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What this tool does
An anchor holds a boat not by weight alone, but by digging into the seabed and resisting a pull. A well-set modern anchor can hold many times its own weight in good sand, while the same anchor in weed or on rock may hold almost nothing. This anchor holding power estimator gives you a planning figure for that holding power from the anchor weight and the seabed type, and it compares the result with the wind load on your boat, so you can see the margin between what holds you and what tries to drag you.
How to use it
Enter your anchor weight in kilograms, exactly as the maker states it, and pick the seabed you expect from the chart or pilot book. Enter the wind speed in knots and a rough windage area in square metres, meaning the area of hull, cabin, and rig that the wind can push on above the water. Twelve square metres is a reasonable starting point for a medium cruising yacht, and larger boats or boats with high topsides should use more. Then enter the water depth and your bow height above the water, pick a scope, and press the button.
Read the three numbers together, not one alone. Holding power tells you what the anchor might resist. Wind load tells you what the wind is doing to the boat. The margin divides one by the other: a margin of 2 means the anchor should hold twice the wind load, which is a comfortable planning figure, while a margin near 1 means you are right at the edge and any gust, wave, or change of wind direction could break you out. If the margin is low, the fixes are a bigger anchor, better holding ground, more scope, or a different anchorage altogether.
Why the seabed matters so much
The seabed type changes holding power by a factor of four between the best and worst cases in this tool. Sand is the gold standard: a modern anchor buries deeply and the sand grips the flukes along their whole area, giving roughly twenty times the anchor weight in holding power. Clay is good too, though it can be hard for an anchor to penetrate at first. Mud holds less, because it shears more easily. Gravel and shell give a middling grip. Rock can snag an anchor firmly or not at all, and it risks jamming the anchor so it cannot be recovered. Weed is the worst common case, because the anchor often skates over the weed mat instead of digging through it.
Wind load and why it grows fast
Wind load grows with the square of wind speed, just like wind pressure on sails. The tool estimates it from the standard pressure formula applied to your windage area, which works out to roughly 0.0165 times area times wind speed in knots, squared, in kilograms of force. At 25 knots on 12 square metres, that is about 124 kilograms of steady force. At 40 knots, the same boat sees about 317 kilograms. At 50 knots, about 495 kilograms. Gusts push higher still, and waves add jerking loads on top, which is why anchoring margins need to be generous.
A worked example
Take a 15 kilogram anchor in sand, a 25-knot wind, 12 square metres of windage, 5 metres of water, and a bow 1 metre above the water, with 5 to 1 chain scope. Holding power is 15 times 20, or 300 kilograms. Wind load is about 124 kilograms. The margin is about 2.4 times. Rode to let out is 5 to 1 times the 6-metre vertical distance, or 30 metres. That is a sensible, comfortable anchoring plan for an overnight stop in settled weather, provided the anchor is properly set and the bottom really is sand.
Limits and safety
Holding power estimates vary enormously between anchor designs, and a modern scoop anchor, a plough, a fluke anchor, and a fisherman anchor all behave differently on the same bottom. This tool uses one typical multiplier per seabed for all designs, which is a simplification. It also ignores waves, tidal current, wind shifts, and the weight and catenary of chain, all of which change real holding. Treat the result as an order-of-magnitude planning figure, and always confirm the set in practice.
This tool is an educational estimator only. It does not replace seamanship, the anchor maker’s guidance, pilot book advice, or local knowledge. Never trust a holding number alone: set the anchor, back down on it with engine power, take bearings on shore marks, set an anchor alarm, and check again after any wind shift. If you drag, re-anchor or move. If the forecast is severe, a harbour berth is worth far more than any calculation.
Common questions
Does chain weight help holding? Yes, in moderation. Chain lying on the bottom keeps the pull on the anchor horizontal, which helps it stay buried, and its weight absorbs jerks. But in strong wind the chain lifts off the bottom and the benefit shrinks, which is why scope matters more than chain weight alone. How much scope is enough? More is usually better up to the limits of the anchorage, because a long scope is cheap insurance, but too much scope in a crowded anchorage means swinging into neighbours, so balance scope against swinging room.
Versión en español: Estimador de Potencia de Agarre del Ancla