avoiding collision by eye or radar 149
Your radar will need to be equipped
with the following controls.
Fixed Range Rings. Turn these
non-movable rings on. Use the fixed
range rings to quickly estimate the range
of the contact between marks.
VRM (variable range marker). Th is
range device moves a dot, called a bug,
out to the target. Use the VRM to plot the
target. If your radar doesn’t have VRM,
use the fi xed range rings for plotting.
EBL (electronic bearing line).
This measures the relative bearing of
the contact.HEAD-UP MODE AND RELATIVE BEARINGS
Most radars show a head-up display, where land and targets appear relative to the
bow of your boat. Some radars show a north-up display, where land and targets
appear relative to true north. We’ll discuss only the head-up mode here.
Relative bearings start at the bow with 0 degrees and run clockwise to
the target. No matter how we turn, the bow is always at 0 degrees. If we have
a target bearing 40 degrees on the radar scope, the target is 40 degrees off our
bow, or 40 degrees relative. If we turn toward the target, its relative bearing
decreases. If we turn away from the target, the relative bearing increases.
On your radar scope, you’ll see a faint fl ash each time the antenna rotates.
Th is heading fl ash stays aligned to 0 degrees relative on the bearing ring, even
when you change course.
PRIORITY #1: VISUAL TARGET TRACKING
If visibility permits, acquire and track the target visually. First, convert the
relative bearing to a magnetic bearing. Always add the relative bearing to your
magnetic heading, and subtract 360 from any number greater than 360.
If your boat’s magnetic heading is 110 degrees, and the radar target bearing is
56 degrees relative, what is the magnetic bearing to target?
110 degrees magnetic + 56 degrees relative = 166 degrees magnetic
Go on deck, hold the handbearing compass to your eye, and turn to
166 degrees magnetic. Acquire your target and check the bearing, then begin
tracking the bearing drift.
If your boat’s magnetic heading is 225 degrees and the radar target bearing
is 190 degrees relative, what is the magnetic bearing to target?
225 degrees magnetic + 190 degrees relative = 415 degrees – 360 =
55 degrees magneticSupplies and Tools
Needed for Radar Scope
Plotting
X grease pencil or overhead
projector marker
X cotton rags
X tongue depressor or
straightedge
X nautical slide rule
X handbearing compass
X binoculars