# Thread: Catenary / Parabola Curve Calculator

1. Originally Posted by jellyfish
What holds up the center? Will you have a center pole? How will a hammock suspension fit inside?
Not a real tarp, so I guess as drawn it would have a pole in the middle, and basic pyramid shape. I guess I need to make a real one.

2. I think this (these?) company(s) MSR and MEC has all this figured out.
The rainfly in the middle MEC Hummingbird 2&3 and bottom MEC Tarn are interesting. What I think I'm understanding is:
The middle rainfly has a slope stretched peak (like most hammock flys) so there's a very noticeable catenary arch along the bottom with the gap.
The fly on the bottom MEC tent however has a ridge that is stretched over a pole and this must allow the bottom to be straight across.
-- Also I think there is something with the line of fabric between the sidewalls and the bathtub that may follow a catenary which gives some depth against water on the bottom, but the arc of the curved seam gives some catenary to the sidewall.
https://image.isu.pub/120118172114-6...pg/page_33.jpgpage_33.jpg

I guess as a noob with a sewing machine I'm a bit too quick to jump in over my head.
This image is interesting in that it shows long established tentmakers sewing a lot of arcs beyond what is easy to grasp (the stretched fabric under the curved poles easy to understand, some of the other curves? maybe more than just aesthetics.).
What the MSR Hum and the MEC Tarn might indicate is that if you can get a tight arc and stretch at the ridge it's easier to achieve a straight across bottom, if the ridge hangs then the bottom should have a catenary with the arc gap. And if you attach the side to the bottom (bathtub), sew the bottom to a catenary.
In the MEC hummingbird 2 & 3 tent the rain fly is interesting in that the upper seam is one catenary, but the lower edge is two. I think the lighter panel is cut out the fabric at width, and the lower parts are to make the panel wider, but also include the curves for engineering reasons.

3. This is another interesting design.
Sierra Designs MOUNTAIN GUIDE TARP https://sierradesigns.com/mountain-guide-tarp/
It's got invert curved sides, 5 panels, one center pole, no bottom, a straight bottom edge, and gray snow flaps around the bottom There's a vent at the top to relieve pressure. The Bernoulli effect of fast moving air over the outside and stationary air on the inside can create low pressure on the outside. The vent is an equalizer. What this design seems to indicate is that negative curves may also allow for a straight bottom end. Nothing in the set-up instructions indicates the bottom edge is part of the structural wind bracing.
I keep thinking of that cold breeze blowing under the arcs in hammock flys. Perhaps a rigorous tie down off the ridgeline of the fly could allow for flat wind blocking bottom edge?
Hammock Wind Fly.pdf

4. I think a good thing to do is spend some nights in a hammock and then troubleshoot. I never really found that wind blowing under the side was a huge problem. It is all about how you pitch it.

Wind blowing through the ends is really annoying, so I like a winter tarp and I close the doors at one end.

5. Originally Posted by WillUpnDown
This is another interesting design.
Sierra Designs MOUNTAIN GUIDE TARP https://sierradesigns.com/mountain-guide-tarp/
It's got invert curved sides, 5 panels, one center pole, no bottom, a straight bottom edge, and gray snow flaps around the bottom There's a vent at the top to relieve pressure. The Bernoulli effect of fast moving air over the outside and stationary air on the inside can create low pressure on the outside. The vent is an equalizer. What this design seems to indicate is that negative curves may also allow for a straight bottom end. Nothing in the set-up instructions indicates the bottom edge is part of the structural wind bracing.
I keep thinking of that cold breeze blowing under the arcs in hammock flys. Perhaps a rigorous tie down off the ridgeline of the fly could allow for flat wind blocking bottom edge?
Hammock Wind Fly.pdf
The flys you mention are for ground structures, which a hammock is not, although I get the concept you're trying to explore. What you lose in the ability to protect from wind traveling beneath you (and your tarp) you more than make up for in your ability to increase your insulation, add an UQP, adjust your hang angle/height/location, etc. So, my hunch is this is a non-issue, unless you've pitched a taut winter cat-cut tarp in 40 mph winds and thought you could use some relief from the Bernoulli effect.

IMO there's no need for anchoring lines. You can always pitch the tarp as low to the ground as you want. Worrying about air moving under the cat cut is like trying to plug all of the gaps in a cabin wall. I personally like some air circulation under my tarp, but HYOH. If you want to be able to pitch low and have the tarp still be high, then use XL fabric.

6. why do you need to do the cat cuts on a tarp?

7. Originally Posted by macheek
why do you need to do the cat cuts on a tarp?
it helps with the wind mainly, less vibration (ie less "flapping") as it keeps good tension on the edge of the tarp

8. I'm really terrible with mathematics and would love if someone could explain this to me as if I were a 2 year old. I am sewing tents and have the need for something like this in order to compensate for the extra material that I am ending up with by just sewing a straight seam. However, since I am using different materials for each tent, I find that some have more stretch than others. Right now I am eye-balling it after erecting the tent and seeing how much sag I have for each different fabric, so I am wondering how we can use an equation to solve the problem. Hope someone sees this and can explain how to input the different parts of the equation. I understand that one has to do with the length of the fabric...what are the other parts? Thanks in advance