Anchor geometry: fluke
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Modeling the fluke

All anchors under consideration are equipped with a fluke. For modeling purposes the fluke can initially be considered as a flat plate with the same surface area as the real anchor. The shape, thickness and the presence of stabilizers have a big influence on the penetration behavior and must therefore be incorporated in the model.

Four properties are involved in modeling the fluke:

  • Surface area

  • Shape

  • Thickness

  • Appendices

Surface area

The fluke area determines part of the anchors friction as well as its holding capacity. The fluke area can simply be defined by the user and used to determine its holding capacity. With the area of the fluke known it can then be use to calculate the surface friction of the anchor.

Shape

The shape of the fluke determines its form resistance; lean sharp fluke will have lower form resistance then chubby blunt ones. Nearly all flukes have a triangular or double triangular shape, and they can therefore be considered as one or two triangles.


Instead of dividing the anchors in to triangular or double triangular the fluke shape can be represented by a shape factor. This factor should be dimensionless and should tell something about the geometry of the fluke, for instance a low value for sharp lean anchors and a higher value for blunt chubby anchors. The shape factor can be determined as follows:   

A shape factor of one represents the best-fit triangle or double triangle with width and length corresponding to the width and length of the fluke (figure 4). By dividing the fluke area by the area of the triangle a coefficient is obtained which matches the triangle to the properties of the fluke. Because the triangle is only a rough approximation the coefficient can be larger than one if the fluke has rounded of edges.

Thickness

The thickness of the fluke will also have an influence on the drag of the anchor; a thicker fluke will have bigger resistance than a thin one. To incorporate this in the model the effect of the thickness of the anchors’ resistance will have to be investigated.

Appendices

Some of the anchors under consideration are equipped with stabilizers to encourage its penetration and consequently its holding capacity. These stabilizers have a negative effect on the drag and come in a variety of shapes and sizes. To model the appendices it’s easiest to use an appendix coefficient, which indicates the influence of the stabilizer on the drag of the fluke.  The coefficient can range between one and two, if for instance the stabilizer ads 20% of drag the coefficient should be 1.2. The influence of stabilizers on the drag should be investigated so that proper coefficient values can be assigned to different types and shapes of stabilizers.

 

Copyright © 2005 Project group 1
Last modified: 10/27/05