piloting tips and techniques 89
For example, from your normal helm position, you take a centerline bearing
of 125 degrees magnetic. Adding 45 degrees to the centerline bearing gives
170 degrees magnetic. Subtracting 45 gives you 80 degrees magnetic. Sight
over the handbearing compass again using these bearings.
You notice that the bearing at 170 degrees magnetic touches one of the
lifeline stanchions to starboard, and the bearing at 80 degrees magnetic touches
a lifeline stanchion to port. Mark both points, and you have your 45-degree
bearing marks.
Now, when you’re underway and ready to fi gure distance off a charted
object, sit or stand at this helm position. When the object “touches” one of your
preset marks, mark your time, then watch your speed and hold your course.
When you can look directly abeam at the object (i.e., when it’s off your shoulder
if you’re facing the bow), mark the time again. Figure the distance sailed from
mark to mark. Th at distance equals your distance off the object when it was
abeam. Follow these steps and work through the example:
- Maintain a steady speed and course (any course will do, as long as you
stay steady). - Mark the time an object touches your 45-degree boat mark.
- Mark the time again when you see the same object abeam (off your
shoulder). - Convert the time (minutes) to fractional or decimal parts of an hour.
Multiply by your speed.
You want to find your distance off of an island off your starboard side. The
chart shows a prominent church spire on a high hill near the shore.
You get onto a course of 090 degrees magnetic and a speed of 5 knots.
At 1035, the church spire touches your 45-degree boat mark.
At 1123, you can look abeam (off your shoulder) at the church spire.
How far are you off the island?
1123 – 1035 = 48 minutes or 0.8 hour (48 ÷ 60)
0.8 hour × 5 knots = 4.0 nautical miles off the island.
Practice this technique using GPS for a precise measurement of the dis-
tance run between the bow and beam bearings. Your navigation won’t depend
on the answer if your GPS receiver is functioning, but the exercise will build
your visual memory of how objects appear from various distances off. Practice
with islands, rocks, navigation buoys, lighthouses, and so forth. You’ll gain
a better understanding of how a bell buoy appears from a distance of 1 mile
versus 2, or how an island appears from 2 miles versus 3. Th is knowledge is
highly useful when piloting by eye (see earlier in this chapter).
To make the exercise even more useful, estimate your distances run (through
the water) between bearings using your knotmeter or an RPM/boatspeed graph,