150 seamanship secrets
Go on deck, hold the handbearing compass to your eye, and fi nd 55 degrees magnetic.
Acquire the target, refi ne the bearing, and begin tracking the bearing drift.RULE #1 OF HEAD-UP RADAR CONTACT PLOTTING
In a seaway, the contact will appear to move if your boat wanders off course. Be-
fore you mark the contact on the radar scope, have the helm steady up on course.
When the helmsman is on course and calls “Mark!,” use your marker or grease
pencil to mark the target on the scope. Do this each time you place a mark on the
target. Th e example below illustrates this procedure. Refer to the diagram as you
read through the scenario. Th e boat at the center of the scope in the illustration
simulates the radar’s heading fl ash.
Your course is 020 degrees magnetic and your speed is 4 knots. Your radar is
set to the 6-mile range. Each fi xed range ring represents 1 nautical mile. Make
your marks on the scope every 6 minutes. Th at way, you can use the 6-minute
rule to calculate contact speed.- Radar contact and the first mark. At 1340, your radar picks up a
target. Use either the variable range marker (VRM) or fi xed range
rings to fi nd the target range. (Note that the VRM allows more precise
measurements because you control the size of the range ring. Th e fi xed
range rings are set to the selected scale by the radar. The radar allows
you to use either method individually or along with the other.) Ask the
helm to steady up and sound out “Mark!” when the boat is on course.
Place a mark onto the target with your marker. You fi nd the range to
be 4.8 nautical miles. Label this first mark 1340. Place the EBL onto
your mark and read the contact’s relative bearing: 22 degrees relative.
Convert this to a magnetic bearing (020 degrees magnetic + 22 degrees
relative = 042 degrees magnetic). - Acquire visual contact and bearing drift. Grab your binoculars and
handbearing compass. Step outside and sight with the handbearing
compass in the direction of 042 degrees magnetic. If you sight the
contact, read and record the actual magnetic bearing. If not, drop the
compass and scan with the binoculars to the left and right of the bearing.
To see a small target 4.8 miles away, you need to raise your height of eye
to 17 feet. If you still see nothing, try again in a few minutes. Remember,
you need to mark the target again at 1346 (6 minutes aft er the 1340 mark).
Return to the radar to get ready for the next mark. - Th e second mark and the DRM Line. At 30 seconds before the second
mark, ask the helm to steady up on course. As soon as the helm says
“Mark!,” place your second mark on the target. Label this mark 1346. Th e
range to the target is now 3.8 miles. Now you have everything you need to
fi nd range, bearing, and time to the closest point of approach, or CPA.
Lay your straightedge along the two dots (1340 and 1346) and extend the
line an inch or two past the center of the scope. Th is represents the contact’s