The force of the wind on an anchored boat and thus the wind-induced load on ground tackle—depends on two factors: the wind speed and the exposed surface area of the boat. While wind speed is easily measured, exposed surface area is more difficult to discern. From the boat’s length, beam, and height above the waterline, we can derive a first-order estimate, but design and gear play a large role as well. A sailboat equipped with roller furling, a large pilothouse, or a bimini—or a power cruiser with a canvas-enclosed flying bridge .
We can calculate forces exerted, or load, due to wind (Fw) on a given vessel by means of the formula:
Fw = 1/2 × ρ × Cd × A × V2
ρ = density of air (1.225 kg/m3)
Cd = drag coefficient
A = frontal surface exposed to the wind
V = wind speed in km/h
A violent scirocco blew through that night, and the next morning Achim climbed above Playa Francesa to document the scene with our camera. All it had taken was for one anchor to slip, and others had followed suit. Two sailboats were laid over on the rocks, their owners wringing their hands on the shoreline. Four other cruising yachts limped into the harbour after having sustained dings and breaks. We heard all their horror stories; in each case, unfortunate circumstances had made it impossible to flee the anchorage and escape damage.
The loads exerted by currents are relatively Insignificant. A 5-knot current (a rare occurrence in an anchorage) would impose a load of around 340 pounds (150 daN) on a 40-foot (12.2 m) boat.
Current loads deserve consideration, however, especially when you’re anchoring in a river estuary or some other area subject to considerable tidal influence. When subjected to strong tidal currents, an anchored boat will swing successively in one direction, then the other. Checking tidal depth will allow you to ascertain if your boat will handle the swing at low tide without running aground. Every anchor type reacts differently to directional changes in the drag force. Some anchors pivot in place without breaking free from the seafloor, realigning themselves to the new direction of pull.