For a small sail use heavy cotton canvas if you can find it, a heavy woven tan or white polyethylene tarpaulin as the next best, layered bedsheets only as a stopgap, and reinforced plastic sheeting last because it tears and holds no shape. A 5.6 square meter sail spreads about 382 N at force 5 over a 3 m foot, an average of only 1.27 N per centimeter, so the cloth matters less than how well the corners and seams are reinforced.
Part of Small Sailboat Construction
Sail material is any flexible sheet that can hold wind and take stitching: the traditional answer is cotton canvas, with woven tarpaulin, bedsheets and plastic film as improvisations.
The Cloth Carries Little Load; The Joins Carry A Lot
Spread evenly along a sail’s foot, the wind load is small. Take the 5.6 m² sail from Spritsail Rig: at the top of force 5 the force is about 382 N, over a foot of 3.0 m. That is 382 ÷ 3.0 = 127 N per metre, or 1.27 N per centimetre of edge. But the load does not arrive evenly. If the sheet takes about half of it (190 N, an assumption) through a patch 5 cm wide at the clew, the patch carries 190 ÷ 5 = 38 N per centimetre, about 30 times the average. A sail tears at its corners, along its seams, and at its stitch holes, long before it tears across the open cloth.
That changes how to compare materials. You are comparing resistance to tearing out around a stitch or a ring, resistance to sun and rot, and ability to hold a shape, not just plain tensile strength.
What You Need
- A candidate cloth large enough for a test seam
- Weights or a bucket to hang from a strip (see Rope Strength Testing for the incremental method)
- A sunny wall to expose a sample for a week beside a shaded control
- A sewing kit with thread of polyester or nylon
Method: Rank Your Cloth
Step 1: Weigh a known area. Canvas is often sold by weight: 1 oz per square yard is 33.9 g/m², so a 10 oz canvas is 339 g/m² and a 5.6 m² sail made of it weighs about 5.6 × 0.339 = 1.9 kg before reinforcement.
Step 2: Test a strip. Cut a 5 cm wide strip and load it until it tears; record the weight at tearing as a figure in N per centimetre of width.
Step 3: Test a seam. Sew two strips with the seam you will use (see Cutting & Sewing) and repeat. A seam that gives way below the strip load is the weak link.
Step 4: Test a hole. Sew a ring or grommet into a strip and pull through it; the load at which it tears out is what a corner can carry.
Step 5: Sun test. Leave a sample in the sun for a week; compare its strip strength with the control.
Worked Example: Comparing Four Materials By What You Measure
| Material | Carrier’s verdict | What to check |
|---|---|---|
| Heavy cotton canvas (tent, awning, artist canvas) | Traditional; ideal | Mildew if stored damp; weight when wet |
| Woven poly tarpaulin, heavy tan or white (not thin blue) | Surprisingly effective | UV degradation; tearing outward from stitch holes |
| Bedsheets, multiple layers stitched | Very temporary | Rot; seam pulling out |
| Reinforced plastic film, taped and grommeted | Light and waterproof, tears easily, holds no shape | Sun damage; the tear that starts at a grommet |
Rank by the lowest of the three tests (strip, seam, hole), not the highest, because the sail fails at its weakest.
Common Mistakes
- Choosing by feel — a stiff cloth can be brittle; a soft one can be strong.
- Skipping the sun test on a tarpaulin — polyethylene loses strength in sunlight (hardware-store poly film lasts about one season, per Plastic Film & Alternatives).
- Dressing the cloth with linseed oil for waterproofing without fire precautions — see below.
Safety
Oil-dressed canvas can heat itself
Linseed oil cures by reacting with oxygen and gives off heat. Cloth that is soaked and then bunched or stored in a heap can ignite hours later without any flame. Hang dressed cloth spread flat and keep rags out of piles; see Oiled Cloth & Animal Membrane and Waterproof Fabric.
What’s Next
- Learn next: Cutting & Sewing — layout and seams for the chosen cloth
- Combine with: Grommets & Attachment Points — reinforcing the corners the test shows are critical