Anchor Sizing
Anchor sizing charts are written for a boat lying at anchor overnight in ordinary weather, with someone aboard. That is a much narrower claim than most people read into them, and the difference shows up at three in the morning.
Three things decide holding, in this order
The bottom. A well-set anchor in mud holds many times what the same anchor holds in weed or hard sand. No chart can know what you are lying over, and this is why local knowledge beats specification.
Scope. The anchor holds because of the angle the rode makes at the seabed. Short scope turns a good anchor into a bad one faster than any other single factor.
The anchor itself — which is where most of the discussion goes, and where least of the variance is.
Weight is the least interesting variable
Modern designs — Rocna, Mantus, Spade and their relatives — hold far more per pound than a Danforth or CQR of the same weight, because they set faster and bury deeper. Comparing them by weight compares products that are not comparable.
What weight buys is the ability to punch through weed and to reset when the wind shifts, and those are worth paying for.
The storm anchor is a different question
Everything here assumes normal conditions. A boat left unattended, or riding out a real blow, wants substantially more — the usual guidance is one size up, plus a second anchor deployed.
Set it, then verify it
Back down at low revs until the boat stops moving with the rode taut. Take a transit on shore and watch it for a few minutes.
An anchor that has not been set is a heavy object lying on the bottom, and it will tell you so at the least convenient moment.
5 articles
"Anchor Weight per Foot: The Rule of Thumb, and Where It Breaks"A pound per foot is a rule that works in the middle of its range and fails at both ends, for two different reasons.
Fluke Anchors: Where They Excel, and the Two Bottoms That Defeat ThemThe fluke anchor has the best holding-to-weight ratio in common use and two well-known failure modes. Both are about the bottom, not the size.
Reading a Modern Scoop Anchor Chart, and What Its Small Print AssumesModern anchor charts look interchangeable and are not. The difference between them is usually one line of small print about assumed wind speed.
Anchoring a 24-Foot Boat: The Questions the Chart Cannot SeeTwo 24-foot boats can need different anchors. Length is the index every chart uses and the one variable that tells you least about the load.
Anchor Sizing for Boats: Why Length Alone Cannot Answer ItEvery anchor chart is indexed by boat length, and every one of them carries conditions in the small print. The load your ground tackle has to hold does not rise with length. It rises with length squared.