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  1. #1

    Tarp, reinforcements and choosing stitch, a couple of questions

    In the process of planning and designing a new tarp (first diy tarp) and a couple of questions have come up which I'll be happy to get your input on.

    Reinforcing corners, pull outs, etc. When using ripstop fabrics that don't stretch in two directions (x/y) but do on the diagonal, do you align any reinforcement panels/patches so they are aligned the same way as the main tarp panels? My thought is that if they are not aligned it could create a stress point where the tarp is being pulled by the guy line on the diagonal of the main tarp panel but the reinforcement patch is aligned so it won't stretch. I hope you can understand what I mean. In my head it just feels more right to have the reinforcement panel be able to also have a bit of stretch to follow the main fabric.

    Second question, related to the first. I've never seen a tarp where zigzag has been used on reinforcement panels on corners etc. To me that would be the obvious choice due to it allowing some stretch to the seam where the fabric is a bit stretchy. It also distributes stress over a wider area in the warp. In sailmaking zigzag stitching is basically all you see in those areas of the sail. A straight stitch that doesn't stretch with the fabric can create stress points/lines.

    Maybe it's a bit unfair to compare a tarp to a sail where the forces are so much bigger but in tarps where you want to be able to use thinner and lighter materials to save weight it feels important to not create any unnecessary potential failure points.

    I'm probably going to go with my gut feeling but were curious about your thoughts on this.

  2. #2
    all secure in sector 7 Shug's Avatar
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    When I made my tarps I used there directions......http://www.tothewoods.net/ImagesStay...ackCatTarp.pdf Pretty beefy corners but they have lasted for years.
    These are very thorough directions.
    Shug
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    ObdewlaX's Avatar
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    Quote Originally Posted by Kalle Mandelstam View Post
    In the process of planning and designing a new tarp (first diy tarp) and a couple of questions have come up which I'll be happy to get your input on.
    Here's how I did mine... Let It Rain

  4. #4
    Quote Originally Posted by Shug View Post
    When I made my tarps I used there directions......http://www.tothewoods.net/ImagesStay...ackCatTarp.pdf Pretty beefy corners but they have lasted for years.
    These are very thorough directions.
    Shug
    Quote Originally Posted by ObdewlaX View Post
    Here's how I did mine... Let It Rain
    Thank you both!

  5. #5
    Found this about the design of corner patches on sails. And yes I think sails are a pretty good analogue to tarps when it comes to what forces they see and how they are made. Might be a bit overkill to apply sail design principles to tarps but hey, if nothing else it'll make the tarp prettier in my opinion

    My thought is to make corner reinforcements with the same fabric as the tarp body but in a few staggered layers to spread the forces over a bit of a larger area.

    Picture source: https://www.precisionsailloft.com/ed...ails/mainsail/
    PS_Mainsail.jpg

    Some sail design principles from https://www.instructables.com/Make-a-Dacron-Sail/

    "Corner patches are consecutively smaller layers of cloth that distribute the strain from the corner ring into the rest of the sail.

    A good rule of thumb for small boat sails is the edge of the patch should be 1in to every 1ft of the edge that it's applied to. The Rookie's luff is 10' 9", so I made the edges of the corner patches on the luff 11" long. The leach edge of the patches is 12" long, and the foot edges are 7".

    The patches in the head and clew are four layers thick, and the tack is three layers thick. In small boat sails, each consecutive layer can be an inch smaller than the one before it. I have a ruler that's about 1 1/8" wide, so that's how far apart I space my layer patches, to make it easy.

    I draw the corner patches onto the sail itself in light pencil.

    Then I place a piece of cloth on top , and outline each of the patch layers for a particular corner. It is important to align the direction of the weave of the patch with the sail, so that they stretch together at equal rates."

  6. #6
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    hey Kalle, interesting questions you have.

    i think you will find (if you haven't already) that tarp designs typically don't take into account strength of the tieouts, i reckon relying on the strength of the material to be a lot in excess of what will be needed in practice. in other words, tieouts are always weak points by design, although it shouldn't have to be so. i also did a bit of research and experimentation in this regard, while modifying some cheap tarps (which came with useless gromets), but i hadn't thought of looking into sail design, that's a brilliant idea, thank you.

    to your initial questions:

    - the different stretch on wheft, warp, and diagonal are actually particular to the weave (so ripstop or not, it will do that anyway), and i think you are right that the reinforcement patches should be aligned to take that into account, otherwise they will be useless at best.
    - the zig zag stitching is recommended for stretch, but i think in this case it might not be so important, as the stretch is relatively little, and the thread you use for stitching has some stretch itself,. you got me thinking though: why not? with a shallow zig zag width, it would add some stretch security, for not much cost in strength, and might look nice too.

    like you, i think it's not unfair or unreasonable, especially as we want the tarps to be light and such but still strong, the temptation to use lighter material is always there, and although some other factors dictate using stronger material, not just tensile strength on the tieouts, i still think it worthwhile.

    there's a lot of "reinforcements" and tieouts out there that very clearly make the material weaker rather than stronger, but are still in use; so i'd say use your own judgement and don't be "intimidated" by what is commonly done/standard, imho you can definitely do a lot better. and let us know how it goes, i for one am interested.

    what i came up with myself so far is a bit "early stage of development", but a few basic (obvious) concepts are: the reinforcement patch should not be square or triangular, it creates hot spots, depending on where the tieout is, it should have some parabolic (actually catenary, to be precise) shape; circle arcs will be good enough if in doubt, and definitely a hell of a lot better than something with corners; the stitching should be predominantly in the direction of tension, not distributed equally between directions (so most stitching should be "radial", or perpendicular to the curve of the patch mentioned above); the stitching should be distributed all over the patch to distribute the load sharing and not create hotspots; the actual tieout should be stitched on the reinforced patch close to the edge of the curve, not close to the edge of the tarp: the farther in you are attaching the tieout, the more threads inside the fabric you can put to work to share the load, so the least likely to rip the tieout off (when i see pieces of webbing attached to the edge of the tarp with box stitching (so close to the edge, material which is much more rigid than the tarp, and a pattern with corners which creates hotspots) it always makes me raise an eyebrow and go "oookay. i guess you dind't care..", and as you might have noted, it's a common way of doing it. probably still a bit better than gromets though.

    the above are _not_ tested and proven, it's what i have done on the first few tarps i modified, based on what i understand about how fabric is tensioned on a tarp, and how it fails, i am still testing these tarps though (rarely :/ ), so i might find that some of my "rules" above are misguided or simply wrong. and now i have to look into sailmaker techniques, that you gave me the idea.

    i have a few other ideas which have to do with catcuts and load distribution, but i haven't had a chance to test those at all yet (i'll share once i have anything worthwhile)

  7. #7
    Quote Originally Posted by nanok View Post
    hey Kalle, interesting questions you have.

    i think you will find (if you haven't already) that tarp designs typically don't take into account strength of the tieouts, i reckon relying on the strength of the material to be a lot in excess of what will be needed in practice. in other words, tieouts are always weak points by design, although it shouldn't have to be so. i also did a bit of research and experimentation in this regard, while modifying some cheap tarps (which came with useless gromets), but i hadn't thought of looking into sail design, that's a brilliant idea, thank you.

    to your initial questions:

    - the different stretch on wheft, warp, and diagonal are actually particular to the weave (so ripstop or not, it will do that anyway), and i think you are right that the reinforcement patches should be aligned to take that into account, otherwise they will be useless at best.
    - the zig zag stitching is recommended for stretch, but i think in this case it might not be so important, as the stretch is relatively little, and the thread you use for stitching has some stretch itself,. you got me thinking though: why not? with a shallow zig zag width, it would add some stretch security, for not much cost in strength, and might look nice too.

    like you, i think it's not unfair or unreasonable, especially as we want the tarps to be light and such but still strong, the temptation to use lighter material is always there, and although some other factors dictate using stronger material, not just tensile strength on the tieouts, i still think it worthwhile.

    there's a lot of "reinforcements" and tieouts out there that very clearly make the material weaker rather than stronger, but are still in use; so i'd say use your own judgement and don't be "intimidated" by what is commonly done/standard, imho you can definitely do a lot better. and let us know how it goes, i for one am interested.

    what i came up with myself so far is a bit "early stage of development", but a few basic (obvious) concepts are: the reinforcement patch should not be square or triangular, it creates hot spots, depending on where the tieout is, it should have some parabolic (actually catenary, to be precise) shape; circle arcs will be good enough if in doubt, and definitely a hell of a lot better than something with corners; the stitching should be predominantly in the direction of tension, not distributed equally between directions (so most stitching should be "radial", or perpendicular to the curve of the patch mentioned above); the stitching should be distributed all over the patch to distribute the load sharing and not create hotspots; the actual tieout should be stitched on the reinforced patch close to the edge of the curve, not close to the edge of the tarp: the farther in you are attaching the tieout, the more threads inside the fabric you can put to work to share the load, so the least likely to rip the tieout off (when i see pieces of webbing attached to the edge of the tarp with box stitching (so close to the edge, material which is much more rigid than the tarp, and a pattern with corners which creates hotspots) it always makes me raise an eyebrow and go "oookay. i guess you dind't care..", and as you might have noted, it's a common way of doing it. probably still a bit better than gromets though.

    the above are _not_ tested and proven, it's what i have done on the first few tarps i modified, based on what i understand about how fabric is tensioned on a tarp, and how it fails, i am still testing these tarps though (rarely :/ ), so i might find that some of my "rules" above are misguided or simply wrong. and now i have to look into sailmaker techniques, that you gave me the idea.

    i have a few other ideas which have to do with catcuts and load distribution, but i haven't had a chance to test those at all yet (i'll share once i have anything worthwhile)
    Thank you a lot for a thorough reply Nanok! There will definitely be some experimenting in these first few diy tarps so we'll see what works and what doesn't. Once I get going and have a design ready I'll start a build thread for possibly more detail questions.

  8. #8
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    An interesting question.

    As with Nanok I think arced reinforcement patches are preferable but the differences in the forces along the perimeter when compared to a straight edged are minimal - the centre of the arc would be maybe a centimetre further from the corner/point of tension. Even if you aligned the ripstop of tarp and patch surely the 300D would stretch much less than the 20D of the tarp? In fact you are relying on that fact to strengthen things. In this instance the tarp material isn't doing much at all other than providing a means of connecting the reinforcement patch to the rest of the tarp, it's stretching capabilities only need to match that of the patch which being much light than that isn't likely to be a problem. Of course if you are simply doubling (or tripling) up the tarp material then everything will/should stretch at similar rates.

    I've a couple of tarps sold here in the UK by Alpkit but I think they are generic Chinese products. They are 30D silnylon. They have circular arc reinforcement patches about 75mm in radius. They haven't failed in several years and storms. The patches are glued on (another possibility for you) with stitching around the perimeter.

    People go to a lot of trouble to align the inside and outside lengths of the tie-out fabric but having them "splayed" would also help spread the load across the reinforcement patch. It would look a bit odd though unless you sandwiched the tie-out between tarp and patch.

    There was a thread on here where someone glued on their reinforcement patches and mid-panel tie-outs, I think it was on a silnylon tarp. Can't remember how long ago the thread was or what long term results were drawn.
    Better weight than wisdom, a traveller cannot carry - Viking proverb

  9. #9
    Quote Originally Posted by Bob-W View Post
    An interesting question.

    As with Nanok I think arced reinforcement patches are preferable but the differences in the forces along the perimeter when compared to a straight edged are minimal - the centre of the arc would be maybe a centimetre further from the corner/point of tension. Even if you aligned the ripstop of tarp and patch surely the 300D would stretch much less than the 20D of the tarp? In fact you are relying on that fact to strengthen things. In this instance the tarp material isn't doing much at all other than providing a means of connecting the reinforcement patch to the rest of the tarp, it's stretching capabilities only need to match that of the patch which being much light than that isn't likely to be a problem. Of course if you are simply doubling (or tripling) up the tarp material then everything will/should stretch at similar rates.

    I've a couple of tarps sold here in the UK by Alpkit but I think they are generic Chinese products. They are 30D silnylon. They have circular arc reinforcement patches about 75mm in radius. They haven't failed in several years and storms. The patches are glued on (another possibility for you) with stitching around the perimeter.

    People go to a lot of trouble to align the inside and outside lengths of the tie-out fabric but having them "splayed" would also help spread the load across the reinforcement patch. It would look a bit odd though unless you sandwiched the tie-out between tarp and patch.

    There was a thread on here where someone glued on their reinforcement patches and mid-panel tie-outs, I think it was on a silnylon tarp. Can't remember how long ago the thread was or what long term results were drawn.
    Interesting input, thank you!

    My thoughts so far was to reinforce the tie out corners with the same fabric as the main tarp body but in multiple layers that are staggered in length. That way it won't be a sudden abrupt step from stretchy too not stretchy but more evened out over a few inches rather than on a single seam. Say you'll end up with four layers at the actual corner and that the length of the patches increases with something like an inch each step.

  10. #10
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    Quote Originally Posted by Bob-W View Post
    An interesting question.

    As with Nanok I think arced reinforcement patches are preferable but the differences in the forces along the perimeter when compared to a straight edged are minimal - the centre of the arc would be maybe a centimetre further from the corner/point of tension.
    i know it looks marginal. i don't have the experimental data to prove it, but my estimation is that the difference is significant. think of catenary curves, the difference between the curve and the straight edge is quite marginal if you think of the scale, but the difference in behavior due to it is spectacular. these are basically cat cuts too (ideally), although depending where they are on the tarp, their ideal shape would be quite complex. but i digress, imho shape matters here, even if we're talking centimeters. if i get the chance to do some sontrolled testing to prove it.. aaah, i need a lab.

    Even if you aligned the ripstop of tarp and patch surely the 300D would stretch much less than the 20D of the tarp? In fact you are relying on that fact to strengthen things. In this instance the tarp material isn't doing much at all other than providing a means of connecting the reinforcement patch to the rest of the tarp, it's stretching capabilities only need to match that of the patch which being much light than that isn't likely to be a problem. Of course if you are simply doubling (or tripling) up the tarp material then everything will/should stretch at similar rates.
    actually, the more similar the reinforcing patch to the tarp material, the better. particularly, if it goes beyond a certain gap in rigidity, it will create hotspots and instead of help with strengthening, they will help the tarp rip exactly at the transition off the reinforcing patch (on the stitches). if memory doesn't deceive me, ripstop by the roll had some experience with this with one of their kits (and because they are awesome, they published the data and supported their customers, iirc)

    I've a couple of tarps sold here in the UK by Alpkit but I think they are generic Chinese products. They are 30D silnylon. They have circular arc reinforcement patches about 75mm in radius. They haven't failed in several years and storms. The patches are glued on (another possibility for you) with stitching around the perimeter.

    ah, i nearly forgot. i think this is a great idea, the problem is finding the proper glue (i'm strongly inclined towards silicone glue for a try). i think it might prove the best solution if done well, thanks for pointing this out.

    (...)There was a thread on here where someone glued on their reinforcement patches and mid-panel tie-outs, I think it was on a silnylon tarp. Can't remember how long ago the thread was or what long term results were drawn.
    iirc it wasn't long ago. it was a european gear (small) manufacturer if i'm not mistaken, might have been in the DIY subforum, maybe slovak, nice detailed pics, no long term conclusions yet (if you're thinking of the same one, that is)

    Quote Originally Posted by Kalle Mandelstam View Post
    Interesting input, thank you!

    My thoughts so far was to reinforce the tie out corners with the same fabric as the main tarp body but in multiple layers that are staggered in length. That way it won't be a sudden abrupt step from stretchy too not stretchy but more evened out over a few inches rather than on a single seam. Say you'll end up with four layers at the actual corner and that the length of the patches increases with something like an inch each step.
    the staggered approach makes sense if you want to put the load very near the edge (which with sails is quite common i guess). for tarps, i decided i will shamelessly cheat: i make rather large radius reinforcement patches, stitch them on the perimeter, and also attach the actual tieout on the perimeter (so long perimeter, relatively far from the edge), this distributes the loads much safer, with very little hassle, and yes, it makes the patch between the outer perimeter of the reinforcement patch and the edge of the tarp mostly "dead", but that's okay, as it is small enough it will not exactly flop in the wind (and yes, you can further cheat and just not reindorce most of that dead patch, save some weight and material, haven't tried that yet)

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