Many home boat builders are producing simple functional sails from white and canvas-colored polytarp simply through rounding the luff and/or foot and (sometimes) placing an 18″-24″ long by 3″ wide dart in the tack.  For most Bermuda, Marconi, and Leg o’ Mutton sails, this simple technique allows home sail makers the advantage of an easily-constructed sail that allows them to get out on the water and put their newly-constructed craft through its paces.

However, a number of experienced builders, having gained respect for the strength and low cost of the white and canvas-colored polytarp material, are now looking for more sophisticated construction techniques that provide more effective sail shapes from a single sheet of material. To assist these polytarp experimenters who purchase our inexpensive sailmaking kits, HR Solutions/PolySails offers two new techniques for shaping sails. The first is a simple method for modeling sails on graph paper. This technique allows the sailmaker to construct several models of a one-piece sail and to achieve optimum shaping suiting the builder’s purposes prior to making the first cut in the polytarp material. The second technique allows PolySail builders to add removable battens to their sails easily without sewing in batten pockets. Let’s begin by exploring the modeling approach.

For the modeling exercise you will need the following materials:


  •  Sail plan
  • Sharp pencil
  • Graph paper (preferably 17″ x 11″ pad)
  • Scissors
  • Ruler (preferably 18″)
  • Scotch tape
  • (Optional) Small diameter wood dowels to represent masts and booms


Before constructing your PolySail, lay out your sail plan to scale on a piece of graph paper. (See figure 1 below. 17′ x 11″ graph paper is available from Staples.) Include any rounding (outward curves) or hollowing (inward curves) of the sail edges that you plan to have in your final sail shape. Once you have your sail outline down on the graph paper, cut the sail out around the perimeter. Lay this master on another piece of graph paper, and mark around the edges to make other copies. Three copies of each sail that you plan to construct should do the trick.

(1) Flat Cut Out of Sail Plan

(2) Rounded Luff Straightened by Mast

(3) V-darts Cut and Taped Plus Rounding

By rounding the edges of your sail, you will provide some curvature to the sail when the rounded edge is straightened against a mast or boom. (See figure 2 above.) This curvature is enough for a sail to work well in most conditions. In fact, sailors will often hoist a flatter sail in windy or stormy conditions. You can see the effect of rounding an edge by taping the luff of one paper copy of your sail directly to a straight edge, such as a ruler or small diameter dowel representing the mast. Make certain that you straighten out the rounded edge as you tape it to the straight edge. If you have a boom at the foot of your sail and rounding in the foot, you can also tape this edge to a straight edge representing the boom. Again, make certain that you straighten out the rounded edge when taping the edge to the straight edge.

For most sailing situations, you can get more power from your sails if they have additional depth or camber. If you want additional curvature or depth in the body of your sail, you can place V-darts in critical locations. (V-darts are sewn or taped overlaps or seams in the edges or corners of the sail. For an illustration, see page three.)

Using this method I found, for example, that the sail illustrated above (a fully-battened, jib-headed sprit boom sail of about 80 sq. ft. intended for light air use on one of the Bolger/Payson designs such as Brick, Cartopper, Gypsy, Junebug, Surf, or Teal) would benefit from using the following placement of V-darts and edge rounding: Start by rounding the luff by 3″ about one third of the way up from the tack with the rounding tapering away to the tack and head. Round the foot by 2″ about one third of the way from the tack with the rounded edge tapering away to the tack and clew. Place an 18″ long x 3″ wide V-dart in the tack with the dart pointing about one-third of the way up the leech from the clew. Curve the dart slightly so that the belly of the curve is at the top of the dart. Next, place an 8″ x 1″ V-dart in the head.  Add an additional 8″ x 11/2″ V-dart in the foot about three-fifths of the distance between the tack and clew. To help secure these taped V-darts, sew each dart (preferred) or place a grommet through the folded material wherever you have placed a V-dart.

For a jib, rounding and V-darts in the foot of the sail are a virtual necessity. Cutting the luff in a slight reverse S curve with a slight hollow at the top and rounding in the lower portion provides a good entry, and long V-dart in the tack assures good shape. A straight or slightly hollowed leech allows a smooth exit for the air in the slot between the jib and the mainsail.

Using the additional copies of your sail plan, experiment with the best locations for V-darts in your sail plan. Once you have a plan you like, transfer these locations directly to the outline of your sail shape on your poly tarp. Make certain that you place the V-darts in the material prior to taping and folding over the edges.  Note that the V-darts will shorten the lengths of your sail edges, so take this effect into account before cutting out the final sail shape. V-darts in edges can also slightly affect the rounding of the edges. You should be able to see these effects in your paper model and adjust your edge cuts in the polytarp accordingly.

Using this simple graph paper method, you can roughly determine the shape of your PolySail before you construct it. Later, while constructing the sail, you can place bags of mulch or lawn furniture pillows under the sail just back from the luff to help check whether you’ve achieved the desired shape you want in your sail. Usually, the deepest draft should occur about 30% to 40% into the sail as measured from the luff.

A second technique might help with shaping the leech of a sail with a small amount of roach. Using some of the tarp edging, make at least three small batten pockets with the ends about 10” apart. Cut the sharp ends off some bamboo skewers (about $1.00 for 100 at Walmart) and sand them smooth. After sewing on the batten pockets at the edge of the leech, insert one or two skewers in each pocket to act as battens. Here are some pictures of a sailboard-like sail that I made using this technique.

Skewer batten pocket sewn onto a lightweight racing sail.

Note how the skewer/battens hold out the leech. There are also a couple of skewers in the head to help stiffen that part of the sail.

Here’s the 58 sq. ft. sail in action on the Z-PDR. A small sprit was needed near the top of the mast to hold out the peak sufficiently.