This brings us to the last but most important parameter that determines your anchor's holding power, that is, the seabed itself, particularly the cohesiveness and type of soil on the bottom.
Having determined from the frontal surface area of your boat and the expected wind speeds to be encountered, the load to be held by the anchor, you must then know the most important thing of all - the nature of the bottom holding ground.
There is such a wide range of bottom conditions which play a vital part of your anchor's holding capacity, that they must be known, understood and catered for. One type of anchor is inadequate for all conditions. Three different types of anchors are usually needed for different bottom classifications such as rock, coral, gravel, weed, sand or mud. Within each class of bottom there are many degrees of variance. Under "weed" for instance, the type and amount of weed can be important. The cohesiveness of the sand, mud or clay has a big bearing - it is gravel or shale? type and size of granules?
Harbour or sea bottoms can vary from fine silt with particle sizes around 6 microns, to coarse silt at 60 microns. Fine sand usually is categorised at a particle size of 200 microns, medium sand at 600 microns and coarse sand (such as on coral beaches) up to 2mm. Fine gravel then starts at a particle size of 6mm, medium gravel @ 20mm, coarse gravel @ 60-70mm and cobblestones at 200mm. These gradings are given in BSS1277 and are more or less recognised internationally. Naval charts have over 93 different Bottom Classifications, each of which has a different effect on your anchor's holding ability.
It is obvious that an anchor designed for a specific purpose may not be so good under different circumstances. For example, a rock grapnel may not be too good in soft mud and a sand anchor may not perform in weed so well as an Admiralty pattern type. The conclusion follows that not one anchor is best for all conditions and that cruising boatmen, rarely knowing what is on the bottom of the anchorages they may visit, should always carry two or three different types of anchors.
If you have a choice of anchorage, always pick one with a bottom you know your anchor (s) will hold in. If you don't know what the bottom is, put out 2 or more anchors of different types. The holding power of a sand anchor in weed or rock, is usually very low - better use a grapnel or Admiralty pattern.
In Australia, the large majority of anchorages are in estuaries or harbours, where sand or mud of varying degrees of cohesiveness is the norm. The majority of boating people live in these areas, so it is no wonder most people carry sand and mud anchors similar to the Danforth or CQR. Outside fishermen will always carry a grapnel rock anchor for holding over reefs, as well.
If an anchor manufacturer claims his anchor will hold X lbs, you must ask, in what soil? under what conditions? The chance is, he will quote the maximum holding power in conditions ideal for his anchor, not the average sand or mud generally found in Australian estuaries. The only real test for comparing the holding power of different anchors, is to test them all on the same bottom.
Going back to the example of a 26 ft Sailboat required to withstand a 60 knot wind, it was determined that a load up to 400 kg could be experienced. You must now answer the next question - what sort of bottom? The following table broadly classifies some of the bottoms and types of anchors, rating each anchor out of 10. From experience, you may know what your anchor will hold, otherwise you may need a guide to the type and size of anchor required - determined from tests of various anchors in various bottom soils. The table below gives an indication of this type of rating.
A nasty problem can always raise its head, especially for yachtsmen and cruising skippers away from their usual sailing ground - that is to know what sort of bottom you have below. Ancient mariners always carried a lead line to plumb the depth. On the bottom of the lead, they applied a piece of lard or sticky material to bring a sample of the bottom up for examination. Modern electronics provide depth sounders that, with some skill and understanding, can provide a good clue to the nature of the bottom. If it's not too deep, and you should never try to anchor in water more than about 10 fathoms, say 15-20 metres, unless you have to, you may be able to see the bottom. Failing that, admiralty charts record the general nature of the bottom that can give you a good clue as to its cohesiveness.
An extract from "Table S 'Quality of the Bottom'" from Chart No. 1 - U.S.A Nautical Chart Symbols and abbreviations is given on Fig 20.
The point is, is that if you have to hold 400 kg (say 900 lbs), a 15 lb (7kg) Danforth anchor or a 5 kg Flook may do the job in ideal hard sand, whereas in mud you will require a 24 lb (11 kg) Danforth and in weed or rock maybe a 50 lb one. For Plough anchors such as the CQR, you may need to add at least 50% to these anchor weights and add a lot more chain as well.
|(Sa)||Co Hd||Coral Head||54||glac||Glacial|
|(Sb)||Vol Ash||Volcanic Ash||57||bl;bk||Black|
|Grs||Grass||(Sc)||S/M||Surface layer and|
|32||Fr||Foraminifera||springs in seabed|
|38||Cir||Cirripedia||only before S|
Source: "Table S. Quality of the Bottom." Chart No. 1: United States of America Nautical Chart Symbols and Abbreviations, 8th ed. (Washington, D.C.: Department of Commerce, 1984).