Seam allowance is the extra material left along the edge of a sail panel beyond the finished seam line, so that adjoining panels can overlap and be glued and stitched together. In dressmaking the allowance is folded inside; in sailmaking the panels are lapped, so the seam allowance is simply the width of the overlap, usually somewhere between 12 and 30 mm depending on the cloth. The beginner needs to know that this strip is what the seam is actually made of. The sailmaker adds that the allowance is drawn after the shape has been designed, that its width and lap direction are chosen deliberately, and that on a laminated sail it is also where the adhesive lives.
Seam Line and Cut Line
When a sail is designed, the software produces each panel as a flat shape whose edges are the seam lines, the lines where one panel meets the next in the finished sail. Broadseam curves are drawn on those seam lines. Only then is the seam allowance added, pushing the cut line outward by a fixed amount along every edge that will be joined. The plotter or laser cutter marks both lines on the cloth: one to cut along, one to align the neighbouring panel to. If the allowance were added before the shape was drawn, the curves would be distorted and the sail would not fly as designed.
How Wide, and Why It Varies
The width of the allowance follows the load the seam has to carry and the number of stitch rows planned for it. A 0.75 oz spinnaker nylon might have a 10 mm lap and a single row of zigzag. A 9 oz woven Dacron mainsail for a cruising boat might lap 25 mm or more, wide enough for a triple-step zigzag and a second row outboard of it. The double-sided seam tape used to hold the lap while it is sewn is bought in matching widths, so the allowance and the tape are chosen together.
On laminates and membrane sails the overlap is often the same or slightly wider, because the bond depends on adhesive area as much as thread. Where a Tape-Drive® or Titanium™ sail relies on load-bearing tapes or yarns running across the join, the panel seams of the base laminate can be modest, since they are not carrying the primary load.
Which Panel Laps Over Which
A detail that separates a good loft from a careless one is the direction of the lap. Convention is to lap the upper panel over the lower on a mainsail and the leech-side panel over the luff-side on a headsail, so the exposed edge of the seam faces down and aft. Rain runs off rather than into the seam, the raw edge is shielded from the sun, and the edge does not catch on spreaders or the pulpit when the sail is hoisted or dropped. If you look at a well-made sail from the leeward side, all the little steps at the seams face the same way.
Allowance and Sail Shape
Because the allowance is applied after the broadseam is drawn, the shape lives on the seam line, not the cut edge. That is why the sailmaker aligns the panels to the marked seam line rather than to the cut edge when sticking the seam. A panel that is lapped 5 mm short along one seam and 5 mm long along the next will produce a sail with a hard spot and a wrinkle that no amount of trimming removes. The same care applies when a sail is recut: the loft opens the seam, redraws the broadseam on the seam line, and uses whatever allowance is available.
A cruising sail is built with generous allowances and enough spare cloth to allow a later recut when the draft has crept aft; a racing sail is built with the narrowest lap that will carry the load, because seams add weight aloft and small ridges in the aerofoil.
Frequently Asked Questions about Seam Allowance
Is seam allowance the same as tabling?
No. Seam allowance is the overlap between two panels that are joined together. Tabling is the reinforced hem along a free edge such as the leech or foot, which carries the leech line, bolt rope or foot cord.
Why does the lap direction matter?
Lapping the upper panel over the lower puts the exposed edge facing down and aft, so water, sunlight and chafe fall on the covered side. It also gives the sail a consistent, smooth surface on the leeward side where the airflow is most sensitive.
Can a sail be recut using the existing allowance?
Usually, yes. A loft can open a seam, redraw the broadseam and restitch it, moving the draft forward or flattening the sail. The amount of change available depends on how much allowance was built in when the sail was made.
The seam allowance is a few millimetres of cloth you never see once the sail is finished, yet it decides how well each seam holds, how the sail sheds water and sun, and whether it can be reshaped in later life. See also seam, broadseam, tabling and panel.