DIY Full-Length Underquilt: Step by Step

**hammock_monk** · 10-25-2012, 22:34

Wolf, thank you; that is very enlightening. It never occurred to me that there might be different kinds of arches and that the geometry would be different. Seems obvious now.

**Wolfman** · 10-25-2012, 23:08

So many years since I did this kind of math!!

OK so what I found and what I remember is;

Setchup is a "Eyebrow Arch" and the

Spread Sheet is an "Elliptical" (Or half of an Elliptical shape)

Still have no idea how to do the math! But oh well at least we have names!

Wolf

**CatSplat** · 10-25-2012, 23:20

That measurement is going to depend heavily on your curve profile. Sketchup will allow you to define whatever shape you want, but that will affect your fabric requirements - for example, using a triangle shape instead of a curve will give you the maximum possible mid-chamber height for a given fabric width, so there's definitely some variation due to shape.

I also note that Sketchup is defining the fabric width as approximately 6", how accurate is it?

For my sheet, I use a basic approximation of the perimeter of an ellipse to calculate the upper fabric per baffle. Specifically, I calculate half the perimeter of an ellipse:

(1/2)P = (2*PI*SQRT((a^2+b^2)/2))/2

For your example (5.25" width, 2" baffle, 3.25" max height), it returns an upper fabric width like this:

(2*PI*SQRT(((5.25/2)^2+(1.25)^2)/2)/2 = 6.46"

Now, if I use the more advanced Ramanujan approximation (which is something I just learned of the other day), it goes like this:

PI*(3*(1.25+2.625)-SQRT(((3*1.25)+2.625)*(1.25+(3*2.625))))/2 = 6.28"

That approximation works better for an ellipse of larger eccentricity (but still a fixed profile), so in this case it's likely the more correct value of the two, but with only 2.8% deviation either approximation should work fine. If we take a fixed upper fabric value of 6.00", with 5.25" baffles, this will return a maximum chamber height of 3.01".

So, I'd go with the Ramanujan on this one. The approximations are pretty close to each other, so for UQs there wouldn't really be a noticeable difference. Either way, I'll update the spreadsheet with a Ramanujan for the next version for accuracy's sake.

I suspect that Sketchup is using a rough approximation for that curve length, considering the "~6" value that's given, with no decimal places to indicate accuracy. Combined with your chosen (non-pure-ellipse) curve profile, that should account for any discrepancies.

**Wolfman** · 10-26-2012, 07:54

OH God my head hurts! Way to early for that level of Math!!

Thanks CatSplat great explanation on the math, even if it is too early.

Wolf

**hammock_monk** · 10-26-2012, 16:07

CatSplat, I'm so glad you joined the discussion. I almost PM'd you yesterday to ask for your input. The accuracy of Sketch up is, I think, to the hundredth of an inch—more than adequate for my purposes. But, as you observe, the more critical element is the curve profile. You and Wolfman both note that the differences in the width of the outer fabric layer will depend entirely on the profile of the curve of the arc.

I love the direction this discussion has taken. I'm a bit of a math geek, though more on the drafting side than computation.

In any event, I am planning to approach the question from the opposite direction: I am going to use 6 inches as the width of the fabric for the outer layer of each chamber and let the curve profile take care of itself. I figured that each chamber will end up with somewhere between 2 3/4 and 3 1/4 inches of height. With 850fp down, I expect that will keep me comfortable down to 20° and maybe even the teens.

The last of my materials arrived in the mail today.

I'm going to spend some time practicing with my sewing machine this weekend before getting started. I picked up a silver sharpie for marking lines and plan to use it in combination with some painters tape.

Looking ahead, I have what may be a new idea for stuffing down. It may not work at all but the concept is to create some degree of vacuum at the bottom of the down chamber. I have ordered a small laptop cooling fan that I intend to attach to the outside of the fabric at the bottom of the vertical chamber that I am trying to fill. I don't know if the idea will work out but I don't think it will cause any problems in any event. If it does help I'll be sure to do a little writeup for others to see.

Thanks again for your input and assistance. I appreciate the support.

PS: I apologize for any spelling or typing errors in my posts. I severed a finger and badly damaged my wrist in an accident seven weeks ago. I am using dictation software for most of my longer posts and I'm sure I'm missing a few mistakes when I review it before posting. it's surely going to make sewing an adventure!

**Wolfman** · 10-26-2012, 19:52

I severed a finger and badly damaged my wrist in an accident seven weeks ago.

Gosh Monk, take it easy with the Thread Injector!!!

OK so maybe that was not very funny, to early? I hope your hand turns out OK. I thank the stars that in all my construction I have been very lucky and only had minor mishaps. I know too many that have really hurt themselves. It is not only painful and debilitating, but it can be a life changing experience, and that is not always a good thing.

Keep us updated I am really looking forward to this. I like the fan Idea, although I would probably try my big shop vac! Where did all that down GO!

Wolf

**hammock_monk** · 10-26-2012, 20:59

Hee hee, all jokes welcome re my hand. Better laughing than crying.

**hammock_monk** · 10-27-2012, 16:23

OK, I started measuring, marking and cutting today. It's pretty scary but I suppose I'll gain confidence as I go along. The first picture I'll show is of my materials: 1.1 ripstop (calendared) and DWR ripstop, 1/8 in shockcord, some grosgrain, mitten hooks, cordlocks and noseeum. Down not shown. Materials from Wilderness Logics and DIY Gear Supply.

And here is a shot of my tools: dining room table, old Singer sewing machine, silver sharpie marker, measuring tape, painters tape, a 48-inch level (as straight edge) and 8-foot metal straight edge. Note the gizmo I made to hold the oversized thread spool. Dan at a local sewing place gave me that idea.

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Here are shots to show the fabric marked and cut.

Finally, the before and after shots as I dialed in the tension on the sewing machine:

Honestly, I'm a bit wiped out just from that much work. I may tape the baffles in place later (I have them cut) but I'm about done for today. I expect to work more efficiently tomorrow, now that I am getting the hang of it. It'd be faster to do both quilts at once, but I want to hold off on the second one, so I can make any corrections or improvements that come to light during construction of the first one.