Editing Anchor (section)

=== Mushroom ===
[[File:Portsmouth with mushroom anchor.jpg|thumb|upright=1.2|Mushroom anchor (right) on the [[United States lightship Portsmouth (LV-101)|lightship ''Portsmouth'']] at [[Portsmouth, Virginia|Portsmouth]], Virginia]]

The mushroom anchor is suitable where the seabed is composed of silt or fine sand.  It was invented by [[Robert Stevenson (civil engineer)|Robert Stevenson]], for use by an 82-ton converted fishing boat, ''Pharos'', which was used as a [[lightvessel]] between 1807 and 1810 near to [[Inchcape|Bell Rock]] whilst the [[Bell Rock Lighthouse|lighthouse]] was being constructed. It was equipped with a 1.5-ton example.

It is shaped like an inverted mushroom, the head becoming buried in the silt.  A counterweight is often provided at the other end of the shank to lay it down before it becomes buried.

A mushroom anchor normally sinks in the silt to the point where it has displaced its own weight in bottom material, thus greatly increasing its holding power. These anchors are suitable only for a silt or mud bottom, since they rely upon suction and cohesion of the bottom material, which rocky or coarse sand bottoms lack. The holding power of this anchor is at best about twice its weight until it becomes buried, when it can be as much as ten times its weight.<ref>[http://www.acegroup.com/us-en/assets/moorings.pdf Moorings] {{Webarchive|url=https://web.archive.org/web/20120320122253/http://www.acegroup.com/us-en/assets/moorings.pdf |date=20 March 2012 }}. INAMAR. acegroup.com</ref> They are available in sizes from about 5&nbsp;kg up to several tons.