For the modeling exercise you will need the following materials:

Before constructing your PolySail, lay out your sail plan to scale on a piece of graph paper. (See (1) below.) 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 (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 (3) above.) 

Using this method I found, for example, that the sail illustrated above (a jib-headed sprit sail with battens of about 80 sq. ft.) 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 1-1/2" V-dart in the foot about a third 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. A V-dart in the clew is also important. A slight hollow at the top of the leech (the edge nearest the mainmast on a jib) is also desirable.

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 important technique developed by HR Solutions/PolySails allows polytarp sailmakers to make fully or partially battened sails easily. After struggling to sew batten pockets into a couple of polytarp sails, the author sought and found a batten alternative that works well on polytarp, but is probably unsuitable for sails made from other materials. 

Our answer lies in a recently-released, industrial strength rolls of Velcro. Available in 20' x 2" rolls in white or black for less than $1.50 a foot, this Velcro product features an incredibly strong backing of waterproof adhesive that adheres easily to polytarp. By placing a strip of the loop material directly on the sail and a strip of hook material on a fiberglass or wood batten, a sailmaker can create an "instant" batten. Besides the ease with which these battens are created and attached, a further advantage of the Velcro battens is that they can be easily removed for rolling and storing the sails.

Velcro battens can be used to make fully battened, high aspect catamaran sails, junk sails, batwing sails, and sharpie sails, to mention just a few applications. On small loose-footed sails, such as the 35 sq. ft. canoe sprit, a Velcro batten can be used to hold out the clew. For sails that need sleeves, such as the lateen sail used by the Sunfish, the Velcro strips might be used to create sleeves for the mast and boom in lieu of sewing. (I haven't tried this application yet.)

Along with the modeling technique, the Velcro strips extend the possibilities for easily-constructed polytarp sail applications for small home-built boats. With continued experimentation, white polytarp sails might soon offer a strong challenge to the newer synthetics as a power source for small boats.

MAKING A V-DART IN POLYTARP SAILS

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