The seabed – where to anchor

Since a good anchor digs itself into the seabed we're interested in more than just the upper layer. Shells, weed and sea grasses might prevent an anchor from grabbing. But once through, the anchor can dig itself into the lower sandy, mud, peat, cobbles, stony or clay bottom, each with different holding characteristics, requiring different anchor types.

Cables on the seabed: telephone, oil and gas. — Submarine cable; submarine power line; oil pipeline; gas pipeline.

Mud, for instance will provide better holding than peat – which is often too watery – yet usually we cannot pick the seabed.

Moreover, the seabed might even be unusable for anchoring, as is the case with cables, telephone lines, high voltage power lines or pipelines, which will be indicated on the nautical chart as illustrated aboveon the right.

Other reasons why anchoring may be forbidden are an explosives dump area or a historic wreck, or simply because anchoring will block the passage.
Also the seabed might be foul, which is indicated on nautical charts by a #, marking perhaps old chains or simply polluted ground below.

Likewise you should avoid naval (submarine) exercise areas, nature reserves, precious coral and areas with divers down (see the “A” flag and “diver down” flag).

Types of seabed

Symbol	Seabed type
S	Sand
M	Mud
Cy, Cl	Clay
G	Gravel
Co	Coral
Cb	Cobbles
Sn	Shingle
P	Pebbles
St	Stones
Rk, Rky	Rock, Rocky
Ch	Chalk
Sh	Shells
Wd	Weed
S / M	Two layers e.g. sand over mud
	Kelp
	Spring in seabed

Qualifying terms

Note that lowercase letters are used for these qualifying terms.

Symbol	Qualifying term
s	small
f	fine
m	medium
c	coarse
h	hard
sm	small
bk	broken
v	volcanic

Thick mud, clay and sand will provide good holding, as will pebbles once the anchor is deep enough.
A rocky seabed might perhaps provide even better holding, but also increases the risk of a permanent mooring when the chain or anchor itself gets jammed under a heavy boulder.
Therefore, especially when dealing with (volcanic) rocks, the use of a trip line or tripping line is recommended to enable you to retrieve your anchor.

Thick layers of shells are rather useless: at least with seaweed and grass the anchor has a chance to penetrate into the more suitable layers underneath.