The essential principle in the action of all anchors is that a surface set at an acute angle to the ground will dig in if pulled horizontally. In order that an anchor may function properly, it must satisfy two conditions.
The first is that in whatever position it may fall as it strikes the sea floor, it must begin to dig in as soon as the pull comes on the chain.
The second is that it shall remain in the ‘digging in’ position while it is dragged into the ground, or, in other words, it shall be stable in the ground. Up to the present, only two essentially different designs have been used.
In the traditional form, with two flukes and shank and stock which is longer than the flukes, the stock serves a double purpose. It ensures that the anchor shall fall on the ground in such a position that one of the digging planes or palms will function as soon as the chain begins to drag along the ground; and it also serves, by lying flat on the ground, to keep the palm set at the correct angle as it buries itself.
In stockless anchors, there are two digging blades set on opposite sides of the shank and hinged to it by a horizontal hinge which allows them to set themselves at the correct digging angle whichever way up the anchor may fall on the ground. There is no difficulty in making them begin to bite as soon as the pull comes on the chain. Unfortunately, however, the simple design, consisting of two blades set on opposite sides of a shank, is essentially unstable in the ground. If one of the blades penetrates slightly more deeply than the other, the difference between the downward pressures on the two blades necessarily brings into playa couple which tends to rotate the anchor round its shank, burying the lower blade still more deeply and raising the upper blade out of the ground. After the upper blade has come out the lower blade continues to rotate around the shank till it also comes out of the ground on the opposite side to that on which it went in.