Calculate loads and tensions in sailboat rigging
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Download NowEvery sail force the rig produces has to end up somewhere solid, and tracing that path is the whole of rigging load analysis. Wind presses on the sails, the sails pull on mast and shrouds, the shrouds pull on chainplates, and the chainplates pull on the deck and hull, so the design question is whether each link can safely carry the peak load it inherits.
Loads start with dynamic wind pressure, which grows with the square of wind speed, applied over the sail area and levered by the height of the centre of effort. That overturning moment is resisted largely by the windward shroud, and the geometry of the shroud angle turns a share of that moment into enormous tension in the wire.
Enter sail area, mast height, beam and design wind, and the calculator estimates the wind force, overturning moment, mast compression and shroud tension, then suggests a wire size for the resulting loads.
Wind Pressure = 0.00256 × V²; Wind Force = Pressure × Sail Area; Overturn = Force × CE Height; Shroud Tension = Overturn ÷ (Shroud Base × cos Angle)| Shroud Load | Suggested Wire | Breaking Load | Use |
|---|---|---|---|
| Under 2000 lb | 1/8 in | ≈ 1,800 lb | Light day-sailers |
| 2000 - 4000 lb | 5/32 in | ≈ 2,500 lb | Small cruisers |
| 4000 - 6000 lb | 3/16 in | ≈ 3,700 lb | Mid-size cruisers |
| 6000 - 8000 lb | 7/32 in | ≈ 5,000 lb | Heavy or offshore boats |
Guide to wire size by shroud load.
Formula verified against standard naval-architecture references · Updated October 2026
It derives from wind pressure over your sail area, levered by the height of the centre of effort, then resolved into shroud tension by the rig geometry. Because shroud angles are shallow, a modest heeling moment becomes a large wire tension. Enter sail area, mast height, beam and design wind and the calculator estimates wind force, overturning moment, mast compression and shroud tension so you can see the true loads the rig carries.
Because they hold the mast up at a shallow angle. The windward shroud must produce a horizontal component large enough to balance the heeling moment, and at a small angle to the mast that means a much larger tension along the wire, with a big vertical component added to mast compression. Widening the shroud base reduces both tension and compression for the same load, which is why spreader length matters so much.
Choose a gust speed the boat will realistically meet, not its average sailing breeze, since rigging is sized against peaks. A typical coastal cruiser might analyse at 30 to 40 knots and an offshore boat higher, with an appropriate safety factor on top. The pressure grows with the square of speed, so a design at 40 knots sees almost double the load of 28, which is why the wind-speed input dominates the result.
Pick a wire whose breaking load exceeds the calculated shroud tension with a healthy rigging safety factor, commonly around three to four. The table maps shroud load to a starting gauge, but terminals, swages and toggles must match or exceed the wire. Always size the whole assembly to the wire's rating, because a rig fails at its weakest connection, not at the strongest one.
Yes. Every rigging load has a vertical component that stacks onto the mast step, and it can exceed the side force that produced it. The mast, its step, and the structure below must carry that compression without crushing the deck or buckling. This is why keel-stepped masts and properly reinforced steps appear on larger boats, where compression reaches serious magnitudes.
Treat the chainplate, its fasteners and its backing as part of the load path, sized to the shroud tension with margin and bonded to solid structure, not just a thin deck. A correctly rated wire that pulls on a small plate bedded in balsa will tear through. Inspect chainplates for corrosion and cracks regularly, since they sit at the waterline where the loads are highest and the decay is fastest.
Forestay and backstay loads follow from the same wind force and stay geometry, and mast section must resist the resulting compression and bending. This tool focuses on the side-load and shroud path; use standing-rigging and mast calculations alongside it so headsail loads on the forestay and the mast's own strength are all checked against the same design wind rather than assumed.