piloting tips and techniques 79
[(square root of the height of eye × 1.17) + (square root of the height of the
object in feet × 1.17)] times 1.17 = Distance off in nautical miles. In our
example:
Observer’s horizon distance = square root
of 9 = 3 × 1.17 = 3.5 miles
Dry Tortugas Light’s horizon distance = square root
of 151 = 12.288205 × 1.17 = 14.4 miles
Geographic range = 3.5 + 14.4 = 17.9 miles
- Draw both range arcs on the chart. Th e range at which you actually see an
object at sea varies with atmospheric conditions and, in the case of lights, full
output of candlepower. You should see a light somewhere between the nominal
range (given on the chart) and the geographic range (computed as above). For
this reason, it is prudent to plot both of these range arcs on the chart. In this
example you would do so by sticking the needle point of your compass into the
light’s position dot on the chart and swinging a 17.9-mile arc (the geographic
range arc) and a 20-mile arc (the nominal range arc) across the TR. Th e next step
is to plot a DR to both of these arcs from your current position. Next set the GPS
alarm or a stopwatch to signal when you pass the fi rst arc. Begin scanning the
horizon at that time. Set the alarm to sound again when you transit the second
arc. If you still don’t see the object upon arrival at the second arc, stop the boat,
plot a position, and check your math. Raise your height of eye and scan again.
Has haze or light fog set in? Try scanning just above or below the horizon.
Making Landfall on an Unlighted Object
You can use points of land, mountains or islands to determine time of landfall,
as long as the chart shows the elevation contours. In this example, let’s assume
you are traveling on a power cruiser toward Angel Island. You want to fi nd out
when you can expect to see the highest point on the island.
- Find your HE and the height of the object. Th e height of the fl ying bridge
deck on your power cruiser is 20 feet, and your own height is 6 feet. So
your HE is 20 + 6 = 26 feet. Th e height of the highest peak on Angel
Island is 500 feet. - At what range might you expect to start picking up the highest point on
Angel Island? To find the geographic (computed) range:
Your horizon distance = square root of 26 = 5.10 × 1.17 = 6.0 miles
Angel Island’s horizon distance = square root
of 500 = 22.36 × 1.17 = 26.2 miles
Geographic range = 6.0 + 26.2 = 32.2 miles - Use a straightedge to extend your TR over Angel Island intersecting the
500-foot elevation contour. Mark this point.