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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » problem 6-1

   
Author Topic: problem 6-1
Sandy Englehart


 - posted January 27, 2005 08:56 AM      Profile for Sandy Englehart           Edit/Delete Post 
using the geographic range table in the light list i get for A) 3.5 + 12.75 = 16.25, and B) 3.5 + 9.5 = 13.0. please help!
David Burch


 - posted January 27, 2005 06:29 PM      Profile for David Burch           Edit/Delete Post 
Race Rks light is 118 ft above MHW. Hence the total geo range is 1.15 x sqroot(118) + 1.15 x sqroot (9) = 12.49 + 3.45 = 15.94. Unfortunately, the factor of 1.15 will vary with the particular geographic range table. I believe the 1.15 is the latest best value.

The nominal range is 15 on the chart and 18 on the light list edition we include. Visible range is always the smaller of the two, so if the chart is right the answer is 15 and if the Light List is right, the answer is 15.94.

The Light List table we have appears to be using a factor of 1.17 (check out the value of H = 100 ft). And with those values, your solution is correct.

Using 1.17 in the table we get (1.17/1.15)x15.94 = 16.22 which is roughly what you got interpolating the table.

For the second, light ht is 67ft, so geo range (by table) = 1.17xroot(67) + 1.17xroot(9) = 9.58+3.45 = 13.03

The answers are wrong in the workbook. Another errata for the tech support section. Thanks for finding this.

From: Starpath, Seattle, WA
David Burch


 - posted January 27, 2005 06:51 PM      Profile for David Burch           Edit/Delete Post 
Summary of geographic ranges

Geo range = K x sqrt(eye height in feet) + K x sqrt (target height in feet)

Geometric with no refraction, K = 1.04

Visible light, K = 1.15

Radar (microwaves), K = 1.22

From: Starpath, Seattle, WA


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