See how much chain it takes before the pull at the anchor is horizontal. Enter depth, bow-roller height, chain weight, and the horizontal load on the boat; get the minimum chain, the pull at the bow, and how much chain is lying on the bottom.
Use this when anchoring on all-chain rode and you want to know whether the chain you have out will keep the pull on the anchor horizontal at a given load — the condition anchors are designed for.
Enter the water depth and the height of the bow roller above the water. Use your chain’s weight per foot from the maker’s data or by weighing a measured length; the page converts it to submerged weight. The horizontal load is an estimate you supply — wind and wave pull on the boat at anchor — not something this page computes.
The result shows how quickly chain lifts off the bottom as load rises. In a blow, a catenary that looked generous at 300 lb can straighten out.
A chain hanging between two points under its own weight forms a catenary. When the chain just touches the seabed at the anchor with a horizontal pull, the curve starts flat at the anchor and rises to the bow roller. The shape is set by one number, a = horizontal load ÷ submerged chain weight per foot: a heavy chain or a light load makes a deep sag; a high load makes a straight, tight line.
For a vertical rise y (depth plus roller height), the length of chain in the curve is √(y² + 2ay). Any chain beyond that lies on the bottom. Steel in seawater weighs about 87% of its weight in air, which is why the page multiplies by 0.87.
y = depth + bow-roller height
w = chain weight in air × 0.87 (submerged steel)
a = horizontal load / w
minimum chain s = √(y² + 2·a·y)
horizontal span x = a × asinh(s / a) · bow tension = load + w·y
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Depth | Roller | lb/ft | Load lb | Out | Min chain | Span | Bow lb | On bottom |
|---|---|---|---|---|---|---|---|---|
| 20 | 4 | 1.1 | 400 | 150 | 143.7 | 141.0 | 423 | 6.3 |
| 20 | 4 | 1.1 | 200 | 150 | 103.0 | 99.2 | 223 | 47.0 |
| 20 | 4 | 1.1 | 800 | 150 | 201.7 | 199.8 | 823 | lifts |
| 10 | 4 | 1.1 | 400 | 150 | 109.1 | 107.9 | 413 | 40.9 |
| 30 | 4 | 1.7 | 1000 | 250 | 217.1 | 213.5 | 1,050 | 32.9 |
| 15 | 3 | 0.75 | 250 | 100 | 118.8 | 117.0 | 262 | lifts |
With the calculator defaults (Water depth (ft) = 20; Bow roller height above water (ft) = 4; Chain weight in air (lb/ft) = 1.1; Horizontal load on boat (lb) = 400; Chain deployed (ft, optional) = 150), the arithmetic runs like this:
y = depth + roller height = 20 + 4 = 24 ft
submerged weight w = 1.1 × 0.87 = 0.9570 lb/ft
a = load / w = 400 / 0.9570 = 418.0 ft
minimum chain s = √(y² + 2·a·y) = √(576.0 + 20062.7) = 143.7 ft
horizontal span = a × asinh(s / a) = 141.0 ft
bow tension = load + w × y = 400 + 23.0 = 423 lb
Result: 143.7 ft minimum chain. The hanging chain spans 141.0 ft horizontally; the pull at the bow roller is about 423 lb at 19.0° below horizontal. With 150 ft out, about 6.3 ft lies on the bottom at this load.
Now change Horizontal load on boat (lb) to 800 and leave everything else alone:
y = depth + roller height = 20 + 4 = 24 ft
submerged weight w = 1.1 × 0.87 = 0.9570 lb/ft
a = load / w = 800 / 0.9570 = 835.9 ft
minimum chain s = √(y² + 2·a·y) = √(576.0 + 40125.4) = 201.7 ft
horizontal span = a × asinh(s / a) = 199.8 ft
bow tension = load + w × y = 800 + 23.0 = 823 lb
Result: 201.7 ft minimum chain. The hanging chain spans 199.8 ft horizontally; the pull at the bow roller is about 823 lb at 13.6° below horizontal.
Doubling the load from 400 to 800 lb raises the minimum chain from about 144 to 202 ft, and 150 ft is no longer enough — the chain lifts off the anchor. This is why chain that looks generous on a calm evening can come up short in a squall.
The catenary is a shock absorber as well as a weight. As the load rises the chain lifts and straightens, and once it is straight the only stretch left is in the chain itself, which is very little. A nylon snubber adds the elasticity that a straight chain no longer has.
Loads are the hard part. Wind load rises roughly with the square of wind speed, and gusts and waves add peaks on top. If you are unsure, run the page at two or three loads to see how quickly the minimum chain grows.
Swing room is the price of more chain. Check the horizontal span plus the chain on the bottom against the space you have, especially in a crowded anchorage where boats on rope rode swing differently from boats on chain.
Units: Feet for lengths and depths, pounds for loads, pounds per foot for chain weight.
Where the model stops: Static catenary for all-chain rode with a horizontal load at the bow. Dynamic wave loads, current on the chain, seabed slope, and rope-chain combinations are not modeled. The load is your estimate.
Not a substitute: Anchor choice, holding ground, and seamanship decide whether you stay put. Use the anchor and chain makers’ guidance and local knowledge.
| Chain (lb/ft in air) | Depth | 200 lb pull | 400 lb | 800 lb | 1,600 lb |
|---|---|---|---|---|---|
| 0.7 | 15 ft | 113.3 | 159.1 | 224.2 | 316.5 |
| 1.1 | 20 ft | 103.0 | 143.7 | 201.7 | 284.3 |
| 1.1 | 30 ft | 124.0 | 172.0 | 240.8 | 338.9 |
| 1.6 | 30 ft | 104.5 | 143.9 | 200.6 | 281.6 |
| 2.4 | 40 ft | 101.8 | 137.1 | 188.8 | 263.4 |
Lengths in feet. Roughly: 5/16 in chain is about 1.1 lb/ft, 3/8 in about 1.6 lb/ft, 1/2 in about 2.4–2.7 lb/ft.
In light air the chain lies in a long curve and only a short length leaves the bottom. As the wind rises, the curve straightens: at the load where the whole chain lifts, the pull reaches the anchor shank at an upward angle and the anchor can break out. Look across a row of the table and you see how fast the required length grows. Doubling the pull adds roughly 40% more chain, not double.
Wind load on a boat at anchor rises with the square of wind speed, and gusts and surging add peaks well above the steady figure. The 1,600 lb column is a strong-wind load for a mid-size cruising boat. A nylon snubber absorbs the peaks that a straight chain would otherwise jerk onto the anchor and the windlass.
An estimate of the horizontal pull on the boat from wind and waves. It depends on the boat’s windage and the conditions; this page does not compute it. Try a calm-day value and a storm value to bracket the answer.
Steel is about 7.7 times as dense as seawater, so in water it loses roughly 13% of its weight to buoyancy.
No. Rope is nearly weightless in water and changes the shape completely. This page is for all-chain rode.
Most anchors are designed to set and hold with the pull along the seabed. As the angle at the shank rises, holding typically falls. Keeping some chain on the bottom keeps the pull low.