4 seamanship secrets
You will want to take advantage of or avoid such effects once you recognize
their potential by examining your charts when planning a passage. These are
the three most common land profile symbols.
Flat beach. Black, smooth lines wrapped around a land mass indicate fl at
beach areas. These lines represent the average high-tide mark. When
the tide goes out, this line expands out toward the water. Study the
chart and give beaches a wide berth to avoid rip currents and dangerous
surf.
High cliff s. Teeth set just inside the land’s edge—tiny black triangles that
look like the serrations on a saw blade—indicate high cliff s. Cartographers
refer to these symbols as hachures. High cliff s might provide shelter for
anchoring, but vessels under sail should stay well clear; high winds or wind
eddies may exist near their bases.
Mountain elevations and peaks. Many charts show mountains with elevations
along their contours. In some atmospheric conditions, tall mountain peaks
can be seen from far at sea. Note the 300- and 500-foot elevations on Angel
Island in the illustration on page 3.
How to Interpret Water Depths and Seabed Characteristics
Nearing Nantucket, fog blanketed the square-rigger. Th e captain ordered
the boatswain forward to begin casting the lead. Each cast solved three
mysteries—water depth, nature of the sea bottom, and the distance off
the ship-killing reefs ahead.DEPTHS
Every tiny number located against a background of white or blue shows the
water depth, or sounding, at an average low tide. On U.S. charts the usual low-
water datum, or reference level, is mean lower low water, or MLLW, which is
the long-term average of the lower of the two low tides that occur in each
24-hour period. (In some regions, such as the U.S. Pacifi c Coast, one low tide is
signifi cantly lower than the other; this is called a mixed tide.) British Admiralty
charts and many other charts around the world use lowest astronomical tide, or
LAT, as the datum for soundings. LAT is a slightly more conservative datum
measurement than MLLW. Both measures, however, are more than ample for
safe navigation.
MLLW and LAT serve as baselines, or starting points, for determining
how much water you’ll have available in any given place, at any given time. Th e
actual depth will be greater at mid-tide or high tide. At an extremely low tide,