 Im still none the wiser, William. 
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Oh good, it's not just me then
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 I remember a recent thread on BPL where somebody claimed that silnylon sag was caused by temperature drop rather than wetness. Found it here
Silnylon is just a little stretchy, that's all. It doesn't need wet or cold particularly.
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It needs 'something', Jerry. If my shelter is nice and tawt, what causes it to sometimes 'sag' overnight?
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 First up, there's silnylon and then there's silnylon. Some have pull coatings, some "impregnations". The fact that some have taped seams and others don't because the coating is too slippy to tape the seams suggests pretty strongly that they're not all equal. An obvious cause of sagging is the degree to which pegging points lose tension over time because the tension on them pulls them against the ground and gradually enlarges the holes. That dooesn't mean that the material itslef may not sag too though. I recall Robert Saunders stating in print that an advantage of polyseter over nylon is less sagging, and RS tents use high quality silicone coated nylon. Pete.
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I've seen a silnylon tarp sag overnight Pete, with no rain, or wind to loosen the pegs. I agree you don't get the problem with polyester. That was why spinnaker became popular, until cuben fiber made spinnaker (and it's low puncture resistance) obsolete.
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It needs 'something', Jerry. If my shelter is nice and tawt, what causes it to sometimes 'sag' overnight?
Just tension, Mike.. this is not rocket science  And who says spinnaker fabric is "obsolete?" It is better than silnylon and half the price of cuben fabric
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Tension? So what if the tension doesn't change, but the tarp still sags? Most cottage manufacturers have dropped spinnaker from their line.
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| Edited: 07/10/11 17:35 |
I don't think it's just down to tension either. Sometimes the sagging doesn't happen, in spite of the tent being under tension; other times the tent goes so slack that it can't possibly be caused solely by the tension PS. I think we've hijacked this thread. Or are we just giving it extended bumps
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I think that's half the fun of OM, William. Threads can drift into other related topics.
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One reason i think temp is involved. I was out with a couple of fellow OM'ers last weekend. It was a wet, but unseasonably warm night. We were camping in 100% humidity. Virtually fog. I was using a silnylon Stephensons 2R, and there was virtually no sag in the morning. The day/night temp was very similar.
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So what if the tension doesn't change, but the tarp still sags? The tension will change. When you pitch it you tension the pegging points and so they'll all be pulling in, which will (over time) enlarge the pegging holes and reduce the tension. Though I agree it's not just that. Pete.
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 Silnylon tarps tied to trees can sag. The trees don't move, so the tension should stay the same.
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What about the guys, or knots, especially if there is a slight wind causing the trees to flex. In case you couldn't tell - I know nothing about this.
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 So, anyone up for building and testing some spring-loaded tarp fasteners?
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I camped at New Year a few years ago - no wind and freezing hard. The temt was as limp as a wet rag in the morning. I wondered if nylon had a negative temperature expansion coefficient but no it does not. Nylon sags because expands as it absorbs moisture. See here for example: http://www.intechpower.com/material-information/effects-of-moisture-absorption
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Yet i've seen other reports on BPL.com where folks have said their tarp has sagged due to heat. Still not sure how the nylon absorbs moisture if it is coated in silicone.
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