Seamanship_Secrets_185_Tips_-_Techniques_for_Better_Navigation-_Cruise_Planning-_and_Boat_Handling_Under_Power_or_Sail_(Re)_e..

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chart and publication secrets 17


LIGHTS ALWAYS CARRY TWO EXTRA DESCRIPTIONS
Both small and large fixed-light structures give height and range informa-
tion after the basic aid information. Take a look, for example, at the charted
description for Dry Tortugas Light:


DRY TORTUGAS LIGHT
Fl W 20s 151ft 20M

Height. The vertical distance of the eye of the light above datum high
water is 151 feet.
Range. The charted visibility of this light, called its nominal range,
is 20 nautical miles. Nominal range depends only on candlepower of the
light, however, and doesn’t take into account the height of the light or the
observer’s eye. The earth’s curvature will often prevent you from seeing
a light at its full nominal range. For example, the distance of the horizon
from the 151-foot-high Dry Tortugas Light is 14.3 nautical miles. If your
height of eye as you attempt to view the light is 9 feet above the water,
your horizon distance is 3.5 nautical miles. Since 14.3 + 3.5 = 17.8, the geo-
graphic range of the Dry Tortugas Light for you is 17.8 miles—more than
2 miles short of its nominal range. Even in clear weather, you will not see the
light from 20 miles away. (Learn more about using height to find range to any
object in Chapter 4, How to Determine When You Will Make Landfall.)


Chart Notes: Nuggets of Hidden Gold


Look all around the land mass and margin section on your chart and you’ll notice
navigational notes. Take time to scan the notes when you fi rst purchase a chart.
Th ey’re specifi c to the geographical area displayed, and you’ll want to use your
highlighter for the most critical data. Here are examples of common notations
and their meanings.


SEA-CRET TIP


X If searching for a wave-actuated buoy in low visibility, send a
crewmember up to the bow, away from engine noise.
X You can also turn your boat in a tight circle to create a wake.
Stop and listen. Your wake may help actuate the buoy’s clappers
or diaphragm.
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