* The Formula Rule: All chain: 15 m + 2 × the depth good for up to about 30 knots. Rope and chain: 15 m + 4 × the depth good for upto about 25 knots. (In feet: 50 ft + 2 × depth, and 50 ft + 4 × depth.) See notes below.

Which depth? The depth the chain actually has to span: from the bow roller down to the seabed. This model assumes a bow roller 4 ft above the water and a keel drawing 5 ft, so the sounder reading below the keel plus 9 ft is the figure the formula uses.

Why it works. Chain lying on the bottom guarantees that the pull at the shank is horizontal, and a horizontal pull drives the plough deeper into the seabed. As the wind builds, the catenary straightens and the boat lies further back: the chain gives up its sag before it gives up its grip. Once the last of the slack lifts off the bottom, the rode starts pulling upward on the shank — the anchor rotates about its buried fluke, the shank end rises, and the plough rolls out.

The formula is a minimum. Up to the marked wind on the slider the formula is the guidance: pay out at least what it calls for. Because light winds now is not a guarantee for light winds later. In higher wind speeds, the formula is no longer enough. Wind load rises with the square of the wind speed, so it climbs fast — doubling the wind quadruples the force. At the formula’s scope the last of the chain lifts off the bottom at about that marked wind, and everything in the red zone past it is wind the formula was never meant to cover. By 50 knots you need roughly one and a half times the formula figure, which is why experienced crews let out more when it starts to blow.

The two rodes have different limits. Switch between all chain and rope plus chain and the mark on the wind slider moves. An all-chain rode holds the formula to about 30 knots: the whole rode is heavy, so there is a long run of chain on the bottom to be consumed before the pull at the anchor starts to lift. A rope rode with a chain leader holds only to about 25 knots. Nylon is close to neutrally buoyant, so it hangs in a nearly straight line and contributes no sag of its own — all of the catenary work is done by the short chain leader at the anchor end, and that leader is a small fraction of the rode’s weight. It is consumed much sooner and thus lifts off the bottom earlier than all chain.

That is the practical difference to carry away: on rope and chain, if the wind gets up past 25 knots, let out more than the formula calls for. More rode out equates to a lower angle of pull on the anchor, giving it more holding power. Wind often rises overnight, and a boat that was well anchored at dusk in 15 knots can be lying to a lifted chain leader by 3 a.m. So the answer to more holding power is more scope, not panic.

In practice this matters less often than it might sound, because the two rodes go with different boats. Rope and chain is mostly carried by smaller boats, where an all chain rode and the windlass to handle it would be too much weight in the bow — and a smaller boat has no business sitting at anchor in 25 knots and the sea state that comes with it. Bigger boats, which do stay out in harder weather, generally carry all chain. So the lower limit on the rope and chain rule lines up with the kind of anchoring those boats actually do. The crossover is somewhere around boats of 30 feet in length, where you will find boats rigged either way.

The formula is only a starting point: many other conditions should be weighed as well — boat windage, weight of chain, type of bottom, anchor selection, gusts, wave action, tide, and more. Use good seamanship skills whenever anchoring.