Originally Posted by
nanok
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)
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