hello All,


In looking at the Coast Guard Light List, if a lighthouse height above water is 85 feet and the nominal range is 9 miles, and the height of eye is 15 feet, what is the maximum distance from which the light house can be seen?

Thanks.

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post #3 of 12 Old 07-18-2011
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This post explains the whole thing pretty well.

Light Lists, Lighthouses, and Visible Ranges

It would be of interest as to why you are asking the question.
The answer would be different if it is a theoretical calculated answer vs an actual navigation question.
I for have not been able to see light house for any where near the charted distance in memory even on clear nights for example.
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post #4 of 12 Old 07-18-2011
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This might be helpful:

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post #5 of 12 Old 07-18-2011
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There three different light ranges:

Quote:
A light’s luminous range is the maximum range at
which an observer can see a light under existing visibility
conditions. This luminous range ignores the elevation of the
light, the observer’s height of eye, the curvature of the
earth, and interference from background lighting. It is determined
from the known nominal range and the existing
visibility conditions. The nominal range is the maximum
distance at which a light can be seen in weather conditions
where visibility is 10 nautical miles.
Quote:
A light’s geographic range depends upon the height of
both the light and the observer. Sum the observer’s distance to
the horizon based on his height of eye and the light’s distance
to the horizon based on its height to calculate a light’s geographic
range.
http://www.irbs.com/bowditch/pdf/chapt04.pdf

Light lists usually provide the nominal range.

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post #6 of 12 Old 07-19-2011 Thread Starter
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Hi, thanks for your replies. I'm currently taking U.S. Sailing Coastal Navigation, and this is one of the exercises on the workbook. My answer is also 15NM using the lighthouse distance calculator. But the workbook answer is 9 miles, which is the nominal range shown on the Light List. By the way, the light house is the Point Diablo light house (Light List, Vol VI, Pacific Coast). Is the workbook answer wrong? Thanks.
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post #7 of 12 Old 07-19-2011
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Quote:
Originally Posted by kwindancer View Post
Hello All,
In looking at the Coast Guard Light List, if a lighthouse height above water is 85 feet and the nominal range is 9 miles, and the height of eye is 15 feet, what is the maximum distance from which the light house can be seen?
kwindancer
The Nominal range of the light is as stated 9 nautical miles. This is how far you can see that light on a dark clear night with a visual range of 10 nautical miles...
So the max distance you can see that light is 9 nautical miles. The USCG would have to punch up the power of the light in order to see it at a greater range.

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post #8 of 12 Old 07-19-2011
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Quote:
Originally Posted by Boasun View Post
The Nominal range of the light is as stated 9 nautical miles. This is how far you can see that light on a dark clear night with a visual range of 10 nautical miles...
So the max distance you can see that light is 9 nautical miles. The USCG would have to punch up the power of the light in order to see it at a greater range.
Absolutely, and may I add that the world may have to flatten out a bit as its curvature is a consideration. Also the range stated on a chart may be more realistic if seen from the bridge of a ship.

However, do your CG tests require a technically correct answer or a practical one?


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Quote:
Originally Posted by Boasun View Post
The USCG would have to punch up the power of the light in order to see it at a greater range.
Not quite.

There an visbility scale that can used in comjunction with a nominal range diagram.


Yards
0 Dense fog . . . . . . . . . . . . . . . . . Less than 50
1 Thick fog . . . . . . . . . . . . . . . . . . . . . .50-200
2 Moderate fog . . . . . . . . . . . . . . . . . .200-500
3 Light fog . . . . . . . . . . . . . . . . . . . .500-1000
Nautical Miles
4 Thin fog . . . . . . . . . . . . . . . . . . . . . . . . 1/2-1
5 Haze . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1-2
6 Light Haze . . . . . . . . . . . . . . . . . . . . 2-5 1/2
7 Clear . . . . . . . . . . . . . . . . . . . . . . . . 5 1/2-11
8 Very Clear . . . . . . . . . . . . . . . . . . 11.0-27.0.
9 Exceptionally Clear . . . . . . . . . . . Over 27.0



from Bowditch online. http://www.irbs.com/bowditch/pdf/chapt04.pdf

An exceptionally clear night would be 15 miles, the geographic range.

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Kwin, the correct answer would appear to be 15 miles IF the question was stated without any mention of weather/visibility conditions.

The 9-mile range is indeed nominal range--and that only applies when the visibility conditions are ALSO defined as nominal, i.e. stated up front that they apply to the situation. If the visibility conditions are not mentioned in the question, then nine miles is wrong because it assumes something that is not so: Nominal conditions rather than optimal or maximum ones.

USSA has been known to make mistakes. Since you are taking their course, it might pay to raise this question with them and ask them to rephrase the question to remove the error or ambiguity.
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