MYOG Silnylon 1P Mid Tarp

Discussion in 'DIY & MYOG' started by Graham, Jan 3, 2016.

  1. Graham

    Graham Thru Hiker

    As requested in another thread :)

    In early 2013, I had a trip planned where I expected to cook in my shelter a fair amount and would encounter mostly small/uneven pitches. I decided that I needed an easy to pitch, lightweight shelter that used my trekking poles and had a small footprint. Decent headroom and vestibule space were needed.

    A “Mid” looked to be the best option but the Mountain Laurel Designs ("MLD") SoloMid at the time was a little on the small side (the SoloMid 2015 ironically has similar dimensions to the following design) so in 2013, I opted for the MYOG route…


    All that effort just to make a packable clothes horse!

    Facts and Figures
    • 1.25m wide x 2.65m long rectangular shape, 1.22m high at the apex.
    • Trail weight: 445g, including seam-sealing, guy lines and silnylon stuff sack.
    • 5.5m Ripstop silicone coated nylon (50 g/sqm)
    • 20cm square piece of spare dyneema material I had lying around
    • Lineloc2 x 9
    • Coil zip, width 6 mm, length 136 cm (two-way)
    • Rasant thread 75 and 120
    • 0.4m velcro
    • Grosgrain (2.5m of 20mm and 1.0m of 6mm)
    • Bungee cord (0.6m of 4mm and 0.5m 2mm)
    • KAM snaps x 3
    • Small delron buckle
    • 5 mitten hooks
    • 2 mini cordlocks
    • Builders merchants clear silicone sealant / white spirit
    • Trimble SketchUp (CAD) / MS Excel
    • Large cutting mat / 1m and 30cm metal rules / Sharpie / Rotary cutter
    • KAM tool
    • New Home domestic sewing machine / size 12 needle. Bigger needle for grosgrain webbing bar tacking.
    • Plastic spatula / Greaseproof paper
    • Cardboard (for templates)

    I used the freeware Trimble SketchUp extensively for the design. This is a fantastic, easy-to-learn CAD tool that I relied on heavily to get the major dimensions of the shelter finalised and then to finesse the design. It’s a great tool for working out material amounts (square metres/yards), for measuring lengths, angles etc. and for playing around with different shapes/sizes. Once I’d got the design finalised in 3D, I transferred the panels onto a 2D representation of the fabric and orientated the required panel shapes to minimise the amount of fabric required. I’ve shown two examples of my use of SketchUp below to give you some ideas of its capabilities.

    This was something I did to work out the shape of the material required for the vent, including the size of the opening – in case you’re wondering about the dimensions shown in the image, I was NOT working to 0.1mm tolerance!

    Starting with the well-known Mid design, my own design work probably took around 2 hours to finalise using SketchUp, including printed versions of panels with dimensions and a step-by-step work list (build notes) so that I could schedule work to avoid being held-up at any point in the project. For example, I got reinforcement patches cut and stuck in place on the main panels so that they would have time to bond without holding up the rest of the build.

    Shelter_3.jpg As this figure shows, SketchUp is really useful for visualising the design and for getting some idea of its practical use. In this case I removed half of the vent and the left door and other details, so that I could ‘fly’ inside the designed shelter and put representations of my standard sized polycryo sheet, an A-frame pole setup, me lying down/sitting up with an offset single pole setup and with my pack etc. in the vestibule. I also created a basic copy of the MLD solo inner and placed that inside the CAD version of the shelter to check the fit and distances between the two walls at all points. At this point I knew the quantities I required and went ahead and ordered the materials from

    While I was waiting for the materials, I copied across the SketchUp dimensions (including seam allowance) onto large cardboard templates (by joining together pieces from cardboard boxes). Although things were greatly simplified because of the simplicity of the four-sided shelter, rather than mark-out directly onto the fabric I liked the idea of using templates. At this point the templates had straight sides but that made it easy to double-check that the ‘seam edges’ were exactly the same length.

    This shot shows the two sections of the (longer and thinner) side panels with catenary cut. For the side panels, the most efficient use of material is to lay two side panel templates head-to-toe on the fabric to form a large parallelogram. However, because of their length and width it wasn’t possible to get complete panels in one piece, without a lot of fabric wastage. By splitting the panel into two and having a separate small triangle section near the apex (see photo), the amount of fabric required is minimised. MLD does the same thing on its Mids, a smart idea that I followed. It does introduce an extra seam of course, although it’s a short one.

    The other panel template was half of the rear panel and was used to mark-out both the rear panel and the doors. This template had a right angle, it was flipped over to mark out the left and right hand doors and for the rear panel, which doesn’t have a seam.

    Seam Allowance
    s: I used a 15mm allowance for the panel-to-panel seams and a 30mm allowance for the hem, which was rolled twice to give a 4-ply (plus another 2 ply with the reinforcement patch) strong hem that the tie-out webbing would be stitched to. Note the angular sections on the templates at the lower right corner and the little V notch in the apex of the small panel due to the seam allowances.

    Catenary cut:
    I used a 50mm deflection catenary cut over the ~1.9m length corner seams after reading various MYOG posts and deciding to have a relatively conservative cut. One concern I had was that with the long seams if the catenary cut was too aggressive then the felled seam might pucker up as it was stitched. Actually this concern was unfounded, the seam lay perfectly flat and I could (probably should) have gone with a little more deflection, say 75mm, to tighten-up the fabric even more. The curve was calculated using an Excel spreadsheet downloaded from one of the MYOG forums (can't remember which sorry). Once I’d marked out the deflection at 40 points and added back on the seam allowance, I cut the new edge and then put packing tape along the length of the template edges to get a smooth line for marking-out the fabric.

    I decided to have a LineLoc for each door, rather than MLD’s single LineLoc so I had flexibility over which door to open, if I only wanted one open. I rejected the idea of a ‘waterproof’ zip and went with a regular zip and storm flap with KAM snaps to secure the flap at the bottom and part-way up. Unlike MLD’s Mids, I wanted a 2-way zip so that I could open the zip down from the top if needed. My design is a bit of a compromise however because zipping down from the top doesn’t allow a flap of material to be opened. Instead, an opening in the unzipped section needs to be created using a stick, toothbrush or other dual-use item. It sounds pretty Heath Robinson but in practice quite a large and effective second vent can be created this way and the shelter is still weatherproof, providing rain is not driven directly at the door. The reason why I took the above approach is due to the limitations of the structural components of the door/vent design described below.

    Vent design:
    I spent quite a bit of time working on different door/vent design options and I’m not sure that I’ve got the best solution, although it does work. I decided not to stiffen the vent peak with a plastic insert or similar and in practice the peak naturally sticks out and doesn’t flap in the wind, so I got lucky on that rather than it being good design. It’s probably because the Velcro strip inside that is used to close the vent is stiff enough to help form the small peak. I used the ‘hook’ length of Velcro on the inside of the peak, as this is usually stiffer than the ‘loop’.
    Beneath the vent peak, the door zip terminates in a closed-end. A piece of grosgrain joins the end of the zip and the apex of the shelter (see left). This short piece of grosgrain is designed to be taut when the doors are closed and the LineLocs are pulled tight. The grosgrain ensures that the material around the base of the open vent is not stressed. It would not be possible to have an open-ended zip at this point, unless the force could be transferred some other way.

    Other design / finish notes:

    • 9 Lineloc tie-outs around the shelter hem (one at each corner, two at the front/centre, one at the rear/centre and two in the centre of the side panels – although the shelter is only 1.25m wide since these panels would normally face into the wind I added them as a belt and braces approach.
    • 3 mid-panel tie-outs, for the rear and two side panels. Thought about a tie-out for the doors (like the DuoMid) but decided against – could be added easily.
    • 6mm grosgrain (in a contrasting lime green) was used for door tie-backs and to attach mitten hooks.
    • A mitten hook was attached in the apex of the shelter for an inner tent/light and 4 mitten hooks were added mid-way up the corner seams for bivy/washing line.
    • External grosgrain loop at apex, in case I happen to see a shelter tie-off so I can go pole-less.
    • Small side-release buckle to secure both doors together.
    • KAM snaps along porch storm flap, to reduce flapping in the wind and to relieve pressure on the zip.

    The complete build phase took around 25-30 hours (going steadily) with an elapsed time of about a week; mainly working in the evenings and allowing for bonding to attain full-strength etc.

    This was the largest scale project for me at the time, so the first step in the build was approached cautiously as I wasn’t sure about accurately cutting-out such large panels. That was sorted by the usual “measure twice, cut once” approach and making sure that the slippery silnylon fabric was held firmly in place. I used bags of rice, sugar etc. to weigh down the material (without stretching it), placed the cardboard templates onto the material and marked out the panels with a Sharpie. I’d then check everything before cutting out the panel shape using a rotary cutter. I prefer to use a rotary cutter as it’s easier for me when working with longer lengths of material.

    Once the main panels were cut, I cut out matching reinforcement pieces for the tie-out points ensuring that the ripstop pattern on the reinforcements were aligned to that of the panels i.e. don’t criss-cross. The reinforcements were bonded to the panels using undiluted clear silicone sealant. Not the quick-drying stuff. I worked on one reinforcement at a time, putting a small amount of sealant on the piece of fabric and spreading it very thinly using a plastic spatula, almost removing all the sealant so it left a thin film. The reinforcement was then stuck to the main panel and sandwiched between greaseproof paper with books and other heavy objects on top. I’d have about 10-12 reinforcements drying at any one time, left for at least 24 hours to fully bond.

    While these main reinforcement pieces were bonding I got to work on cutting out the smaller pieces of material for the vent, storm flap and various reinforcement pieces I’d identified e.g. the dyneema reinforcement for the inside of the apex, small circular silnylon reinforcements around the 4 mitten hooks, door tie-outs and the seam joins near the vent (used an egg-cup for sizing of these). For the mid panel tie-outs, I used small saucer sized circular patches with square patches, I’ll describe this later.

    Once the reinforcements on the main panels had bonded it was time to start sewing. I’d never sewed such long seams before – at almost 2 metres in length I worried about getting the bottom and top stitches even, particularly as I was working with silnylon. I used a felled French Seam or Top Stitched French Seam for the main seams, it’s relatively simple and produced even seams (this video shows this really well)

    All the major sewing work was done with Rasant 75 thread. I used 120 on the lighter stuff only. A few test runs were done of some long, thin silnylon off-cuts to reassure me that I wouldn’t end up with an embarrassing mismatch at the end of the seam. I only bothered to pin at the very start and finish of each seam and at the quarter and half-way points of the seam edge. The catenary cut was very gradual so it was basically like working with a straight seam. I took my time, making sure to pull the material apart to help lay the felled part of the seam flat (see video) and the sewing machine produced a neat and strong felled seam. The stitch length on the seams was 3 stitches per 1cm.
    The build steps went as follows:
    • Sew all reinforcement patches
    • Sew side panel apex pieces to the main side panel sections.
    • Completed side panels sewed onto the back panel, sewing from corner almost to the apex, with a small gap left at the apex itself.
    • Doors sewed to side panels (by this time the whole project was getting pretty big and I’d learnt from earlier MYOG projects that I needed to be very careful to make sure that renegade sections of fabric didn’t get sewn by accident).
    • Install the zip and zip/apex grosgrain.
    • Install loop Velcro section (for vent closure) on top of door.
    • Sew on the pre-fabricated storm flap to left door (no KAM snaps at this point but reinforcement patches added).
    • Sew pre-fabricated vent peak along the two front corner seams. Vent had ‘hook’ Velcro strip sewn already.
    • Dyneema cone was sewn inside the shelter apex with a small gap at the very apex left open.
    • Grosgrain with mitten hook and the zip/apex grosgrain were then inserted through the remaining small gap in the apex and bar-tacked. Excess was cut and heat-sealed.
    • A small 75mm long sided cone of silnylon was then bonded with silicon sealant around the outside of the shelt er apex and allowed to cure. I also bonded the pre-fabricated external mid-panel tie-outs. Frankly, that was the stressful part over and I took a day’s rest I seem to recall!!
    • When cured, an external grosgrain hang loop was added at the top of apex.
    • The 3 mid-panel (square) reinforcement patches were sewed.
    • Perimeter hem was rolled and seamed.
    • All perimeter tie-outs added using two horizontal and one vertical bar tack.
    • Mid-panel tie-out internal reinforcement patches were bonded and left to cure. Then stitched.
    • Door side-release buckle added.
    • Small circular silnylon reinforcement patches around vent peak, door tie-backs, mitten hook attachment points and where KAM snaps would be placed were bonded and stitched when cured.
    • Add the smaller pieces of shelter furniture. Door tie-backs were simple design: bungee cord with cordlock, latter threaded through grosgrain loop and adjusted to tie-back.
    • KAM snaps installed on zip storm flap.
    • Apply external seam-seal to all stitching.
    Additional build notes
    Two reinforcement patches are used for each of the three mid-panel tie-outs.
    A square piece of silnylon had a grosgrain loop bar-tack stitched to it first. This was then glued to the side panel (using a thin layer of undiluted silicone sealant). When the silicone sealant was cured (24 hours minimum) the square reinforcement was then stitched to the panel (the loop was not stitched again). A circular saucer-sized, silnylon reinforcement was then bonded to the inside of the panel. After, curing this was also stitched. I used pretty small length stitches on the circular section to follow the circular shape – this was going through 3 ply of material. The ripstop grid for both reinforcements is aligned in the same direction as the grid on the side panel material. The short length of bungee cord dissipates the stress in a strong wind but even so I don’t pull this tie-out too tight. I used to have it connected with one length of cord to the Lineloc at the hem but now I have separate cords so I can really pull the hem Lineloc tight and have the mid-panel cord just taut.


    As mentioned earlier, I might have gone a little deeper on the catenary deflection than the 50mm that I ended up using, particularly with silnylon being so stretchy. Really though, before this thought recently entered my head I’d not had a concern about flapping material or creases in the material. The Mid tightens up nicely and when I’ve had material stretch when wet, simply raising the trekking pole sorts it out instantly. Still, I’ve looked at some photos of the catenary cut on my DuoMid (which I acquired after the project) and the deflection is quite aggressive by comparison.
    The seams of the DuoMid really do look super curvaceous!

    This was the first project that I’d used a rotary cutting tool on. I found it made cutting long lengths very easy. Large circles/arcs were easy to cut but small circles were best done with scissors, although tricky.

    The door/vent design didn’t break any new ground. I feel it’s not that elegant but it does work. Nowadays, I might consider an overlapping door arrangement without a zip like ZPacks is using, that might weigh less anyway (weight of zip plus storm flap would be lost). I’d maybe also think about replacing the KAM snaps and going with a toggle design, although that might be more fiddly?
    The KAM snaps have held up really well, I’m gentle with them but they must have gone through approaching 1k open/close cycles by now. They go through 4 to 6 ply of silnylon fabric and when I installed them I did go the extra step of removing the rubber end tip of the KAM tool and giving the snaps an extra hard squeeze.
    I attach the two door guys to the one peg and the two Linelocs plus the side-release buckle is overkill. I hardly ever use the side-release buckle nowadays, I just haven’t got around to cutting it off.


    It delivered what I first asked of it. It has never leaked - held up well in a couple of big, intense storms with torrential rain lasting several hours. I wouldn’t chose it for prolonged windy conditions but it sheds the wind well when it needs to and I’ve had it pitched in a few exposed places on windy nights without thinking that I needed to collapse the pole and use it as a 'flappy bivy'. Never felt bothered by condensation, though it gets some, sometimes.

    Nothing’s broken on the shelter, the material, thread/seams and smaller bits and bobs don’t require any maintenance. The extremtextil silnylon has aged well. I was trying to do a count-back of how much use it’s had, at least 40 nights over the nearly three years. So let’s say at £1 per night to date, it owes me nothing.
    Last edited: Jan 4, 2016
    Dave V, FOX160, JKM and 18 others like this.
  2. Meadows

    Meadows Section Hiker

    Great finished product and a thorough write up.

    Nice one
  3. Mole

    Mole Thru Hiker

    That's Brilliant Graham. Really impressed at the documentation.

    Thanks for sharing so much detail.
  4. Graham

    Graham Thru Hiker

    Many thanks both!

    It's a bit long-winded so will likely require a few edits e.g. I neglected to mention that I used a sewing machine in the list of 'Tools' or maybe should add myself to that list :joyful:
  5. paul

    paul Thru Hiker

    Lovely work Graham.
  6. Lady Grey

    Lady Grey Thru Hiker

    That's something to be proud of Graham.
    Mightily impressed..:thumbsup:
  7. Whiteburn

    Whiteburn Thru Hiker

    Really nice job :thumbsup:

    Don't think your comparison between the catenaries on your MYO Mid & your Duomid is apples to apples. Cuben is significantly stiffer than Silnylon so needs little, if any, catenary cut.

    Did you use the Dyneema to reinforce the peak??

    If you were to design/ built another what would you do different?
  8. Teepee

    Teepee Thru Hiker

    Cracking detailed write up and a really nice project. :)

    You'd obviously hard thought about this before tackling it and got a decent bit of kit from the effort. :)
  9. Graham

    Graham Thru Hiker

    Cheers :)

    Yes, I agree - I wasn't intending to make a direct comparison. It may be an optical illusion but my Duomid appears to have a significant deflection in its catenary cut or at least more than the 50mm I used on the 1P Mid project. When I get the chance I'll measure it and we'll see <50mm>. I think measuring the deflection of a commercial shelter is probably best done by pitching it. For the Duomid maybe tie some string to the apex loop, then to one of the corner Linelocs, pull it taut and measure the deflection at a right angle to the string. Any other ideas folks?

    Yes, as described in the post a Dyneema 'cone' was sewn inside the shelter apex - it seems to be wearing well.

    Just the suggestions I made in the post. Of those, probably it would be a zipless design. Shelter design has moved on since '13 and some interesting ideas have emerged that I would likely clone if I were building it today. If I weren't designing a Mid, a TrailStar clone would have been massively simpler to plan/build.
  10. Graham

    Graham Thru Hiker

    Huge thanks for the feedback :)

    I do like MYOG stuff - over the Holidays I became a proud owner of a home-made polycryo "Shewie sleeping mat inflator doodah" - fantastic :smuggrin: That's coming with me on my SWCP trek as I'll be using a full-length mat.

    If someone can find a link to the catenary cut spread sheet it would be good to have a thread on that? Otherwise I can provide the formulae from my Excel ss.

    Here are the shelter panel dimensions, including midpoint to apex lengths.

    Panel Dimensions.jpg
    Mole likes this.
  11. slovhike

    slovhike Thru Hiker

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  12. slovhike

    slovhike Thru Hiker

    @Graham "TrailStar clone would have been massively simpler to plan/build" ...more work as standard width of the fabric only gets to 150cm and you would need 180cm for the TS panels...but what would happen if only 150 width fabric would be used? Lower pitch? Smaller floor area? How to re dimension or scale down standard TS panels of 180cm h/210cm w/214cm length side ... to fit 150cm fabric?

    Much appreciated for the write up..mind sharing how exactly you've add catenary curve to the fabric..

    Mole likes this.
  13. Graham

    Graham Thru Hiker

    A TS clone only needs one panel template. Also it has no vent, no zip, no storm flap and fewer tie-outs. All these elements added significantly to the overall complexity of my project.

    I would have got the widest silnylon I could find. I looked into this a while back and 72" is sometimes available for Retail buyers but it does get snapped-up quickly. It's possible to get up to 60" usable width silnylon without much bother.

    I would have kept to the standard TS dimensions - by extending each TS clone panel as needed.

    In my project I added a triangular piece of silnylon near the apex just as MLD does on the side panels of its SoloMid and DuoMid. It would mean a horizontal seam near the apex for all 5 panels and a little extra weight but the seam would be short and, done carefully, it wouldn't significantly weaken the structure. That short seam has been no problem on my shelter and it's the side of the shelter that faces the wind.

    I measured and marked-out the catenary on a cardboard template. Used 10cm intervals between the points, joined the dots between intervals, cut the template to shape and then used masking tape to provide a smooth edge. I then used the template to mark the panel shape onto the silnylon and cut it using the rotary cutter.

    Hope that helps :)
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  14. slovhike

    slovhike Thru Hiker

    Graham likes this.
  15. Graham

    Graham Thru Hiker

    No problem, happy to help @slovhike. Not sure if you were just being curious or thinking about a project yourself but they have 64-65" usable width 1.3oz silnylon (w/o selvedge) here although the hh is quoted as "~1000mm" which is not in the same league as MLDs 1.5oz Pro Silnylon. If you are looking I'd just keep an eye on what the US fabric suppliers have in stock.
  16. Chip

    Chip Trekker

    Nice work and thank you for the excellent write up!
  17. FOX160

    FOX160 Thru Hiker

    Your more than welcome to check out my (when it comes) for templating catenary curves.

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