A functional sail for a 4-5 m boat is made by laying out the shape on a flat surface, cutting cloth with a 5 cm hem allowance on every edge, joining panels with overlapping flat-felled seams in polyester or nylon thread, and reinforcing every corner with extra layers because the corners take the highest loads. The cloth can be canvas, a heavy woven tarpaulin, several layers of bedsheet or reinforced plastic sheeting, in descending order of durability.
Part of Small Sailboat Construction
Sail making is the craft of cutting and sewing cloth into a shape that holds wind, with seams and corners strong enough for the load.
The Corners Decide How Long The Sail Lives
The cloth is rarely the first thing to fail. A sail is a flexible surface pulled at three or four corners by ropes, and nearly all the force passes through those points. A sail with sound cloth and weak corners tears out at the clew within a few outings, so the order of work is: pick a shape, pick the cloth, join it, and spend most of the effort on the corners and edges.
The four edges have names from the carrier: luff (mast edge), head (top), leech (outer edge) and foot (bottom). A rope stitched along the luff and foot, called a bolt rope, takes the load that would otherwise strain the cloth edge.
What You Need
- Cloth for the area, plus 5 cm hems all round (see Sail Materials)
- Strong thread: polyester or nylon upholstery thread; cotton thread rots
- Needle and palm or sailmaker’s awl, and a heavy needle for the canvas (see Needle Making)
- A flat floor large enough to lay the whole sail
- Rope for the bolt rope and a few metres of cord for lacing
Method: From Shape To Finished Sail
Step 1: Draw the shape full size with chalk or a taut cord (see Cutting & Sewing).
Step 2: Cut the panels with 5 cm added to every edge for hems, and orient them so the weave runs along the edge that takes tension.
Step 3: Sew the seams flat-felled: fold, fold again, stitch through four layers.
Step 4: Reinforce the corners with extra patches, then fit grommets or hand-sewn rings (see Grommets & Attachment Points).
Step 5: Fit the bolt rope and hem the edges. Run a rope along the luff and foot.
Step 6: Test on land. Hoist it in light wind and look for puckers or loose areas that indicate a mismatch of seam to load.
Worked Example: Cloth For A 5.6 m² Sail
The trapezoid from Spritsail Rig has foot 3.0 m, head 1.9 m and height 2.3 m. With the 5 cm hem all round the cut shape measures 3.1 m, 2.0 m and 2.4 m, or (3.1 + 2.0) ÷ 2 × 2.4 = 6.1 m². If the cloth is 1.0 m wide and the strips run parallel to the foot, three strips with an assumed 4 cm overlap at each seam cover 3 × 1.0 − 2 × 0.04 = 2.92 m, enough for the 2.4 m height. Cutting every strip at the 3.1 m maximum width gives an upper bound of 3 × 3.1 = 9.3 m of cloth to buy; the strips above the first are shorter because the sail narrows, and the offcuts become corner patches.
Common Mistakes
- Spending the effort on the middle of the sail — the cloth in the centre is almost never the weak point.
- Cotton thread — it rots and the seams fail while the cloth is still good.
- No hem allowance — the cut edge frays and the stitching pulls out.
Safety
Oiled or tarred cloth can ignite
If you dress the finished sail with linseed oil, hang it spread flat; never leave oil-soaked cloth in a heap, because the curing oil gives off heat and can ignite it. See Oiled Cloth & Animal Membrane.
What’s Next
- Learn next: Sail Materials — choosing the cloth
- Combine with: Cutting & Sewing — seams and layout in detail