138 seamanship secrets
Imagine that it’s nighttime, or that a fog and haze have set in. You turn to your
radar and study the path ahead. You need to pick out that next buoy along your
courseline. Up ahead, however, the radar shows a cluster of dots where your buoy
should be. Which is the buoy, and which are other boats? To solve this puzzle,
you fi rst must understand relative motion.
Try some relative-motion practice in clear weather. Pick out a single buoy
ahead and watch your radar as you move toward it. Th e buoy “appears” to move
down the scope toward you. Th is movement is called relative motion, or relative
speed. Now stop your boat and look at the buoy again on the radar scope. It
has stopped all movement, so it not longer has any relative motion.
So how can you pick out motionless objects ahead, such as buoys, drifting
boats, boats at anchor, fish stakes, and small islands? Remember these two
secrets of a motionless contact.
Heading is the same, but opposite. On a radar, a stationary contact shows a
reciprocal (opposite) heading, exactly parallel to yours. No matter how you
turn, the stationary contact follows this rule.
Speed is the same as your speed. A stationary contact’s relative speed is exactly
equal to your boatspeed. Modern radars show range in nautical miles and
tenths of a mile. In radar plotting you can use an easy concept called the
“6-minute rule” to compute speed. Follow these steps:
QMeasure the distance in nautical miles that a contact moves in 6 minutes.
QMultiply the result by a factor of ten.
For instance, if a contact appears to move 1.2 miles in 6 minutes, it has a
relative speed of 12 knots (1.2 × 10). If a contact appears to move 3.2 miles in
6 minutes, it has a relative speed of 32 knots (3.2 × 10).
Your speed is 6 knots. A contact appears to be on a parallel heading to your
course. At 2110, the range to the contact is 7.3 miles. You wait 6 minutes,
and then take a second range. At 2116, the range to the contact closes to 6.7
miles. How can you tell if this is a motionless contact or not?
During that 6-minute span, the object appeared to move 0.6 mile (7.3 miles –
6.7 miles = 0.6). To find its relative speed, multiply 0.6 × 10 (or move the
decimal one place to the right). The relative speed is 6 knots, the same as
our own boat. This confirms that we are looking at a boat at anchor, adrift
(drift ing boats rarely have enough relative motion to matter in this calculation),
or an aid to navigation.Avoiding Collisions the E.A.S.A. Way
I honestly believe that a combination of early action and substantial action, or
E.A.S.A., may be the cure-all for preventing collisions at sea anywhere on the