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XTrekker
12-23-2013, 18:30
Catenary / Parabola Curve Calculator (https://drive.google.com/file/d/0BwE-L0KzzInzSkVSY2Q2ZXBMWXc/edit?usp=sharing)


So I have used various curve generators in the past successfully after getting use to how they function. Over the past projects I have repeatedly tweaked them until I have finally ended up with a condensed ultra simple version. This will generate your basic catenary/parabola curves needed for cat-cut-tarps and bridge hammocks. And to the math experts, yes, I know it is not an exact parabola or catenary curve but I am confident that it is more than accurate enough for hammock and tarp applications. :)

With this calculator/generator you only need to input the length of the curve and the desired depth of the curve. It will output the x and y cords for the curve. The Y cords is the length of the curve in increments of 1 and the x cords adjacent to it are the measurements for the depth of the curve.

I hope this curve generator/calculator comes in as handy for someone else, as it has been for me on my projects. I tried to upload it to the site but the site only accepts xls docs and not xlsx docs which is the newer versions of excel docs. I didnt want to save in the old format because it would lose some of the conditional formatting. I have also uploaded this to my google docs so if you dont have Microsoft excel you can still use it on google.
Here are the two links.

Curve Calculator - Google Doc (https://docs.google.com/spreadsheet/ccc?key=0AgE-L0KzzInzdG5hUEdlbzBQXzlqdlphS3l0ZXQwVlE&usp=sharing)

Calculator - Download (https://drive.google.com/file/d/0BwE-L0KzzInzSkVSY2Q2ZXBMWXc/edit?usp=sharing)

Deadphans
12-23-2013, 21:29
Ohhh wow. Perfect timing! Should be getting a sewing machine from Santa, and materials shortly thereafter. Cat cut tarp for me! Possibly cuben fiber...not sure though,bi hear it is tough to sew and work with....

XTrekker
12-23-2013, 23:57
Ohhh wow. Perfect timing! Should be getting a sewing machine from Santa, and materials shortly thereafter. Cat cut tarp for me! Possibly cuben fiber...not sure though,bi hear it is tough to sew and work with....

Awesome. We'll at least one person found this useful so it was worth the post. The tarp projects are fun And fairly easy to do. Although I have never worked with Cuban fiber so I'm not sure how hard it is to work with. Good luck with your project and I hope it turns out great. Can't wait to see the finished product.

Meanderin'Speed
12-24-2013, 01:58
This is going to be perfect. I'm planning on making some mini bridges for my kids when I get back home, so this will be really handy.

Do I sense a sticky here?

XTrekker
12-24-2013, 17:10
This is going to be perfect. I'm planning on making some mini bridges for my kids when I get back home, so this will be really handy.

Do I sense a sticky here?

Great! Mini bridges for the kids is awesome.

Mrprez
12-24-2013, 17:15
I like it. Nice and easy to use. Many thanks!

XTrekker
12-26-2013, 11:59
I like it. Nice and easy to use. Many thanks!

Thanks.

If anybody has ideas for improvements, feel free to share and ill see if I can incorporate it into it.

Fronkey
12-26-2013, 12:00
Excellent job! It looks great!

Fronkey

Brancher
12-26-2013, 18:38
Catenary / Parabola Curve Calculator (https://drive.google.com/file/d/0BwE-L0KzzInzSkVSY2Q2ZXBMWXc/edit?usp=sharing)


So I have used various curve generators in the past successfully after getting use to how they function. Over the past projects I have repeatedly tweaked them until I have finally ended up with a condensed ultra simple version. This will generate your basic catenary/parabola curves needed for cat-cut-tarps and bridge hammocks. And to the math experts, yes, I know it is not an exact parabola or catenary curve but I am confident that it is more than accurate enough for hammock and tarp applications. :)

With this calculator/generator you only need to input the length of the curve and the desired depth of the curve. It will output the x and y cords for the curve. The Y cords is the length of the curve in increments of 1 and the x cords adjacent to it are the measurements for the depth of the curve.

I hope this curve generator/calculator comes in as handy for someone else, as it has been for me on my projects. I tried to upload it to the site but the site only accepts xls docs and not xlsx docs which is the newer versions of excel docs. I didnt want to save in the old format because it would lose some of the conditional formatting. I have also uploaded this to my google docs so if you dont have Microsoft excel you can still use it on google.
Here are the two links.

Curve Calculator - Google Doc (https://docs.google.com/spreadsheet/ccc?key=0AgE-L0KzzInzdG5hUEdlbzBQXzlqdlphS3l0ZXQwVlE&usp=sharing)

Calculator - Download (https://drive.google.com/file/d/0BwE-L0KzzInzSkVSY2Q2ZXBMWXc/edit?usp=sharing)

XTrekker, this is PERFECT! I just got finished (like, this instant) sewing my ridgeline flat on the 11X10 Winter Tarp I'm building...So this is gonna be a lifesaver for me!

Btw, I know there's a 'general' rule-of-thumb that you cat-cut about 1 " for every foot of curve...So that would mean that on a 5-foot side section (between the corner tie-out and the middle one) I need to go to a 5-inch depth. That seems pretty deep for my taste.


Where did that guideline come from? Is there a 'minimum' depth you should go (ie, 1" / 1ft) or is it more personal taste? I know the JRB 11X10 isn't cut nearly as deeply as some others. Anybody?


Thanks!
sc

XTrekker
12-26-2013, 20:15
The cat cuts just help spread the load across the fabric evenly. It's up to the designer to decide how much to cut depth to use. It comes down to preference really.

XTrekker
12-26-2013, 20:17
Excellent job! It looks great!

Fronkey

Thanks Fronkey

Mrprez
12-27-2013, 15:56
I used the calc today for an 81" cat cut. At first, I was thinking that I didn't have anything that would measure in 10ths of an inch until I remember my digital calipers. Piece of cake after that revelation. Much easier than trying to bend a piece of pvc and draw at the same time! Much more accurate as well I am sure.

XTrekker
12-27-2013, 16:18
I used the calc today for an 81" cat cut. At first, I was thinking that I didn't have anything that would measure in 10ths of an inch until I remember my digital calipers. Piece of cake after that revelation. Much easier than trying to bend a piece of pvc and draw at the same time! Much more accurate as well I am sure.

Awesomeness. Yeah I used the pvc and string method few times but found that the pvc didn't bend evenly. Think it was due to defects in the manufacturing process of the pvc or something but one side would end up with a slightly tighter curve than the other.

You can also skip an inch or two to make it easier to measure for the depth of the curve. Less dots along the curve but will still get the job done. I may tweak the calculator to allow the user to decide on how many points of measurement are displayed. Someone may only want to place a dot every few inches rather than every inch.

bubba74
12-27-2013, 16:31
You, Sir, just saved my day :)

Excellent!

Mrprez
12-27-2013, 17:20
Not to overly complicate things, but maybe a choice between inches and mm?

netjox
12-27-2013, 19:03
This rocks! Just earlier today I was contemplating how to plot out a desired sag on a DIY tarp. Perfect timing on your post! Thanks!

XTrekker
12-27-2013, 19:13
You, Sir, just saved my day :)

Excellent!

Awesomeness! Glad it helped. Its proving helpful on my current project also. Let me know if there is ways to improve it.


Not to overly complicate things, but maybe a choice between inches and mm?

It will output the measurements based on any standard of measurement inputed. Just input the length and curve depth in mm and it will give you correct measurements in mm.



This rocks! Just earlier today I was contemplating how to plot out a desired sag on a DIY tarp. Perfect timing on your post! Thanks!

Awesomeness. Can't wait to see the finished project. Be sure to share with us when your done.

Brancher
01-02-2014, 12:13
Well, after the holiday chaos I finally had the chance to take a break yesterday and finish up my Superfly copy using the Cat Cut Calculator. I did two cuts on each side, 63 inches long and 3" deep.

Hats off to Xtrekker, it worked great, and I have nice, even cats in the new shelter! I'll post pics as soon as I have a chance to hang it.

Thanks!

Yukon
01-02-2014, 12:18
Added this thread to the sticky sub-forum. Nice work XTrekker.

XTrekker
01-02-2014, 12:48
Added this thread to the sticky sub-forum. Nice work XTrekker.

Thanks Yukon. I hope this helps out those who dare to take on those DIY Bridge Hammocks and CatCut Tarps....:D:D

onfire
01-29-2014, 07:48
Thanks for posting this - I am going to try a cat cut hammock out of 1.6oz ripstop nylon!

XTrekker
02-17-2014, 12:51
Just wanted to add a direct link to downloading the file. See attachments below.

62110

kayak4water
06-16-2014, 13:02
Thanks for posting the Curve Generator. It seems to work well with openoffice software.

Rockdawg69
09-05-2014, 18:57
Just wanted to add a direct link to downloading the file. See attachments below.

62110

Thanks. This solved my Google Docs problem. You may see a request for access to the Google doc.
Rockdawg69

DestinedToHang
09-05-2014, 20:43
Thank you for posting this.It will be extremely helpful when I make my tarp.Also thanks for the great DIY videos on YouTube.

Matt65
10-12-2014, 18:11
Helpful tool that I will be using on my next tarp. Thanks for sharing this here.

fltlndr
06-11-2015, 13:18
I was thinking about this last night as I'm planning to make my first hammock setup real soon. A true catenary results from hanging a string from two points. Any thoughts on the effectiveness of temporarily mounting your fabric to the wall and hanging a string from point A to point B and tracing? Depth is a simple adjustment of the string length. Again, I'm still in the planning stage so I'm not sure how easy it would be. Another thought for quickly developing a parabola is to project a circular light source onto the fabric. The boundary between light and dark should be parabolic.

This is a little off topic, but can anyone direct me to a location for hammock plans with a complete material list? I've seen some really great tutorials and will likely try to put together a complete list from investigation. I'm trying to avoid multiple online orders and trips to the store. My end goal is a hammock with suspension and bug net, tarp, top quilt and under quilt.

Thanks!

ylnfrt
06-14-2015, 20:35
To drape your material on the wall would require you to support the fabric in such a way that it was not sagging from any of the hanging points. your idea of using a shadow/ light is interesting- I am going to play around with that.
Your last request is off topic but look in the forums, under camping hammocks or DIY, to any number of build threads on hammocks. A lot of different options. also all your questions can be more fully answered in the specific related forum. HLFL


I was thinking about this last night as I'm planning to make my first hammock setup real soon. A true catenary results from hanging a string from two points. Any thoughts on the effectiveness of temporarily mounting your fabric to the wall and hanging a string from point A to point B and tracing? Depth is a simple adjustment of the string length. Again, I'm still in the planning stage so I'm not sure how easy it would be. Another thought for quickly developing a parabola is to project a circular light source onto the fabric. The boundary between light and dark should be parabolic.

This is a little off topic, but can anyone direct me to a location for hammock plans with a complete material list? I've seen some really great tutorials and will likely try to put together a complete list from investigation. I'm trying to avoid multiple online orders and trips to the store. My end goal is a hammock with suspension and bug net, tarp, top quilt and under quilt.

Thanks!

Boston
06-15-2015, 07:27
You're better off making a posterboard or cardboard template. That way you only need to use the string once, and its easy to trace as many times as you need to.

compman2
11-11-2015, 10:53
I recently wanted to make a tarp with catenary cut so I downloaded your spreadsheet to get the curve values. Before building I plotted the curve in a CAD program. I also compared it to a pure arc as drawn in CAD. Much to my surprise the straight arc and the catenary curve varied by only hundreths of an inch at the plot points. The reason the two are so closely matched is the relatively shallow cut used when making tarps. A much deeper cut would then have the catenary curve and arc significantly deviate.

I chose to use 5.75 inch cut over a length of 80 inches. I then used CAD to generate full sized templated that could be printed out and then taped together to get a full scale curve. I attached it to 1/4" plywood and cut a full size template that I used to make the tarp. You could also use cardboard. The cool thing about this template is that you can use any section of the template for the shorter cuts on the tarp.

I have attached multiple pdf files with different cut depths over different lengths. These can be printed on regular sized paper and then taped together to form a full sized template. One of these templates should work for almost any tarp. Hopefully this will save someone the trouble of createing their own.

You can view my final result in this post http://www.hammockforums.net/forum/showthread.php/117547-My-DIY-Tarp

TinCanFury
11-11-2015, 11:27
I recently wanted to make a tarp with catenary cut so I downloaded your spreadsheet to get the curve values. Before building I plotted the curve in a CAD program I use. I also compared it to a pure arc as drawn in the cad program. Much to my surprise the straight arc and the catenary curve varied by only hundreths of an inch when plotted in CAD. The reason the two are so closely matched is the relatively shallow cut used when making tarps. A much deeper cut would then have the catenary curve and arc significantly deviate.


I'll go one blasphemy further. I don't think the catenary shape is any better than any other even shallow curve for non-ridgeline cuts, and for the ridgeline I think the catenary cut is actually a detriment.

There's a lot of pseudo science/engineering in backpacking DIY, and the strict adherence to the catenary curve is one of the most egregious. That said, for those making tarps that don't have access to CAD, the hang a rope and trace the curve method is the easiest way to get a shallow even curve, so it's a wash at the end of the day. When I design my tarps and tents in CAD, I don't bother with a catenary formula, I just do what you did.

XTrekker
05-29-2016, 23:55
The curve generator is for those who wish to just plot the marks on fabric and cut. There alot of methods that lead to the same end. None are necessarily wrong. Even if you just eyeball it, your tarp will be just fine.

Sent from my SM-G900V using Tapatalk

GadgetUK437
11-25-2016, 08:34
Are you happy to share the unlock sheet password?

I want to add a "X inches per foot" input that will generate the depth of curve input, based on the length and the inches per foot requested.
Like this,
Depth of curve = ((inches per foot)/12)*(length of curve)

blackbishop351
02-10-2017, 14:20
Just FYI.

There's no such thing as a printable "perfect catenary curve," since the functional form is analytically intractable. All that can be done is exactly what you did, provide point-specific values (coordinates).

I'm glad someone else made one of these. People were having strange issues with the one I provided with my BlackCat tarp instructions.

Also - what do you mean by "parabola?" Afaik, a parabolic curve won't provide the same stress dispersal as a catenary.

Thanks!

kitsapcowboy
02-10-2017, 14:48
Afaik, a parabolic curve won't provide the same stress dispersal as a catenary.With your legendary reputation, I have no doubt that you understand these engineering points better than I do, but I was under the impression that while a true catenary curve used in typical hammock applications will outperform a parabola, for certain common aspect ratios in those same applications a parabola is usually a sufficient approximation to the appropriate catenary curve to provide some of the same engineering benefits and has the virtue of being easier to describe and compute mathematically. My $0.02...

blackbishop351
02-10-2017, 14:56
for certain common aspect ratios in those same application a parabola is usually a sufficient approximation to the appropriate catenary curve to provide some of the same engineering benefits and has the virtue of being easier to describe and compute mathematically. My $0.02...

If your goal is to simply remove some material to avoid sag, then yes the parabola is an acceptable substitute. If you're after actual stress dispersal, the catenary is the way to go.

For example, a parabola on a tarp isn't going to get the job done due to the large surface areas and types of stress you encounter. But for a hammock maybe (I have zero experience using a curved cut on a hammock btw), the parabola might work fine.

kitsapcowboy
02-10-2017, 16:32
If your goal is to simply remove some material to avoid sag, then yes the parabola is an acceptable substitute. If you're after actual stress dispersal, the catenary is the way to go.

For example, a parabola on a tarp isn't going to get the job done due to the large surface areas and types of stress you encounter. But for a hammock maybe (I have zero experience using a curved cut on a hammock btw), the parabola might work fine.I stand schooled; like I said, you are the expert.

One must conclude that your construction skills and standards are indeed as refined as your knowledge with regard to tarps -- you yourself having invented a particular kind of tarp that has been widely copied -- because when I compute a true catenary curve 72" long by 5" deep -- which would be a relatively common shape for a cat-cut on the side of a hammock tarp -- and compare that to a parabola of the same depth and span -- specifically y=(5x^2)/1296, intersecting the points (-36, 5), (0,0), and (36, 5) -- it looks to me like the catenary and parabolic curves are approximately congruent to a precision of about +/- 1/16 of an inch. That is to say, if you draw the two computed curves -- catenary and parabolic -- on the same paper template, the two respective lines stay within about 1/16" of each other along the entire curve.

I know for a fact when I sew complex curves or hems on the bias, my sewing skills admit variation of at least that magnitude. If I had known beforehand that variances of +/- 1/16" would mean the difference between a tarp that hangs and a tarp that fails or between a hammock that holds its rated weight and one that blows up under load, I would probably never have attempted any of my DIY shelter system projects to date. I guess I have been lucky so far.

blackbishop351
02-10-2017, 17:42
I'm no expert, I just took some math in college and played around with some tarps. Obviously I don't have a Master's in sarcasm like you do.

Do whatever floats your boat.

kitsapcowboy
02-10-2017, 17:52
I'm no expert, I just took some math in college and played around with some tarps. Obviously I don't have a Master's in sarcasm like you do. Do whatever floats your boat.No sarcasm (or offense) intended -- although the degree I hold in sarcasm is technically doctorate-level... :cool:

My operational definition of "hammock legend" is anybody who has entire categories of hammock gear named after her/him. That's you on at least a couple of counts.

Your reputation on HF is beyond reproach, and it is a fact that you are an old guard pioneer in DIY hammock tarps.

I really do want to understand the physics as you do that explain how variances that seem so minor to a layperson like myself, i.e., presumably beyond the limits of precision for at least some segment of the DIY community here, actually lead to meaningful differences in performance on a tarp build. You pointed out that you have the technical background that afforded you this understanding; in truth and in deference, I am just hoping you can explain it to the rest of us who have been taking a short cut in error.

(PS:


"Physics is the only true science. All else is stamp collecting." - J. J. Thompson

I had always thought that your signature line quote was attributable to Rutherford (https://en.wikiquote.org/wiki/Ernest_Rutherford), not Thomson (https://en.wikiquote.org/wiki/J._J._Thomson), under whom Rutherford worked. Learn something new every day...)

Kroma
02-10-2017, 17:54
Does this have anything to do with the parable of the cat and the canary?

blackbishop351
02-10-2017, 18:03
The easy answer is that I prefer to be precise when precision doesn't cost me anything. Using an approximation when the physically supported data are so easy to obtain just rubs me the wrong way.

The slightly harder answer is that, if memory serves, the greatest variation in functional form occurs at the ends of the curved cut. Which, on a tarp, corresponds to the corners. Small differences at a corner can add wrinkles and wind-dams across the whole body of the tarp. Again, I'm not sitting here doing the math, so this is back-of-the-napkin. And relying on 10 year old memory.

blackbishop351
02-10-2017, 18:03
Does this have anything to do with the parable of the cat and the canary?

Yes, absolutely.

kitsapcowboy
02-10-2017, 18:31
The easy answer is that I prefer to be precise when precision doesn't cost me anything. Using an approximation when the physically supported data are so easy to obtain just rubs me the wrong way.And that's what makes you a legend. See?

Honestly, I appreciate the response.


The slightly harder answer is that, if memory serves, the greatest variation in functional form occurs at the ends of the curved cut. Which, on a tarp, corresponds to the corners. Small differences at a corner can add wrinkles and wind-dams across the whole body of the tarp. Again, I'm not sitting here doing the math, so this is back-of-the-napkin. And relying on 10 year old memory.I think the reason we disagree stems less from the numbers and more from a matter of perspective, on which I hope you'll grant me latitude in the spirit of HYOH.

I believe you are exactly correct; the greatest deviation between these two curves would occur at extremes furthest from the line of symmetry.

However, just as you prefer to be precise with numbers, I prefer to be precise with words, which is why I qualified my original statements exactly as I did...


...for certain common aspect ratios in those same applications a parabola is usually a sufficient approximation to the appropriate catenary curve to provide some of the same engineering benefits and has the virtue of being easier to describe and compute mathematically...

The ends of these curves would vary more greatly across a different span, but for long, flat curved edges such we find on tarps and bridge hammocks the biggest differences are proportionately pretty darn small...

Also, for me, the latter point is likewise important, if you have the same appreciation of the difference between accuracy and precision that I do. The calculated values using the catenary curve may be more precise and model more closely the true relationships at work in the physical world, but if the variance introduced when rendering a set of precise theoretical coordinates as a practical application in the real world -- with small errors introduced during successive steps of rounding, measuring, templating, transferring, cutting, and sewing -- puts the final result of using the precise method and the final result using a quick-and-dirty approximation within the same bounds, then the approximation is a valid one, and in this case it seems to be not only sufficiently precise but also sufficiently accurate, as well as being something one could calculate quite easily with nothing more than a pencil and a cocktail napkin, given nothing more than the correct parabolic equation -- no spreadsheets or laptops needed. (Newton's laws of motion were rendered technically "incorrect" by Einstein's work, but Newtonian kinematics were sufficient to put a man on the moon and still work well in virtually every day-to-day situation involving bodies in motion that we encounter because the errors remain so small.) To me, avoiding anything "analytically intractable" in the calculation of the curve has practical value at virtually no cost, making the parabolic curve used in this application a logical alternative. I understand if your priorities and criteria for evaluation may differ, but I don't think mine were in error, unreasonable, or misleading to anyone else.

Thanks for furthering the discussion.

blackbishop351
02-10-2017, 18:43
I think at the end of the day, HYOH is the operative idea. You recognize that there are differences in the two curves (some wouldn't), so if you can live with those differences on your kit, great. You may well be correct with regard to the scope of those differences; in a real-world application, using one over the other may offer exactly zero advantage. I'll stick with what I'm doing though, it just feels more..... right.

Duncanslam
02-25-2017, 09:33
The calculator maxes out at 160 Is that because most tarps are less than that? I'm making a group tarp that's 15 feet long. Would I be better to have two curves and a middle tie out? Or can I trick the calculator into giving me a longer curve?

Thanks
Steve

blackbishop351
02-28-2017, 09:37
The calculator maxes out at 160 Is that because most tarps are less than that? I'm making a group tarp that's 15 feet long. Would I be better to have two curves and a middle tie out? Or can I trick the calculator into giving me a longer curve?

Thanks
Steve

For a tarp that long, you might be better off using three tie-outs per side. And in that case, yes you'd have two cat cuts per side too.

WillUpnDown
12-29-2017, 20:48
I know this is an old thread, but.... Question. The height of the curve seems to be somewhat arbitrary. And the rule of thumb is that it's 1" of height for each running foot (width) of the span, so a 12' wide tarp would need a 12" depth. The calculator allows height to be an independent variable.
Seems to me that this curve should be based on the stretch of the fabric and the width, so a very stretchy fabric would need more height of curve.... But I'm really just guessing at this.
Second thought: What if instead of cutting and hemming the catenary, instead you just ran a raised gather(?) using very sturdy series of stitches, following the catenary curve. Then you wouldn't have to cut the fabric. These 'gathers' would be wider near the corners and wouldn't have to meet in the middle. This would stretch the fabric out more along the baseline, to match the tension along the catenary.... I'll include my sketch. (My sketch has two catenary gathered seams because Xtrekker's cool calculator makes it easy to calculate.)

Keep in mind the 'gaps' (white areas) along the catenary lines are to be pulled together and stitched. The top drawing was my original idea, using different panels, but then I realized you could probably just gather together fabric and stitch it. The 'gaps' at the bottom would be bast on the specific stretch of the particular fabric, so polyester would have a smaller gather than nylon (because nylon stretches more).

It seems to me that the issues with a flapping tarp are 1. the bottom edge, and 2. an general area in the tarp where tension seems to drop because it's spread out. It's an interesting puzzle that once sorted should allow for tarps of any shape. (and likely this has already been figured out and I've been googling the wrong terms while trying to reinvent the wheel.....)
Will

158987

jellyfish
12-30-2017, 13:33
I know this is an old thread, but.... Question. The height of the curve seems to be somewhat arbitrary. And the rule of thumb is that it's 1" of height for each running foot (width) of the span, so a 12' wide tarp would need a 12" depth. The calculator allows height to be an independent variable.
Seems to me that this curve should be based on the stretch of the fabric and the width, so a very stretchy fabric would need more height of curve.... But I'm really just guessing at this.
Second thought: What if instead of cutting and hemming the catenary, instead you just ran a raised gather(?) using very sturdy series of stitches, following the catenary curve. Then you wouldn't have to cut the fabric. These 'gathers' would be wider near the corners and wouldn't have to meet in the middle. This would stretch the fabric out more along the baseline, to match the tension along the catenary.... I'll include my sketch. (My sketch has two catenary gathered seams because Xtrekker's cool calculator makes it easy to calculate.)

Keep in mind the 'gaps' (white areas) along the catenary lines are to be pulled together and stitched. The top drawing was my original idea, using different panels, but then I realized you could probably just gather together fabric and stitch it. The 'gaps' at the bottom would be bast on the specific stretch of the particular fabric, so polyester would have a smaller gather than nylon (because nylon stretches more).

It seems to me that the issues with a flapping tarp are 1. the bottom edge, and 2. an general area in the tarp where tension seems to drop because it's spread out. It's an interesting puzzle that once sorted should allow for tarps of any shape. (and likely this has already been figured out and I've been googling the wrong terms while trying to reinvent the wheel.....)
Will

158987

I am not sure that I understand your diagram. Is the ridgeline at the top?

I like a 1:12 cat cut, but the curve is not 12í long. It is in segments with tie outs. It is windy here and the luffing is minimum. If I ever get a problem it is because a gust has pulled a stake, lol. I like the added coverage of 1:12.

I think Black Bishop likes a 1.5:12 and his tarps look nice and pitch tight. His tarps are 12í ridgeline, which is too long for my hammock stand, so Iím using an 11í ridgeline and then I get the extra coverage I need with a more shallow cat cut.

If you did some gathering or rouching, that would transfer the strain from the fabric to the thread. Iím not sure you would need much to start popping stitches.

WillUpnDown
01-07-2018, 14:16
Hi Jellyfish,
This would be one side of a four sided fly, the dotted line is the ridge between side panels. I'm just puzzling this out. It's an interesting problem. Yes the stitches would be under a lot of tension.
There are two things that interest me. First I think the underlying idea is probably sound and second the mentions that some tent/fly makers make non-flapping full panel tents and flys--without any information how they do it. I'm heading up to camp in snow, I have a Warbonnet SuperFly, I thought maybe I'll stretch some thin line between the attachment points and see how changing the tension on it affects things.

There is more than enough going on in just hammocks, flys and tents to make it a field of engineering.

jellyfish
01-07-2018, 16:48
Hi Jellyfish,
This would be one side of a four sided fly, the dotted line is the ridge between side panels. I'm just puzzling this out. It's an interesting problem. Yes the stitches would be under a lot of tension.
There are two things that interest me. First I think the underlying idea is probably sound and second the mentions that some tent/fly makers make non-flapping full panel tents and flys--without any information how they do it. I'm heading up to camp in snow, I have a Warbonnet SuperFly, I thought maybe I'll stretch some thin line between the attachment points and see how changing the tension on it affects things.

There is more than enough going on in just hammocks, flys and tents to make it a field of engineering.

What holds up the center? Will you have a center pole? How will a hammock suspension fit inside?

WillUpnDown
01-07-2018, 18:12
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.

WillUpnDown
01-07-2018, 22:47
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/120118172114-62e8d8fc95ea49c297fbaa4221650e89/jpg/page_33.jpg159331

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.

WillUpnDown
01-07-2018, 23:37
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?
159333

jellyfish
01-08-2018, 06:25
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.

mrcheviot
01-22-2018, 11:09
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?
159333

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.

macheek
02-09-2018, 00:03
why do you need to do the cat cuts on a tarp?

ilikemyhammoc
05-26-2019, 23:00
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

godavidcoleman
08-10-2019, 11:33
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