92 seamanship secrets
- Plot range arcs and size the grid based on your speed. Use your plotting
compass to measure 1 nautical mile (or a distance appropriate to your
speed, see below) on the latitude scale. Th en center the plotting compass
needle point on the waypoint and scribe a 1-mile range arc over the bearing
lines. Now go back to the scale and pick off 2 nautical miles, then plot a
2-mile arc in the same manner. Draw arcs for each successive mile (or
whatever increment of distance you have selected).
Power vessels should extend the grid outward from the waypoint to
the distance that will be covered in 30 to 60 minutes at the intended speed
of the run. For example, if you plan to make the run at 14 knots, give the
grid a radius of 7 to 14 miles. Sailing vessels need a grid size equal to or
greater than their average speed. If you anticipate sailing at an average of
4.5 knots, make the grid radius at least 4.5 miles (as in the accompanying
illustration). Sailing vessels beating to windward should make the outer
boundaries of the grid 10 to 20 degrees on each side of the center trackline.
In steady winds, use 10 degrees; in gusty, shift ing winds, use 20 degrees.
(See “Sail a Tacking Cone to a Windward Destination” below.) If under
power or sailing a reach, make the boundaries narrower.
A chart like this will help you plot your GPS position in less than 5 seconds.