18 seamanship secrets
“LOCAL MAGNETIC DISTURBANCE—Diff erences of as much as
3 degrees from the normal variation have been observed on Cuttyhunk
Island between Buzzards Bay and Vineyard Sound.”
What it means: Near this island, the normal compass variation indicated
in the chart’s compass rose may be off by as much as 3 degrees. Check your
position frequently and adjust your course as necessary. (See Chapter 2
for more on magnetic variation and how to account for it when steering
compass courses.)
“CAUTION RIPRAP—Mariners are warned to stay clear of the protective
riprap surrounding navigational light structures.”
What it means: Riprap is erosion-reducing material such as shale, brick,
concrete pieces, or rock built up around the base of a light structure.
Without this protection, the base would quickly crumble from the
pounding of waves and swell. It’s best to stay well clear of the squiggly circle
surrounding the light’s position circle.
“FOR OFFSHORE NAVIGATION ONLY—Detail within the 10-fathom curve
is not shown on this chart except on the off -lying shoal and the Bahama
Islands.”
What it means: Th is one tells us not to rely on any depth equal to or less
than 10 fathoms or 60 feet. Shoal banks surround the Bahama Islands,
extending many miles to sea, showing soundings. Th e cautious navigator
should pull out a larger-scale chart before attempting a passage over these
areas. (A large-scale chart covers a small area in great detail; for more on
chart scales, see Chapter 2.)
“TRAFFIC SEPARATION ZONE—Mariners are requested to stay outside the
circular separation zone centered on the San Francisco Sea Buoy.”
What it means: You’ll fi nd traffi c separation zones in the approaches to
most major ports that accept deep-draft ships. A zone is charted with
inbound and outbound lanes separated by a centerline. Both inbound and
outbound ships stay to the right, just as on U.S. highways. Small vessels not
using the traffi c lanes should stay clear of the zone and maintain a careful
lookout.
Are Your Charts Up-to-Date?
When is the last time you checked your chart edition dates? Before using
any chart, glance at the lower left-hand corner. This lists the edition number,
month, and year of issue. You probably won’t see the current year. This doesn’t
mean the chart is outdated; governments find it too expensive to resurvey and
reissue charts annually unless major topographical changes have occurred.
But you’ll need to check for the most current edition.