MYOG 1P Silpoly Tent Cat Cut Ridge?

[Edgar M]

I’m planning out a 1P tent out of RBTR’s 1.1 Silpoly PU4000.   It will be very similar to the Protrail, but with a removable inner net/floor.  Here are a couple of pics of a paper model.

The Ridgeline will be 84″ to 90″ and the front height will be about 45.”  Simple question:  with the Silpoly, how much curve, if any should I put in the ridgeline?  How about where the front “door” attaches?

 

[bjc]

If you haven’t seen this it might help. I think it has a template linked in the article.

http://backpackinglight.com/myog_tech_note_catenary_curves/

 

 

[Edgar M]

Thanks.  I hadn’t read that particular article,  but have read others and understand the basic concepts.   I also have used a couple of the cat cut calculators for other projects.

My concern in that the rules of thumb for the depth of the cut are silnylon.   Since the Silpoly has a lot less stretch than stillborn, does the optimum depth of the cat cut change?

[jimmyjam]

I put about 2-1/2″ in my silmembrane tarp and silnylon tarps.search this myog forum and you will see their build.

[James Marco]

Edgar, because there is little stretch or sag with SilPoly, bad weather conditions does not exacerbate the problem as SilNylon does, I would say offhand, basically guessing, that it needs nearly none. Cat cuts allow a certain amount of pre-loading for other materials, usually SilNylon or PUNylon. This helps to redistribute loads if/when it stretches. BTW, SilNylon is considerably stronger than SilPoly. I don’t think you will need much more than a very light cat cut to set up a SillPoly tent. It just doesn’t need the stretch resistance (also making it far more fragile than SilNylon…I very much doubt it will survive a downed branch for instance.) Maybe no cat cut, since the reason for it has been removed. However a light cat cut will work to help in setting the tarp up.

[Edgar M]

Thanks for the input.  I understand the tradeoffs with the Silpoly.  On the flat tarp I made, I really like the lack of stretch and water absorbtion.

I think I’ll go with a little cat cut.  As you say, it may make it easier to get a good pitch, even if not truly necessary.   I can just sew the first row of stitches of my seam an see what I get. More depth in the cut can be added without ripping it all apart if needed/wanted.

[Peter H]

This may help. It’s very similar except for the entry

http://www.trekkertent.com/home/home/3-stealth.html

[Edgar M]

The entry looks like a YMG Cirriform.  Trekker makes it in nylon and poly.  I wouldn’t expect then to have different patterns and the pictures show some curve in the ridgeline.  This led me to look at the YMG site.  The Cirriform in Cuben has a significant curve in the ridgeline.

I’m satisfied that 2 to 3 inches of depth in the ridge  will be fine.

[Jerry Adams]

I’ve done 2 to 3 inches deflection with silnylon

Maybe 1 inch with sil poly?

Maybe you actually want more deflection for silpoly because it doesn’t stretch?

Also, with nylon, the stretchiness hides minor measuring errors.  With silpoly you have to be more careful?

Silpoly won’t stretch very much parallel or perpindicular to the threads, but probably will in the bias directions

Sorry, no answers, just questions.  I haven’t used poly at all.

[Jerry Adams]

You could make the ridgeline a flat felled seam, but do the felled part at only the ends, like 1/2 inch.  With no cat.

Set tent up and see if it’s good.  Maybe decide at that point to do a 1 inch deflection and look at that.  Then maybe 2 inches.  When it’s good, fold over twice and sew to complete the flat felled seam.

[Ryan Smith]

“I’m satisfied that 2 to 3 inches of depth in the ridge will be fine.”

Perfect. Silpoly will still stretch, it’s just not affected by moisture the same way nylon is. So, in order to get that perfect pitch a cat curve will make it happen. I use the 1/3 of an inch per foot rule for cuben, silnylon etc and it always works fine.

Ryan

[Edgar M]

Thanks Jerry I have made tarts with SIL poly with no cat curve. These were a simple diamond and square tarp for hammocks. Although there is minimal stretch with the cell Paula there is enough to be able to take out any wrinkles when pitching them.

I like the idea of starting with no deflection and gradually adding more deflection until  the optimum pitch

 

[Jerry Adams]

“tarts” “Cell Paula”

I think I will write a software algorithm to determine whether someone is using a real keyboard or a smartphone : )

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