With their apparent simplicity, small boat anchors are the least understood of sailing equipment. Everyone knows what they are intended to do but there is little scientific knowledge of how they perform in all circumstances because of the difficulty of observing their actions underwater. When they are buried in the seabed, their behaviour depends primarily on the consistency and strength of the seabed itself, factors which vary greatly.
The boat anchor does not actually 'hold', but its ability to penetrate the seabed and that seabed’s substantial consistency determines a good anchorage. Anchor rode cable and chain variations play an important part in the holding power of each boat anchor type. Trials in artificial test tanks can be misleading and it is uncertain what type of boat anchor design is best overall.
There will be a preference, due to experience for a certain type of boat anchor design, in a particular locality. Classification Societies recommend weights of anchor in relation to size of boat, but avoid the controversy of specifying types. A large amount of weight in the boat anchor chain is a solution whatever the boat anchor, as heavy anchor chain absorbs much of the energy of a tossing boat before this force can reach the anchor. The need now is to save weight carried aboard and many boats now carry nylon anchor rope in preference to chain.
Nylon anchor rope can absorb energy by its elasticity, but has little weight in the water with loads going direct to the boat anchor therefore putting more reliance on the anchor’s holding performance. An efficient anchor rode has a few metres of marine anchor chain, called the leader, attached between the anchor rope and the boat anchor to absorb the effects of snatch loading.
The minimum scope for chain is 3:1 and for rope 5:1 but it is much better to increase these ratios to 5:1 and 8:1 respectively if you have sufficient anchor rode. If anchoring a boat in an exposed anchorage in rough conditions, pay out ten or more times the depth of water to avoid dragging, remembering to allow for the rise of tide if anchoring at low water.
Anchoring a boat in light weather is less problematical and for this, a kedge anchor is used. A kedge anchor is lighter than the bower (main) boat anchor and is used in conjunction with the main anchor to move a boat by deploying the kedge anchor and pulling on it.
Anchoring a boat in heavier wind and wave conditions requires a heavy anchor so choose the heaviest that be handled and carried, particularly so for emergency anchoring where a sheltered anchorage is unavailable.
One of the universally-recognized boat anchor types is the [ Fisherman Anchor ], and is still popular for many applications. Either way up the Fisherman anchor lands on the seabed, one of its flukes (points) will be in a downward position able to penetrate the ground. Keeping the flukes vertical is accomplished by the stock (crossbar) at the opposite end of the shank lying horizontally on the seabed.
As most of the weight is felt at the tip of the fluke, the fisherman anchor design is good on rock, weed and firm soils but has its limitations in soft ground due to the small amount of fluke area able to resist dragging. The attributes of the fisherman anchor is its dependable performance in a variety of sea beds and its ability to reset quickly if dragged out.
The [ grapnel boat anchor ] , another boat anchor type, has four flukes on cruciform arm does not need a stock and is suited in areas of hard sand and rocks, but the grapnel boat anchor's disadvantage is that they are not easily stowed on yachts.
Most other forms are classed as ‘high holding power’ or ‘stockless’. Having large fluke areas they bury deeply, achieving a much higher performance but on certain sea beds the grapnel anchor types are unpredictable in their holding power - the ratio of the force they can sustain compared to their weight.
Tumbling fluke boat anchor types, as the [ Danforth anchor ] , or the similiar Fortress anchor have a hinge allowing the flukes to ‘tumble' either side of the shank, pointing downward whichever side they land on the bed. The Danforth or Fortress anchor's advantage is that this form stows flat on the deck ready for use. The Danforth or Fortress anchor is usable on rock and soft ground where its large flukes are effective, but unsuitable on hard ground.
How to use a Danforth anchor to set correctly, is to lay a shorter scope initially, i.e. 3 or 2 to 1, when setting the anchor in a soft bottom. This keeps the shank from sinking deeper into the bottom and ahead of the flukes. When the anchor sets, then let out the normal 5 to 1 or greater scope.Big boat anchors are stowed easily by drawing them directly into a hawse pipe. At the limit of their holding power these designs tend to roll out of the ground and may not reset if they have collected a stone between the shank and the fluke.
The [ plow anchor ], although being cumbersome, has a large fluke area and is popular. The best known of the plow anchor is the CQR anchor, which features a strongly forged hinge between the ploughshares and the shank. This causes the plow anchor to ‘screw' its way into the ground assisting the digging-in phase. The plow anchor is a good all-round performer, especially in softer soils where they can penetrate deeply.
A different anchor type is the [ Bruce anchor ] , with its two curved horns which rotate the main fluke into the ground. The Bruce anchor is ideal on softer ground but is uncompetitive with other designs on harder ground as it cannot penetrate deeply. The Bruce anchor's advantage, on intermediate ground, is even if it rolls out, it retains some holding and a fair chance of digging in again.