Introduction: Polytarp Sailmaking - Crab Claw

This instructable describes the construction of a simple sail from cheap material. The sail shown here is a "crab claw" or Oceanic lateen, which is being built for my nearly-finished proa. Anyone who has been following Tim Anderson's excellent series about canoe building might be interested in this tutorial, until Tim does a better one that is.

The techniques shown here come partly from Gary Dierking's book "Building Outrigger Sailing Canoes", and my experience building a few sails in the past.

If you have any questions, suggestions or criticisms, please comment.

Step 1: Preparation

- Polyethene tarp fabric. I got mine from a canvas supplier. It's 230g/m2, 2m wide and cost me about AU$7 per metre. An alternative source of new material is pre-made tarps which can be cut down, and this way you might get some usable metal rings, webbing etc. You could probably get scraps from any business that does heavy sewing, like upholsterers, sailmakers, places that do awnings and canvas work.

- Nylon webbing. This is used for reinforcements and in my sail, for lacing points. I bought it cheaply when I got the tarp but you could get old car seatbelts for nothing from a wrecker and slit them down the middle with a hot knife. Leather or heavy fabric would work too.

- Thread. I used various thread that I already had. Use the heaviest your sewing machine can handle, or whatever seems reasonable. Probably best to get this from a sailmaker or online. Make sure it's polyester.

- Stainless steel rings. I had some of these already. If you found some scrap stainless rod you could roll your own and weld the join pretty easily.

- Eyelets/grommets: I didn't use any of these, but if you do make sure they are brass or stainless steel, not brass or chrome plated steel. Steel will rust, break and make your sail ugly.

- Double sided tape: I bought some stuff intended for laying carpet. It's super sticky and great for holding things in place while you sew. I had to cut the roll widthwise with a knife as the stuff I bought was too wide.

- Sewing Machine: Someone you know has one they don't use. Older is better generally. When you find a machine spend lots of time sewing samples of your fabric/webbing with the thread you'll use. Fiddle with upper and bobbin thread tension until it works reliably. You may need to experiement with different sized meedles too.

- Scissors: Some heavy shears and some thread nippers are a good combination, but whatever you have is fine.

- Unpicker: Just get one.

- Small, sharp knife: Use this for everything.

- Measuring devices like a tape measure and ruler

- Permanent marker.

Step 2: Marking Out

Read Gary's book, do some sketches and look at your boat. Work out what you want your sail to do and look like.

Mine is for a 6.9 metre long proa. It's symmetrical about the centre seam. The luff and foot are both 5m long with 15cm of rounding at mid span. That was a guess. The leech is about 3.8m I think, and is scalloped for looks and to prevent flapping.

We used taut strings to mark long straight lines, a piece of aluminium extrusion for shorter ones and a piece of �6mm steel rod as a flexible batten to draw long, fair curves. When we needed a right angle, we did a trick with some string and equidistant marks along a baseline.

When we marked out the first panel we forgot all about adding a seam allowance. This just meant we added a double-width seam allowance on the second panel so no great problem. Think ahead though.

Be careful, don't sit on the nails sticking out of your straightedge.

Step 3: Broadseaming and Edge Curvature

If I was to cut out a perfect triangle of cloth and attach it to straight, inflexible spars, the sail would be flat, and form a conic surface when under wind force. Since my spars are somewhat flexible, and I want some belly in the sail for improved power, the shape will be modified in two ways:

1. The edges that will be laced to the spars are bulged out to make a convex curve. For my 5m luff and foot, I added 15cm with the centre of the curve mid-way along. When sailing in light wind, the spars will stay straight and the curves will make the sail full. As the wind increases, the spars bend to hopefully match the cut-in curvature, flattening the sail.

2. The width of the seam created by overlapping the straight panel edges is gradually increased towards the tack. I added 25mm to the seam, tapering back to the normal width for 600mm along the seam. This process is called "broadseaming" and will create fullness in the sail near the tack.

In the photo, the oblique line is where I aligned the edge of the second panel. The far line is there to be confusing.

Step 4: Cutting Out

Pretty simple, use your sharp instrument to cut along the lines.

Step 5: Pre-assembly and Sewing the Main Panels

Use short pieces of double-sided tape to assemble the main panels. You don't need to use tape the full length of the seam, that will gum up your sewing machine. Sew a flat lap seam to join the panels, two rows of stitching, the widest and longest zig-zag your machine can do. Reading this it sounds easy, but unless your machine has super-powerful feed and is sunk flush into the floor like sailmakers have you're wrestling a stiff, slippery piece of plastic, trying to keep things accurate and even. Good luck. My stitching does not look pretty.

Step 6: Edging and Corner Patches

The edges and corners of the sail are subjected to the highest stress, and are reinforced with more fabric.

Cut some 65mm wide strips. You need enough to do the luff and foot of the sail but they don't need to be continuous, just overlap where the pieces join. Fold and crease the strips down the centre. These strips are sewn over the raw edges of the main panels with two rows of zig-zag stitching.

The corner patches can be any shape that makes sense to you, considering the loads in these parts of the sail. I used the shape Gary Dierking shows in his book. Big triangles sewn on both sides would be fine, and you could attach them under the edging strips.

Step 7: Finishing Off - Webbing Reinforcements and Leech Hem

The points where the sail attaches to the sail are reinforced with nylon webbing. The head and clew use a simple strip of heavy webbing looped over the corners and sewn on both sides. The tack uses three pieces of narrower webbing passing through a stainless steel ring and sewn to the sail so as to distribute loads.

To allow the sail to be laced to it's spars, I used little loops of webbing spaced at roughly 40cm centres. I splayed the ends of the webbing into a "V" to distibute loads better. Eyelets would be faster here and probably more convenient to use. Double sided tape was used to stick all the webbing pieces in place before sewing.

Photo 1: The tack of the sail. Also visible are the reinforcing patches and two of the lacing loops.
Photo 2: The tools used to cut make webbing pieces. Melt the ends or they'll fray quickly.
Photo 3: Sewing the heavy webbing at the head of the sail. This webbing came from an old backpack. There are close to 10 layers of cloth plus two of heavy webbing here and the machine could just barely do it. Much frustration and broken thread.
Photo 4: Sewing webbing at the tack. The very corner was cut off the sail, allowing the stainless steel ring to fit in compactly.
Photo 5: Close-up of a lacing loop. You can probably see the stitching which shows where the webbing is on the other side.
Photo 6 and 7: The leech was cut to a concave curve for looks and to stop flapping as the tarp stretches. I hope I curved it deeply enough. The edge was folded over and sewn into a simple 1cm hem.

Step 8: Lacing to the Spars

The sail was triumphantly brought out onto the lawn in the heat and we laced it to the spars. I didn't have any cord thick enough so we used some thin cord doubled over.

Before lacing, I attached and tensioned some in/down and out/uphauls to pull the corners of the sail away from each other. I still need to drill some holes or mount some saddles on the spars to attach these properly.

Lacing progresses like this: Tie cord off to tack, through first loop, around spar, though loop again in the same direction and on to the next loop. Keep a reasonable tension on the cord as you go and tie it off at the end. You could probably play with sail shape by using individual ties and adjusting their length.

There are a few other photographs, showing some details of the rigging of my canoe. None of this is very refined yet, it's just been thrown together to see how it looks.