Asymmetrical tarp

Make your own gear? Talk about it here!

Moderators: Bearbonesnorm, Taylor, Chew

Post Reply
User avatar
whitestone
Posts: 7013
Joined: Thu Dec 04, 2014 10:20 am
Location: Skipton(ish)
Contact:

Asymmetrical tarp

Post by whitestone »

I've been using an Alpkit Rig7 as my hammocking tarp but it's not ideal: in A-frame mode it's not long enough to fully cover my 3.3m (11ft) hammock and in the various diamond modes it's almost too long without providing a lot of side cover. I've bought material to make a specific tarp from RipstopByTheRoll but have been waiting on my mum's old sewing machine to be repaired before starting on that but we do have a lightweight sewing machine. Plus I haven't yet decided what style of tarp to make.

Last year Hilleberg were selling off end-of-roll scraps as a kit for £30. There was quite a variety of material in there, including nearly 6 metres of silpoly or silnylon, from the weight I think it's 1.6oz. Not enough to make a Hex tarp but certainly enough to make a couple of Asym(metrical) tarps - my wife's hammock is shorter than mine and because of this there was just enough. With these sort of packs it's pot luck as to what you get - we'd got dark green, it could have been Canary yellow! I'd also bought the starter pack from RBTR and that included a sizeable patch of silpoly, big enough to be used for the reinforcement patches. I'd enough Grosgrain from the RBTR order to do the tie-outs on these tarps as well.

Why go for an asym? Well partly because I hadn't made a tarp before so wanted to sort out the various construction techniques without messing up what may well be my main tarp. While modern materials and designs mean even full coverage tarps aren't that heavy (all the base material for the proposed tarp weighs just 470g, way lighter than the Rig7's 650g, and I won't even be using all of that), weight and particularly bulk when bikepacking are pretty important, so having something a third of the size would help. Finally I like the connectedness to your surroundings that a tarp brings and bigger tarps are more like tents in that regard. To provide adequate coverage an asym tarp sits very close to the hammock, it's almost like the top and bottom halves of a bivy bag, albeit one with half the insulation slung underneath you.

While looking for suitable patterns for the postponed main tarp I came across this - https://www.hammockforums.net/forum/sho ... Wide-Asym). That used a wider fabric than I had to hand but the concept should work with standard width fabrics, I hoped. Another thread discussed bonding the reinforcement patches rather than sewing - https://www.hammockforums.net/forum/sho ... -questions (there's a link to RBTR on this issue - https://ripstopbytheroll.com/blogs/the- ... d-shelters). I did some test sessions with the material to make sure I knew what to do and what to avoid.

Other design points and wants and want-nots:
+ I wanted pockets at the side tie-outs to hold the lines.
+ The ridge line tie-outs would have Beastie-D rings while the other two corners would each have a Linelok3 and a D-ring. Why two devices? Well if I wanted a simple attachment I could use the D-ring but the Linelok would also let me tension/re-tension the line from under the tarp.
+ Rather than a rectangle I went for a parallelogram, just to make things even more minimalist.
+ To aid with tension I added (well removed) a shallow catenary cut along the short edges, I went with a 1:20 depth for this, just a guess rather than anything scientific - the usual 1:12 depth seemed a bit excessive. I used this downloadable spreadsheet to work out the values - https://drive.google.com/file/d/0BwE-L0 ... sp=sharing I just marked up every 10cm and straight lined the cut between those points, the slight errors would be masked by the hem.
+ I also wanted it to be usable as a ground hammock so went for two extra reinforcement patches at 1/3 & 2/3 along one long edge and one patch at 1/2 way on the other long edge. There were also three small circular patches along the "ridgeline" with Grosgrain loops.

Generally with tarps, and particularly with tarps when hammocking, the smaller the tarp the more accurate and careful you have to be with its setup and usage. To provide maximum cover asym tarps need to sit as close to the hammock's structural ridge line as possible and then you need about 300mm extra coverage at either end to keep the rain off. It's rare to get truly horizontal rain, 45deg seems about the worst you'll get unless you pitch on an exposed ridge so while the "rain shadow" on the ground might not be very large, at hammock height you can be well protected. While bigger is often better, increasing the length of the tarp's ridgeline actually gives less coverage since the hammock's suspension rises up at 30 degrees and interferes with that of the tarp which is near horizontal meaning a longer ridgeline forces the tarp higher which isn't what you want. This tarp is intended to be a summer/three season tarp so really only has to deal with showers rather than full on storm. Since an asym's ridgeline is the diagonal across the rectangle and the width of the material is known, it's simple Pythagoras to work out the length of material required: Length^2 = Hypotenuse^2 - Width^2. In my case the width was 156cm and my effective hammock length, i.e. the length structural ridge line which prevents overtightening of the hammock, was 280cm so add 60cm to that to get 340cm. Plug in the values: Length = sqrt(340^2 - 156^2) or 302cm.

Asym tarps are also directional - you lie diagonally in a hammock, either head left/feet right or vice versa so you need to take this into account. With materials like silpoly there's no "right" side but having the reinforcement patches on the underside would look better and since my patches were a different colour to the tarp ...

Tools required
- cutting board
- roller cutter or sharp knife
- tape measure
- straight edge
- sewing machine
- fabric clips
- pieces of plywood plus clamps or weights for holding the patches when gluing
- lighter (to seal the cut grosgrain)
- Silnet glue
- paper/light card for patterns for reinforcements.
- pencil or sharpie

Build

The main material still had the selvage along the edges so first job was to remove that leaving as much width as possible by finding the first grid that looked "normal" when working in from the edge. The ripstop pattern helps here as you can just follow one of the threads, you just have to make sure that the thread you are following goes all the way. Any "roughness" would be covered by the hem. I ended up with a piece 156cm wide.

MISTAKE! I forgot to seal the cut edges with the flame from a lighter and because I left it a while between cutting and doing th rest of the work handling the threads at the edges would start to unravel. However see a subsequent mistake ...

Then I cut out the main shape. Since I was creating a parallelogram there'd be a bit of wastage at the starting end (I actually used this for reinforcement patches on the second tarp) but there's no need to cut square and then remove the taper at the other end as that would match with the first end of my wife's tarp. The parallelogram was created by cutting a diagonal from one corner to 350mm along the other edge. Finally I cut the shallow catenaries. These went corner to corner, not corner to secondary ridge tie-out, since the secondary tie-out simply makes a shorter cat curve, there's not much in it but it just felt neater to do it the way I did.

The reinforcement patches were next. I made up some paper templates to aid in cutting. The patches are in Royal Purple so a subtle contrast to the green of the tarp material (the three patches on the ridgeline were the same green as the tarp).

Image

When doing this you are working with the tarp "upside down" so it's one of those: check twice, cut (or glue) once moments. I applied the glue to the patches: the patch was placed on old newspaper and a small amount of Silnet applied then spread towards the edges, if I went past the edge I moved the patch down the paper a little way so there was no "bleeding" onto the visible surface. Generally take care: clean hands and a new bit of newspaper for each patch. The patch was then glued to the tarp, set inboard by 1cm to allow for the double rolled hem. With my fingers I applied pressure from the centre to the edge of the patch to remove any air bubbles and/or excess glue which was then cleaned off with tissue paper. Again just take time. Each patch was sandwiched between greaseproof paper and then the whole lot was placed between two table mats with a smooth plastic surface, weighted down and left for 24hrs.

MISTAKE! Being a bloke of course I had to do the two largest patches first rather than make my way up to them! Consequently these are a bit messy. By the time I got to the last patches I was much neater.

MISTAKE! I was a bit random in the distance from the edge of the tarp that I applied the reinforcement patches, a couple were a bit too far to be caught by the hem. So I decided to trim the long edges again by about 5mm. This had the effect of removing the fraying from the first step/mistake noted above. I sealed the edges this time.

The last subassemblies were the tie-outs. These were 6 x 220mm lengths of 25mm Grosgrain, the cut ends were sealed with a lighter to stop fraying. Four lengths were folded in half and had Beastie-D rings fixed in place in the centre of the length by a captive loop, the other two had a combination of a Linelok3 and a Beastie-D. The Grosgrain will sit either side of the tarp material, sort of sandwiching it when sewn.

Once the reinforcement patches had cured the edges of the tarp were tidied up using a double rolled hem. The biggest time here wasn't the sewing but folding the hem and holding it in place using the clips, the very slippery fabric meant that it probably took three times as long to do this as the actually sewing. Cath did suggest using pins rather than the clips but my fingers are so clumsy I can hardly pick the damn things up! :lol:

Image

Image

This process captured the bottom edges of the patches. Again being a bloke I had to do one of catenary cut sides first rather than a straight one but it wasn't that difficult, I just took my time. The four vertices would be covered by the Grosgrain tie-outs which helps with tidying things up.

The next task was to sew on the tie-outs. The tie-outs were sewn on either side of the tarp material, it's quite awkward getting the two legs aligned, easiest way is to mark the centre line of the tie-out on the patch then a parallel line to the side for one edge; put a ruler under the tarp in line with this second edge and move the lower leg up against this; simply align the top leg with the second line then push a pin through the whole assembly (being close to the edges there's no need for seam sealing and you are going to be pushing a couple of hundred holes through the material to do the stitching anyway).

MISTAKE! I was so used to working from the underside of the tarp that I sewed the first tie-out on from that side so the bare threads were on the outside! I then turned the tarp around and did the others a lot better.

The shot below shows the "underside" of my first tie-out on the left and that of the fourth one I did on the right, a much better job.

Image

I've not made a stuff sack yet but just to get an idea of what size I'd need to make I tried putting it in an XXS Exped dry bag and there was plenty of room. (standard can of beans for scale)

Image

I'm not quite finished. The motor on the sewing machine packed up just as I'd done the two main ridgeline tie-outs and the two tie-outs at the sides. So I've still the two supplementary ridgeline tie-outs, the three tie-outs that will be used for ground usage and the noseeum line pockets to fix. However the tarp is actually usable as is, given the current blustery weather I'll wait a while before setting it up.

Total weight just 250g. The main material weighs 220g. The reinforcement patches and line pockets weigh 5g in total. The tie-outs with the hardware weigh 6g. I used about 10g of silnet for gluing the patches. Not sure what the exact cost is as much of the material was just part of various packages, let's say it was a third of the material in the Hilleberg pack and I used 60% of that so £6; the reinforcements similarly, let's say £2; grosgrain - paid $3 for 5yds so about 60p; hardware - £3; Silnet - about a third of an £8 tube so about £2.40. Maybe £2 for the thread. Grand total of £16 or thereabouts!

Timewise: cutting the main material - 40 minutes; 5 minutes for the reinforcement patches; five minutes for the noseeum; gluing the patches - about 5 mins each then 24hrs to cure; folding and pinning the hem - 1hr 30 minutes; sewing the hem - 30 minutes; making and sewing the tie-outs - 10 minutes each. So about four hours of actual work.

The result is reasonably neat but it's certainly not commercial grade. It's very much a prototype and a first sewing project for me - my wife only showed me how to use the sewing machine fifteen minutes before I started doing the sewing on this. The tie-outs on the side corners probably could be a bit smaller and in future I might put all of the Grosgrain on the underside. Once we've a working sewing machine again I'll do the second one.
Better weight than wisdom, a traveller cannot carry
User avatar
TheBrownDog
Posts: 1993
Joined: Thu Mar 14, 2013 8:46 pm
Location: Chilterns

Re: Asymmetrical tarp

Post by TheBrownDog »

Brilliant, particularly this little piece of insight which, when I read it, had me slapping my forehead.
Asym tarps are also directional - you lie diagonally in a hammock,
When done, pix of it hung please ... and when are you taking orders :-bd

Also keen to know if having the tarp so close to the hammock limits its sitability.

Nice one chap.
I'm just going outside ...
User avatar
whitestone
Posts: 7013
Joined: Thu Dec 04, 2014 10:20 am
Location: Skipton(ish)
Contact:

Re: Asymmetrical tarp

Post by whitestone »

If you do a search on hammockforums for posts by a chap called Kitsapcowboy, he's posted lots of designs from DCF ultra lightweight asym models weighing just 90g all the way up to massive winter tarps weighing 800g that completely enclose the hammock and are basically a huge tent. I've a list somewhere of threads he started with various tarp designs, the first link in my post is to one of them.

Did you mean "sitability" or "suitability"? Siting wise, so long as there's the length of the tarp's ridgeline plus a little bit for the suspension between two trees you are good to go. Suitability? My intention is for this to be a summer tarp where there's not much likelyhood of rain, at least not rain like that currently battering the window :shock: I need to set it up along with the hammock and work out what's best. Lower and closer to the hammock is going to give more protection but it will feel more like a suspended bivy bag. Higher will feel more open but it will also be more exposed.

One point I've read is that the hammock suspension should be on the short side of the tarp to provide most protection. It's those sort of things I need to work out.

I'm pretty sure the material I used is standard 1.1oz/square yard or 35g/m^2 silpoly. Ripstopbytheroll do a lighter version called "Membrane" which is only 25g/m^2. Using that would knock 70g off the weight of this tarp! Meant to be quite hard to sew though so I might leave that a while.
Last edited by whitestone on Tue May 10, 2022 3:21 pm, edited 1 time in total.
Better weight than wisdom, a traveller cannot carry
User avatar
TheBrownDog
Posts: 1993
Joined: Thu Mar 14, 2013 8:46 pm
Location: Chilterns

Re: Asymmetrical tarp

Post by TheBrownDog »

Did you mean "sitability"
Yep sitability, as in would the tarp be so low its function as a seat might be limited. But of course, it wouldn't anymore than a square tarp. I usually hang my DD Hammocks 3x3 tarp diagonally and pitch one corner up with a long stick to give me a big space for faffage.

I'll take a look at Kitsapcowboy. Really like the idea of a lighter set up that doesn't compromise coverage. :-bd :-bd
I'm just going outside ...
User avatar
whitestone
Posts: 7013
Joined: Thu Dec 04, 2014 10:20 am
Location: Skipton(ish)
Contact:

Re: Asymmetrical tarp

Post by whitestone »

A couple of shots of it pitched. No hammock as I forgot to take the tree straps with me :roll: :roll: Possibly just as well as it was so windy (a big tree or two came down in this wood the other night) I could only just get the thing pitched and the Groundhog pegs were getting pulled out of the ground!

It's hard to tell from these if it is an asym and not square or a diamond but that's just the angles of the shots.

Image

Image
Better weight than wisdom, a traveller cannot carry
User avatar
whitestone
Posts: 7013
Joined: Thu Dec 04, 2014 10:20 am
Location: Skipton(ish)
Contact:

Re: Asymmetrical tarp

Post by whitestone »

A small update now that we've got a new sewing machine.

I finally got round to making the pockets for the corner lines. They are made out of noseeum mesh so a little interesting to work with, it catches really easily on the feed dogs of the sewing machine so handling it takes a bit of getting used to and you do need to "pull" it a little through the machine.

My initial cuts for these made no allowance for hems so were way, way too small so I cut some more but these are still only just large enough. The hypotenuse which will be the opening is a double rolled hem, the other two sides are single rolled hems.

Image

Doing the above hemming took most of the time as the mesh is really tricky to work with in small sizes, another reason for making them a bit bigger again. Here's one of them attached to the tarp with the line coiled up inside it.

Image

I've only done pockets for the two "outer" corners not those at either end of the ridge. I may add those at some point but will see how these perform first. Now just need to make a stuff sack for the tarp.

Then it's finish off Cath's tarp :grin:
Better weight than wisdom, a traveller cannot carry
User avatar
whitestone
Posts: 7013
Joined: Thu Dec 04, 2014 10:20 am
Location: Skipton(ish)
Contact:

Re: Asymmetrical tarp

Post by whitestone »

Another update. I mentioned in the original post that there was enough material for two tarps, well I finished the second one this afternoon. Slightly shorter because it's for a shorter hammock and I tried some different ways of attaching the side tie-outs that didn't involve a reinforcement patch - basically a twisted loop of grosgrain stitched across the corner.

This one (with stuff sack but no lines or pegs) comes in at 210g :-bd

No pictures because apart from the different tie-outs it's identical to the first!
Better weight than wisdom, a traveller cannot carry
Post Reply