A Catenary-Cut Tarp
Jeff Manion (Sorefoot)
The finished tarp setup over a rope in a low configuration.
General description: My goal was to make a lightweight durable dining tarp for canoe/kayak trips in e.g. Quetico Park, Canada, where it often rains with occasional high winds, or for backpacking in the Pacific Northwest. The tarp is a seven-sided design with catenary cuts on the edges and ridgeline. The catenary cuts help keep the tarp taut so that rain runs off smoothly and also prevents the tarp from flapping annoyingly in the wind. The present design is about 100 sq. ft. and is spacious for 2 people and adequate for 4. It would be relatively easy to scale the design. It is not intended to be a primary shelter, though I suppose it could be used as such if bug-proofness is not an issue. However, if that is the main goal, there are probably more suitable designs (such as Henry Shire’s TarpTent).
The design is similar in some respects to commercially available tarps such as the Moss/MSR Hepta/VistaWings (65 and 120 sq ft, respectively). It is not a direct copy, however, and has different dimensions and edge shapes. I have tried to make what I think are some improvements over the commercial designs. My silnylon design will of course be somewhat lighter. Silnylon is quite strong with excellent tear strength, better or equal to 1.9 oz. polyurethane coated nylon. Compared with commercial tarps, my impressions are that its main weaknesses are primarily in puncture and abrasion resistance, owing to the thinner nature of the material. It may have greater flammability for the same reason, although all nylons will burn readily. These are impressions only, however, I have seen no hard data regarding abrasion or puncture resistance, or flammability. I have used significantly less scalloped edges than used in the Moss/MSR VistaWing designs. My feeling is that this gives more usable space, since true rain protection only begins a couple of feet in from the edges.
Weights: My completed 100 sq. foot version weighs 20.1 oz without any stakes or guylines. The batch of silnylon used will affect the final weight by up to about 2 ounces. My silnylon tested out at 1.4 oz/sq.yd, in the center of the typical 1.3 to 1.5 oz range. One could reduce the weight by up to about 4 ounces by not using any grosgrain reinforcement of the perimeter or ridgeline and by using grosgrain instead of webbing for the tie-outs. Those measures would also reduce durability, but may be acceptable for some people. I would not skip reinforcing the tie-out areas to save weight. I use homemade stakes from aluminum arrow tubing (7 g each) so six stakes would add 1.5 oz. For guylines I suggest Kelty Lightline (1.5 oz/50 ft) or 200 lb test Dacron kite line. The tarp does not stay folded well because of the slippery nature of silnylon. I recommend making a small silnylon stuffsack and stuffing rather than folding.
Scaling the Design: The design should scale easily, although some sizes will be much more efficient material-wise, since silnylon is sold only in about 5 ft widths. A Powerpoint file is available to help if desired. Some other possible designs are included. I would be interested in hearing comments from anyone trying alternate designs or with ideas for improvement.
Set-up: The tarp is designed to be set-up with the pointed end higher than the "flat" end. I like to set up the tarp over a rope running down the ridgeline (through loops on the underside to keep it centered). I think this helps support the tarp weight (reducing seam stress) and also helps stabilize it to the wind. The low flat end should face into the prevailing wind. Set this end lower to the ground for better protection from driving rain, or higher for more space in low wind conditions. One could easily add grommets to the tie-out loops for use with hiking or other poles.
Time: This was my first large sewing project. Prior to this I had sewn only a few stuffsacks. I’m not exactly sure how much time it will take to make because I spent a lot of design time on each step. I was also fairly slow and deliberate since I was learning as I went and silnylon is expensive. I would guess it would take a fairly inexperienced sewer about 20 hours if they simply follow the directions. If you have no experience I strongly recommend making a few stuffsacks first. You will need the following sewing skills and techniques: straight stitch, bar tacking, knowing how to sew a flat felled seam.
(no cotton blends!)
(Not fabric store GG ribbon,
which is polyester)
Options and other ideas:
(1) I used scrap silnylon for reinforcing the tie-out areas. Some people have suggested using uncoated material for reinforcement to prevent water from being wicked in and trapped between two waterproof layers. Possibly a good idea. If you go this way I would suggest either uncoated 1.1 oz ripstop or a microdenier taslan. The latter will be stronger and heavier. These materials will absorb water and add weight, however. A DWR coating is recommended to help prevent this.
(2) I have seen a few tarps with pockets sewn in by the tie-outs to hold the guy lines for packing. Could be interesting, although I just stuff everything into a silnylon stuffsack. If you go this route I would use 1.1 oz ripstop or possibly noseeum netting to make the pockets.
(3) Additional guy-out loops or small plastic (Delrin) D-rings. Sewn on either side of the main tie-outs for high wind conditions. Could also use for very light weight main tie-outs, although less versatile than the design I used.
4) 3M Reflective tape. This can be sewn on various places (e.g at intervals around the perimeter and at guy-out points) for increased visibility at night if desired. Available from Outdoor Wilderness Fabrics (and elsewhere).
Step 1: Decide on tarp size. Scale plans if desired – the Powerpoint blueprint file will be helpful for this. Remember to adjust material amounts, especially for larger tarps if the added side seams become longer than 5 feet. I don’t have a good feel for how large a tarp one could make with silnylon before it can’t handle the wind loading. The tongue tear strength of 1.3 oz silnylon (13 lbs) is actually greater than that of 1.9 oz. urethane coated nylon (5.5 lbs), whereas the grab tear strengths are about the same at 140 to 150 lbs for a 3 inch section.
Step 2: Order materials. I used silnylon seconds to save money.
Step 3 (1 hr 15 min): A catenary line describes a free hanging chain strung between two points, so templates can be made as follows: Tape together some cheap poster board to the length of the desired curve. Stick it to the wall and string heavy LIMP line or thin chain the desired distance apart. I used ca. 200 lb test Dacron (polyester) kite line (ca 3/32" diam.). It is designed to be limp and worked fairly well, although there may be better choices. Trace the curve on the board and cut for a perfect pattern. Some tips. Note that the end points MUST be at the same height for a symmetric curve. Draw a straight line the length of the poster board and use a level. You need tension relief at the ends to prevent any extra line length from distorting the curve. Use a few extra pins to put a slack section just after the end points. For the main curves I originally used a 65 1/8" chord length with a 4" drop at the curve center, but you could go to about 65 ½" with same amount of material. Note: Remember that reducing the drop will require a longer side-seam – make sure it fits! Save both the positive and negative curves. Having both will help with the layout. A third will be useful.
Step 4 (45 min): Make template for smaller curve at flat end of tarp. I originally used 40" with a 2" drop, but I think that increasing to about 48" with a 2" drop would provide for more useful area and a more efficient tarp. This would of course also add additional weight. This Step could be delayed until after Step 6.
Step 5 (15 min): Squaring the silnylon end. Place the roll of silnylon on your hotcutting surface, unroll it a couple of feet and use the carpenter’s square, hot-cutter, and long straight edge to square off the end of the silnylon. Unplug the hotcutter so you don’t start any fires!
Step 6 (45 min): Laying out the main curves. Lay out a 65" x ca. 13 foot length of silnylon. The silnylon will have an unusable uncoated or poorly coated strip about 1 inch wide along each selvage edge. Make a mark 2.5 times the seam allowance in from the usable fabric edge. (about 2.5 to 3" from the true edge if you use a 7/16" wide seam like I did) and about 8" from the end. This will be the "pointed" end of the tarp. The extra usable fabric along the selvage edge is important and will be needed later for the ridgeline seam. Using the templates made in Steps 3 and 4, lay out the tarp. You the will need either a tape measure, or a third template to mark the third 65" chord. Note that the templates will extend past the edge of the fabric at the widest points. Use the sharpie to trace the portions of the curves that are on the fabric.
Step 7 (1 hr): Cutting the wings. Use the hot cutter, carpenter’s square, and long straight edge to cut off the silnylon after the far (flat) end of the tarp. It is important that this cut be square. Leave yourself 3 to 4" of extra fabric in case anything is slightly misaligned. Unroll a duplicate section of silnylon and hot cut it to length. You should now have two identical 65" x ca. 13 ft pieces of silnylon, one of which has the tarp layout marked. Returning to the marked piece, hot cut off the triangular piece of excess material extending from the pointy end of the tarp. This can be done either free hand or with the straight edge. Do not cut along the actual tarp outline, leave a couple of inches or so of excess fabric. The triangular piece will be the material used for the side "wing". Repeat with the second piece of fabric. Use the sharpie to mark on the second piece where the curves extend off the fabric.
Step 8 (1 hr 30 min): Sewing on the wings. Place one of the triangles that you just cut atop one of the main fabric pieces so that it extends in from the edge of the main fabric. Make sure that it extends past the pattern marks (Step 6) where the wing curves extend off the fabric. Also make sure that the ripstop grain of the triangle is aligned with that of the main fabric (i.e. that the 65" width of the triangle piece now runs along the length of the main fabric). The narrow end of the triangle should be towards the pointy end of the tarp. Slide the triangle in about 1.5 inches so you are not on the unusable selvage of the main silnylon piece. Making sure the triangle edge is parallel to the edge of the main piece, use the hot cutter and straight edge to cut through both fabric pieces. They will now be slightly melted together which will make them very easy to sew together. Sew together with a 3/8" flat-felled seam. Repeat with the second piece. You should now have two pieces that look something like this:
Step 9 (1 hr 30 min): Cutting out the tarp. I cut the main tarp outline separately for each piece, but one could possibly do it in one step by laying the two pieces on top of each other. There might, however, be difficulties in keeping the layers aligned and cutting through the wing seams, which will be many layers of fabric thick. If you cut both at once, pin the two pieces fabric together along the selvage edge to maintain the alignment. Starting with the "pointy" end lay out the first curve template. It is best to use the "positive" curve template so that any slips of the hot cutter will be on scrap fabric. Weight the template and the fabric to prevent them from shifting during the cut. I used one to 7.5 lb weights from a set of barbells. Following the template with the tip of the hot cutter, cut the curve. I found the wing seam difficult to cut through entirely, but I just kept going and later used a pair of scissors to make the final cut. At the pointy end of the tarp extend the curve by hand to the fabric edge, but mark the true end of the pattern curve with the sharpie. This mark will designate the final true midline of the tarp and will be needed later. Hot cut the other curves. At the flat end also extend the curve by hand to the fabric edge, again marking the true end (tarp midline). Repeat for the second piece. If you cut the two pieces together, gently separate the two sections.
Step 10 (45 min): Making the ridgeline template. Measure the length of the tarp between the true end marks using the tape measure. Make a catenary curve template as described in step 4. I used a 5" drop for a 12 ft ridgeline. Some posters on Backpacking.net have suggested a drop of ¼ inch per foot based on their experiences (i.e. 3 inches for 12 ft ridgeline). The greater the drop, the more material will be lost and the tarp area reduced accordingly. Moss/MSR used much larger drops. I have not experimented with different designs, although if the drop is reduced too much I would at some point expect loss of tautness.
Step 11 (1 hr): Cutting the Ridgeline. It is slightly tricky to end up with the ridgeline seam perfectly centered after sewing the flat-felled seam. However, it is more difficult to explain than to do. Refer to the figures to help with steps 11 and 12. If you are not sure you understand, I would suggest doing a short trial seam on scrap fabric. To begin, lay the two pieces of fabric on top of each other with the flat edges aligned. The top piece should have the sharpie marks you made earlier that indicate the true tarp midline While maintaining the lengths even, slide the top piece away from the flat selvage edge 1x the desired seam width. The flat edges will now be misaligned by the seam width. Thus for a ½" seam the top piece would overhang the bottom by ½ inch on the wing side. On the straight selvage edge carefully place pins or staples through the waste material on the selvage to keep the two pieces aligned in this manner. Place the ridge curve template on the sharpie marks you made earlier on the top fabric piece that indicate the true tarp midline. Now slide the template straight over 2.5x the seam width toward the flat selvage edge. Carefully weight the template and fabric to keep them aligned and hot cut the seam. Extend the ends of the curve out by hand. The two pieces will now be melted slightly together. Do NOT separate the two pieces as this will keep the seam aligned in the next step. See Figures A and B. In these drawings the two pieces of fabric are shown in different colors for clarity (pink, blue, and purple when overlapped).
Step 12 (1 hr): Sewing the Ridgeline. One piece will be slightly wider than the other. This piece should now be on top. Fold the edge fabric up and in (not under) 1 seam width (I used 7/16" but skilled sewers may wish to go smaller) and sew a seam the length of the tarp. You will be sewing through 4 layers of fabric. You will probably need to increase the thread tension to get a good seam (see Thru-Hiker.com for some tips here). Test tension first on 4 layers of scrap fabric. Depending on your experience, you may wish to use pins, staples, or binder clips along the folded edge to keep the seam width even. Because the edges are melted together by hot cutting, the fabric pieces should stay nicely aligned. When you are through, fold the entire upper piece of fabric back and sew the second seam to complete the flat felled ridge seam. You will need to roll up the inner side of the tarp as you feed it through the sewing machine. Be careful not to sew through any excess layers of fabric during this step. The translucent nature of silnylon will help here. Trim off the small amount of excess fabric at the seam ends. Congratulations, the outline of your tarp is now complete. See Figures C and D.
Step 13 (3 hr): Reinforcing the tie-out attachment areas. The area where the tie-outs are attached should be reinforced to help distribute the force. From some reading on sail repair, I decided to do a double reinforcing patch near the point of attachment of the side tie-outs and a single layer further out. I used only a single layer on the ridgeline tie-outs, as the ridgeline is also reinforced with nylon grosgrain. I could find no information on what size patches were best to distribute the load and so just guessed. Each patch will form a triangle. For strength, the base of each triangle should be cut along the grain of the fabric (i.e. not on the bias). Hot cut seven large and six small triangles. These will be sewn on the underside of the tarp. Mark out 10 inches from each side of the point, fold the long edge of the triangle under 3/8 of an inch and sew between the marked points. Run a second line of stitching for strength and sew around the edge. Cut away the excess fabric with scissors. Mark out 5 inches and sew on the small patch in the same manner. Reinforce the small patch with a box and X stitch pattern (see photos). Reinforce the flat end of the ridgeline with a 2 x 10 inch rectangular patch and reinforce with a box and diamond stitch pattern (see photo). Make sure all patches are sewn on the underside of the tarp.
Step 14 (3 hr): Reinforcing the perimeter with grosgrain. Use ¾" nylon grosgrain (GG) ribbon to reinforce the entire perimeter. This will help the bias cut curves maintain their shape under tension and make the tarp easier to handle. Each curve section will be sewn separately. Fold under ¼ inch of the end of the GG and sew a line to hold it. Now fold the GG in half lengthwise around the edge of the tarp and attach it with a single line of stitching. When you get to the end of the curve, cut the GG ¼ inch longer than needed and fold the end under to seal. Start the next curve as before, overlapping the start with the end of the previous curve.
Note 1: As shown in the photos, I cut off the points at the tie-outs to make small flat sections. In retrospect this just made more work and I would not repeat this.
Note 2: A lighter, but less strong and durable alternative used in tarps is to simply double fold the silnylon edge under and sew a seam. I have not tried this. You will also lose a few square feet around the perimeter (3.25 sq. feet for a ½" seam).
Step 15 (1.5 hr): Reinforcing the ridgeline with grosgrain. Use ¾" nylon GG ribbon to reinforce the under side of the ridgeline. Also cut eight 2 inch pieces of ¾" GG. Cut eight each 1.75 inch mating pieces of 5/8" Velcro. These will be used to form loops through which to run a rope along the underside of the tarp. Making sure you are working with the under side of the tarp face up, use your Sharpie and straightedge to draw a line 3/8 of an inch out from the ridgeline seam center. (I did not do this and found it difficult to keep the GG centered, as it is a blind seam) Starting about 3 inches in from the edge make marks about every foot or so the length of the seam. These will mark the placement of the GG and Velcro rope loops. Begin sewing on the GG with a single line of stitching near one edge of the GG, using the line you just drew as a guide. Starting 3 inches in and at every other mark thereafter insert one of the 2 inch strips of GG about 3/8’ under the ridgeline ribbon and sew to attach. At the next mark, sew on a piece of Velcro in the same manner. Alternate GG and Velcro at each mark thereafter. Sew the complete line of stitching. You will now sew a second line of stitching near the opposite edge of the GG. When you come to the short GG pieces, tuck the free end under the ridgeline GG and sew across to complete the loop. When you come to a Velcro piece, place the mating Velcro half under the GG and sew on. Make sure the Velcro pieces will mate properly when they are joined in a loop.
Step 16 (2.5 hr): Attaching the Tie-outs. I used 1/2" and 3/4" webbing in a 3-dimensional pattern designed to (optionally) take canoe paddle handles for support. The eight tie-outs weigh 3/4 oz total. I know some people have used nylon GG for tarp tie-outs with good success and say they are durable. Going with the GG would save about 0.4 to 0.5 ounces. I opted for durability, since I find canoe trips to be rougher on equipment than backpacking, but the choice is yours. I made each tie-out from one 8 inch section of ½" nylon webbing and one 5.5 inch section of ¾" webbing. To slightly ease set-up, I color-coded the pointy end of the tarp with red webbing. Fold the end of the ¾" webbing around the center of the ½" webbing and bartack with a box pattern. You should now have a tee shaped piece of webbing. Repeat for all eight tie-outs. You will sew the tie-outs to the underside of the tarp first. Begin by placing the two ½" webbing ends side-by-side about 1.2 inches in from the edge of the tarp. Using a straight stitch, sew the webbing to the tarp with a box pattern. Now turn the tarp over and center the 3/4" webbing between the box stitching pattern. Feel through the fabric to align the ends of the ¾ and ½ inch webbing. Now sew a box pattern using a bartacking stitch. Repeat for all tie-outs.
Step 17 (2 hr): Seam sealing. Thin silicone sealer with mineral spirits and paint on with cheap disposable brush. I found it easiest to set the tarp up outside and do this. This will help with drying and prevent killing too many brain cells with the solvent.