86 seamanship secrets
radar beam from your antenna to a pebble dropped into a tub of water. Con-
centric circles emanate from the pebble’s splash point (your antenna), bounce
off the tub sides, and return to the point of origin. So when we plot a radar
range as an arc on the chart, we are actually plotting a segment of a circle of
position. Th e following steps show how to use this secret to plot radar ranges
to clear any danger in your path:
- Determine from the chart a range from a prominent point, cliff ,
headland, or other feature that will keep you clear of off -lying shoals.
If passing between two headlands, pick the one with the more prominent
point. Th is gives a sharper radar picture. In the illustration, we chose
the prominent point to starboard. - Draw a line parallel to your courseline and tangent to the prominent
point. Measure the distance from the tangent line back to your courseline
(0.5 NM in the illustration). - Adjust the variable-range marker (VRM) on your radar to that
distance. Th e VRM measures distance (range) on a radar. Th e illustration
shows what the radar will look like as you pass between the two shoals.
Keep the VRM circle tangent to the prominent point to starboard to
clear both shoals.
How to Clear Hazards without Plotting on a Chart
Sometimes we need a quick solution to avoid hazards such as shoals or ships
that lie ahead. One blazing-fast solution allows you to do this without plotting
on a chart! The Rule of 60 shows how many degrees to alter course to the right
or left to clear a danger. You need to know two terms to use this rule:
Using a radar-range circle of position to clear a dangerous pass.