76 seamanship secrets
help locate yourself. This method may not be that convenient, but if you find
yourself in poor visibility without functioning GPS, it’s a good technique to
know. You have to correct the sounding from an electronic depth sounder
by measuring the distance of the transducer below the vessel’s waterline and
adding this distance to the instrument sounding (unless the instrument has
a programmed offset correction). For instance, if your depth transducer is
mounted 4 feet below the waterline and the sounding reads 10 feet, you are
in 14 feet of water. You then correct this number by adding or subtracting
the present height of tide relative to low-water datum (see Chapter 5). If you
measure a 14-foot sounding at low water and the tide tables tell you to expect
a low-water height of 1.2 feet (i.e., 1.2 feet above low-water datum), subtract
1.2 from 14 to get 12.8, then round up to 13. Look for a 13-foot sounding on
the chart near your DR position—maybe that’s where you are. This works
best when you’re near a rapidly shoaling coast. In such a case, find the 12-foot
depth contour, and you can be fairly sure you’re just outside it.
Combine soundings with bearings, and take action as needed to keep your vessel
in safe water. Use what you have. Advanced techniques include using a sextant.
Piloting by Electronics. Radar is the most versatile electronic navigation
instrument in existence. Not only can it provide distance and bearing from
SEA-CRET TIP
X In narrow channels, always glance astern every minute or so.
Study the set of the boat (how it is moving) relative to aids on the
channel edges. This quickly shows if you’re in danger of wandering
out of the channel.Piloting by eye is one technique for staying on a safe course. When entering a channel, it
might mean delaying the turn until the left -hand markers line up, as here.