6 seamanship secrets
Height Measurements for Safe Passage
Nautical charts show heights of bridges, powerlines, and overhead cables at
mean high water or MHW. This datum is the average of all high waters over a
nineteen-year period. British Admiralty and many other international charts
Bottom Type Bottom Texture Color/Contrast
S—Sand f or fne—fi ne wh—white
M—Mud bk or brk—broken bl or bk—black
Cl or Cy—Clay sy or stk—sticky bu—blue
Sh—Shells so or sft —soft gn—green
Grs—Grass h or hrd—hard yl—yellow
K—Kelp gy—gray
Rk or Rky—Rocky br—brown
Co—Coral lt—light
Co Hd—Coral Head dk—dark
Blds—Boulders
Oys—Oysters
Ms—MusselsChart Seabed AbbreviationsSEA-CRET TIP
X You’ll sometimes fi nd a bottom texture—such as so, sft, h, or hrd—
alone on the chart, with no bottom type indicat.ed. Look around the
symbol out to about a half mile for an S, M, or Cl.
X As an alternative, discover the nature of the seabed yourself with a
lead line—a traditional but handy tool that should be carried aboard.
This instrument consists of a cylindrically shaped piece of lead with
a hollow bottom. A long length of light line is attached to the top.
Mark the line every fathom or every two meters. Smear a bit of
grease or peanut butter inside the hollowed bottom. Cast the lead
out ahead of the boat. When the line goes slack, read the marks for
water depth. Pull the lead back up and turn it over. You should fi nd a
sample of the sea bottom embedded in the grease or peanut butter.
Now you know for sure what kind of sediment is down there.