History of drag embedment anchors
History traces the use of anchors to China as far back as 2,000 BC, though it is quite probable that they were used prior to this. At that time the general tendency was to use large stones, baskets of stones, bags of sand or even logs of wood loaded with lead which were then fastened to lines. It was this weight as well as a certain degree of friction on the bottom which secured a vessel in position.
With the introduction of iron into anchor construction, teeth or flukes were built on the anchor, allowing penetration into the seabed, thus offering additional stability. Yet these primitive anchors were of poor construction and often broke under pressure. Curved arms were introduced in 1813, and from 1852, the so-called ‘Admiralty Anchor’ was used for ships of
the Royal Navy. Another refinement in the 19th 15 century was the elimination of the stock, the crosspiece
at the top of an anchor which ensured that the positioning of the anchor would allow the flukes to penetrate the soil. A stockless anchor was invented in 1821 and became popular, primarily as a result of the ease of handling and stowing, qualities still valued today.
A large number of anchor types has been designed and commercialised over the years. Some have prospered, others not. The most recent designs are the results of vast experience and extensive testing, and are far more efficient than their historical predecessors. A short overview of the anchors in use today, is presented on the following pages.
Characteristics of anchor types
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Based upon certain charateristics such as fluke |
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area, shank, stabilisers, it is possible to classify |
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the various anchor types. |
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To allow a rough comparison of anchor type |
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efficiency, an indication (*) is provided for a 10 t |
Class A |
Stevpris |
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anchor as (HOLDING CAPACITY = WEIGHT * EFFICIENCY). |
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Class A efficiency range *33 to 55 |
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slender anchors with ultra-penetration. |
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Class B |
Bruce SS |
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Class B efficiency range *17 to 25 |
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anchors with ‘elbowed’ shank, allowing for |
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improved penetration. |
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Class C efficiency range *14 to 26 |
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anchors with open crown hinge near the |
Class C |
Stevin |
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centre of gravity and relatively short shank |
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and stabilisers or built-in stabilisers. |
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Class D efficiency range *8 to 15 |
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anchors with hinge and stabilisers at the rear |
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and relatively long shanks and stabilisers. |
Class D |
Danforth |
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Class E efficiency range *8 to 11 |
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anchors with very short, thick stabilisers; hinge |
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at the rear and a relatively short, more or less |
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square-shaped shank. |
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Class E |
AC14 |
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Class F efficiency range *4 to 6 |
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anchors with square shank, no stock stabilisers. |
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The stabilising resistance is built-in the crown. |
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Class G efficiency range *<6 |
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anchors with small fluke area and stabilisers at |
Class F |
US Navy Stockless |
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the front of the shank. |
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Class G Single Fluke Stock
Characteristics of anchor types
Stevshark |
FFTS |
Bruce TS |
Hook |
Stevfix |
Stevmud |
Flipper Delta |
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LWT |
Moorfast - Stato - Offdrill |
Boss |
Stokes |
Snugstow |
Weldhold |
Beyers |
Union |
Spek |
Stock |
Dredger |
Mooring Anchor |
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History of vryhof anchor designs
A brief chronological summary of the types of |
Stevin |
anchors vryhof has designed for use in the offshore |
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and dredging industries: |
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• 1972 - The Stevin anchor: The original design. The wing was not yet enlarged. The anchor had a square shank. It is no longer manufactured.
• 1974 - The Hook anchor: originally designed for |
Hook |
permanent moorings. This design was sur- |
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passed in 1980 by the Stevpris design and is |
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no longer manufactured. |
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• 1977 - The Stevin Mk3 anchor: is the improved |
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Stevin Mk3 |
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version of the original Stevin anchor. It was |
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equipped with an enlarged crown and fluke |
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area and a streamlined shank for more effi- |
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cient penetration. This anchor is still manu- |
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factured and in use in offshore and dredging |
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activities. It has all classification societies |
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approvals. |
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• 1978 - The Stevfix anchor: this anchor was |
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Stevfix |
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designed with special fluke points for harder |
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soils and a larger fluke area than the Stevin, |
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but has been surpassed by the Stevpris |
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anchor. It is no longer manufactured. |
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History of vryhof anchor designs
•1990 - The Stevpris Mk5 and Stevshark Mk5 |
Stevshark Mk5 |
were introduced. The improved versions of |
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the original Stevpris and Stevshark anchors. |
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Improvements have concentrated on two |
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features: higher holding capacity and easier |
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handling. |
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•1996 - Introduction of the Stevmanta VLA |
Stevmanta |
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(Vertical Load Anchor). Based on industry |
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demand for an anchor that could withstand |
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vertical loads, the Stevmanta VLA was deve- |
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loped. The Stevmanta VLA is a new design in |
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which a traditionally rigid shank has been |
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replaced by a system of wires connected to a |
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plate. The anchor is designed to accept verti- |
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cal (or normal) loads and is installed as a |
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conventional drag embedment anchor with a |
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horizontal load to the mudline to obtain the |
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deepest penetration possible. By changing |
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the point of pulling at the anchor, vertical (or |
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normal) loading of the fluke is obtained thus |
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mobilising the maximum possible soil resi- |
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stance. As a VLA is deeply embedded and |
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always loaded in a direction normal to the |
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fluke, the load can be applied in any |
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direction. Consequently the anchor is ideal |
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for taut-leg mooring systems. |
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