| | | THERE are many reasons why some anchors tend to roll but this month I show how the holding of a roll-stable anchor depends upon scope, anchor weight and the rate it's dragged through the seabed. For those who missed last month's article, I'd simply say that my experiments were conducted in a shallow tidal pool at Longniddry beach on the shores of the Firth of Forth, where the bottom was medium-hard sand. Anchors were dragged through the sand by a 5 or 10-part pulley system that enabled forces of up to 700 kg to be applied at the anchor. The cable was either nylon multiplait or, for the heavier anchors, wire plus K-inch chain. It passed over a pulley held | | one metre above the sand by means of a pyramidal frame. The scope, S/D (S=cable length from pulley to anchor, D=the height of the pulley above the sand), was therefore equal to the length of the pulling cable from the anchor to the pulley, measured in metres. How we measured them The tension on the anchor cable was measured using ANCHORWATCH, a device invented by Kevin Scott and myself which uses a load cell to measure forces up to a ton. The CQR type, HLBlade, Delta, Brittany, Danforth, Danforth copy, Bruce, claw and SPADE anchors tested ranged in weight from 1 kg to 15 kg. | | |