Chain Catenary Calculator
What the sag is worth: the wind speed at which your chain's catenary pulls straight and stops protecting the anchor — weight, depth and load in one curve.
Catenary is fair-weather magic: at anchor-test loads the sag vanishes and only geometry holds. The snubber's real job begins exactly here — stretch absorbing the surge loads the straight chain now transmits like a steel bar.
Formula
⚠️ For planning and education only — verify with official charts, tide tables and your vessel's documentation. Not for navigation.
What the sag is worth: the wind speed at which your chain's catenary pulls straight and stops protecting the anchor — weight, depth and load in one curve.
About Chain Catenary Calculator
All-chain devotees credit the catenary — the heavy sag that keeps the anchor's pull horizontal — and in 15 knots they're right. This calculator finds the load at which they become wrong: the horizontal tension that lifts the last link and pulls the curve straight, computed from chain weight, length and depth against your boat's wind load. Past that point, chain holds exactly like rope of the same length — which is the storm-scope lesson in one formula.
How to use Chain Catenary Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula T_straighten ≈ w(L² − h²)/2h — the horizontal load that fully suspends the chain substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Chain Catenary Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula T_straighten ≈ w(L² − h²)/2h — the horizontal load that fully suspends the chain with sources cited on the page
- ✓Catenary is fair-weather magic: at anchor-test loads the sag vanishes and only geometry holds. The snubber's real job begins exactly here — stretch absorbing the surge loads the straight chain now transmits like a steel bar.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
How do I estimate the wind load input?+
The planning rule: load(lb) ≈ 0.004 × frontal windage area (ft²) × wind speed (kt)². A typical 36-ft cruiser presents ~250 ft² → 250 lb at 16 kt, 640 lb at 25, 2,500 at 50 — the squared term is why storms rewrite anchoring. Surge in waves multiplies the static figure transiently, which is the snubber's department.
What does the snubber actually do once chain is tight?+
Provides the elasticity the straight chain lost: a 20–30 ft nylon snubber stretches 10–20% under surge, converting shock loads (which break gear and trip anchors) into longer, gentler pulls. All-chain without a snubber in weather transmits every wave's jerk straight to the windlass and the anchor's set. It's not optional equipment; it's the system's missing spring.
Does a kellet (sentinel weight) restore the catenary?+
It deepens the sag at moderate loads — improving pull angle and damping yaw — but the same arithmetic applies: by the time loads approach the chain-straightening tension, the kellet's 20–30 lb is a rounding error against hundreds of pounds of pull. Verdict from the formula: kellets help below the transition zone, placebo above it. Scope and snubber are the storm tools.
Chain or nylon — what does this math actually favor?+
Chain wins on chafe immunity, weed/coral bottoms and windlass handling; nylon wins on weight aboard and built-in elasticity. The catenary advantage is real but evaporates exactly when holding matters most — so the engineering answer is the hybrid most cruisers converge on: a chain leader for chafe and bottom contact, generous scope for geometry, nylon (rode or snubber) for the spring.
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