Two main factors determine the holding power of anchors in mud and sandy bottoms:
Penetration is determined by several factors:
Anchors with elbowed shank such as Bruce, CQR, Hook and Flook allow for deeper penetration.
In sand and harder soil, it is the shank that restricts penetration. It is the surface area of the flukes that will be in the soil before the shank makes contact with the bottom surface that determines the holding power.
With spade or hinged fluke anchors, such as the Danforth, Stevin, Brittany, those with the hinge point closest to the centre of the gravity give the best penetration, with the hinge point right at the crown, the anchor will trip and tend to roll up on its points.
In soft mud where the shank may penetrate below the surface, it is usually the chain that restricts further penetration. In such circumstances, a steel wire allows deeper penetration and results in a higher holding power.
So many claims have been made by different anchor manufacturers, that the actual holding power of various types and brands of anchors is difficult to believe.
Many tests have been conducted over the years in all parts of the world by manufacturers and by independent testing authorities.
Most manufacturers quote holding power under the most favourable conditions for their particular anchor, but they rarely disclose what those conditions are.
Big ship anchors are specified and tested by authorities such as Lloyds or Det Veritas, under conditions which do not necessarily reflect those met in actual practice. There are no standard test procedures, conditions or places for small leisure boat anchors to be certified for holding power, so it is difficult, of not well nigh impossible, to compare one anchor against another.
Attempts have been made, such as at Beg-Rohu & Binic in France, Edinburgh and Glasgow in U.K. and recently at Palm Beach in Sydney by "Australian Boating Magazine" to test and compare a number of different anchors under uniform conditions. Some tests were conducted on dry land on beaches; others under water; some from fixed rigs; others from small boats. Whilst in each case, the conditions may have been the same for all anchors tested there has never been a proper record of the cohesiveness of seabed soil (which was probably never measured) nor of the scope of rode, type and size of rode, rate of application of load and many other parameters normally defined in engineering test procedures.
It is therefore impossible to compare one test with another, and as each was conducted in different ways in different places with different people, it is perhaps logical that the results are also different.
|Type||Test Results Efficiency Index (Holding Power to weight ratio||Average|
|Admiralty Pattern||5, 7, 14||8|
|Bruce||16, 11, 9, 24, 22, 17, 25, 39, 30||21|
|CQR||36, 9, 12, 40, 26, 70, 12||30|
|Danforth High Tensile||34||34|
|Danforth Standard||17, 38, 34, 20||27|
|Flook||45, 55, 27||42|
Anchors with high holding power, originally so called because their holding power was three to four times greater than normal anchors of equivalent weight, have recently been developed with an efficiency many more times their own weight.
Whilst a concrete block for instance may have a holding power only 1/2 its dry land weight and a mushroom anchor twice its dry weight, modern anchors can hold up to 50 or 60 times their own weight.
Anchors are assigned as "Efficiency Index" which is based on the anchors average ability to hold a load in sand and mud in relation to its weight. A 5kg anchor that will dig in and hold an average 300kg, has an efficiency index in sand of 40. This is a simple way to rate and compare the holding power of different types of anchors - in sand and mud.
When it comes to other bottom soil conditions those which are best in sand, like the burying anchors, may not hold so much as the "picking" or "hooking" anchors such as the old Admiralty Pattern or Grapnel anchors. These types may perform better in weed, gravel and hard clay. The efficiency index assigned for performance in sand and mud, must be modified for these other conditions.
For Example - A 10kg anchor with an efficiency index of 30 is equal to a 5kg anchor with an efficiency index of 60 or a 20kg anchor with an efficiency index of 15.