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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.

Mud, for instance will provide better holding than peat - which is often too watery - yet usually we cannot pick the seabed.
Cables on the seabed: telephone, oil and gas. Moreover, the seabed might even be unusable for anchoring, as is the case with cables, telephone lines or pipelines, which will be indicated on the nautical chart as illustrated on the right.

Cables on the seabed: telephone, oil and gas. 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

SSand Planitis bay on Pelagos
MMud
Cy, ClClay
GGravel
CoCoral
CbCobbles
SnShingle
PPebbles
StStones
Rk, RkyRock, Rocky
ChChalk
ShShells
WdWeed
S/MTwo layers   (example: sand over mud)
Kelp chart symbol. Kelp
Seabed spring chart symbol.Spring in seabed

Qualifying terms

Note that lowercase letters are used for these qualifying terms.
ssmall
ffine
mmedium
ccoarse
hhard
smsmall
bkbroken
vvolcanic

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 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.
 
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 Anchoring course :

 Seabed - where to anchor
 Anchors & anchor parts
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5 Jan 2013
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