Forces due to wind, current and waves
The forces experienced by an anchor arise from a combination of wind, current, and waves.
The force due to wind is in principle calculable and increases with the yacht LOA to a power between 1.5 and 2. Although the wind force varies widely in practice, the peak wind force for a 12 metre (40 ft) yacht in a Force 8 wind will typically be in the region of 400 kilograms force (kgf). A good 15 or 20kg NGA should resist this without moving in the seabed.
The force due to current is relatively small provided a yacht is head-on to the current. It can be estimated from the speed at which a yacht can be driven by its engine and the force developed by its screw in smooth water. Typically the force required to drive a 12 metre yacht at 5 kt would be around 150kgf. Its resistance when anchored in a 5kt current would the similar, roughly equivalent to the peak wind force at Force 5-6.
The force due to waves is another matter. There is no simple formula for wave forces. Wave forces depend critically upon the nature of the anchor rode. If this is all-chain, and if the chain becomes taut, snatching or snubbing will occur. This can impose enormous stress on the anchoring equipment. The older anchors coped with this to some extent by ploughing in the seabed. With NGAs, the forces are increased several-fold since they provide a much greater hold. So, when forced to plough, they put more strain on the equipment. It is then vital to take appropriate measures to reduce snubbing which occurs, especially in waves, when the anchor cable suddenly becomes bar taut. A good spring is essential. Snubbing is immediately detected by loud banging of the anchor cable at the stem head.