Standing rigging is the set of fixed ropes, shrouds to each side and a forestay to the bow, that keep a small boat’s mast upright, and the tension in a shroud is set mostly by its angle: in the worked example a 382 N sail load at force 5 produces about 1,030 N in a shroud attached 0.6 m outboard of the mast and about 532 N when it is attached 1.2 m outboard. A narrow shroud angle also pushes about 1,018 N straight down on the mast, so the shrouds must be sized from a computed load and a tested rope, not from appearance.
Part of Small Sailboat Construction
Shrouds and stays are ropes or wires in tension that hold a mast against the push of the sail, carrying the load into the hull.
The Shroud Angle Multiplies The Load
A shroud carries only tension. The wind pushes the sail sideways, the sail pushes the mast, and the windward shroud pulls the masthead back. Because the shroud is almost vertical (the chain plate is only a little way out from the mast), most of its tension is not holding the mast sideways; it is pulling down. Only a small fraction, the horizontal component, resists the wind. To get that small horizontal force out of the shroud, you must put a much larger total tension into it. The leeward shroud goes slack and carries almost nothing.
The forestay balances forces fore and aft; a backstay is optional (per the carrier). For a short mast, shrouds alone may do, which is a reason to keep the mast short.
What You Need
- Rope or wire whose breaking strength you have tested, or read from its tag (see Rope Strength Testing)
- Eye splices or secure loops at the masthead (see Eye Splice)
- Chain plates: metal straps bolted through a rib or frame, not through a single plank
- A way to tension: a lashing with a few wraps (a lanyard), or a block and tackle
Method: Compute The Load, Then Size The Rope
Step 1: Set the geometry. Masthead height above the hull attachment H = 4.0 m; chain-plate offset from the mast e = 0.6 m (half the 1.2 m beam); sail force F = 382 N, the top of force 5 on the 5.6 m² sail from Spritsail Rig; sail centre 1.6 m above the step.
Step 2: Find the horizontal pull needed at the masthead. Taking moments about the step: T_h × H = F × 1.6, so T_h = 382 × 1.6 ÷ 4.0 = 153 N.
Step 3: Scale up by the shroud angle. Shroud length = √(0.6² + 4.0²) = 4.045 m, so tension T = 153 × 4.045 ÷ 0.6 = 1,030 N. The downward pull on the mast is 153 × 4.0 ÷ 0.6 = 1,018 N, the weight of about 104 kg (1,018 ÷ 9.81).
Step 4: Move the plate out and recompute. At e = 1.2 m: length 4.176 m, T = 153 × 4.176 ÷ 1.2 = 532 N, and the downward pull is 153 × 4.0 ÷ 1.2 = 509 N. Doubling the offset halves the load. A spreader (a short strut at the mast that holds the shroud away from it) can have the same effect.
Step 5: Multiply by a safety factor. Use the corpus’s 5:1 for shelter lines and 10:1 where a failure could hurt a person: 1,030 × 5 = 5.15 kN and 1,030 × 10 = 10.3 kN of breaking strength needed. A knot at the end cuts the strength by about 35-60% (see Eye Splice), so with a knot a 5:1 target needs 5.15 ÷ 0.65 = 7.9 kN or even 5.15 ÷ 0.4 = 12.9 kN.
Worked Example: Is Hand-Made Rope Enough?
Rope Making gives 100-200 lbf for a quarter-inch (6 mm) cord, which is 445-890 N (1 lbf = 4.448 N). A 6 mm cord is therefore a long way short of the 5.15 kN needed. If strength scales with cross-section area (an assumption that holds only for equal quality), the diameter needed is 6 × √(5.15 ÷ 0.445) = 20 mm for the weaker cord and 6 × √(5.15 ÷ 0.890) = 14 mm for the stronger. At 10:1 it is 29 mm and 20 mm. That is why a loaded small rig is best rigged with salvaged wire, a tested rope, or kept to light wind with a reduced sail.
Common Mistakes
- Tying shrouds at the masthead with plain knots — see the loss above; splice or use a rolling hitch with a tail.
- Chain plates through a single plank — the vertical pull (1.02 kN here) can lift the plank off its fastenings.
- Tight shrouds on a loose hull — the compression of the shrouds can deform a lightly built hull.
Safety
A failed shroud drops the mast
When a windward shroud parts, the mast falls to leeward with the sail attached. Inspect splices and rope condition before every outing, check the chain plates for loosening, and replace any rope showing chafe. Wet plant-fibre rope loses 10-30% of its strength and rope in direct sun about 10% per week, according to Rope Strength Testing.
What’s Next
- Learn next: Running Rigging — the ropes that move
- Combine with: Mast Construction — the mast the shrouds relieve