So you want to sew an underquilt, sewn thru style.
Pins, pins and more pins! I found out that sewn thru is more difficult than a baffled quilt because the two layers of nylon want to swim all over the place while you're trying to sew them together.
That being said, I did make one and it works. It kept me warm one night in Texas into the upper 30's while wearing my hiking clothes. It has 3 ounces of 900 fill power down from Ed Speer (now Tree to Trail Gear).
So here goes: Basically, you are simply making a bunch of semi-circles in a row. The “flat” sides of the semi-circles will be next to the hammock (we will call this the “inside”) and the curved sides will be away from the hammock (we will call this the “outside”).
The total size of quilt I made was 40” wide by 51” long with the stitching running longitudinal to the hammock. Pick whatever size you like. I am 6'-1” tall and just shy of 200 pounds and this is a perfect size for me with a foam pad under my feet and knees.
The first thing you do is pick the average thickness of quilt you want to make. Then calculate the diagonal of your semi-circle which will be 8*thickness/pi. I other words. The average thickness multiplied by 2.55. This will be the width between stitches on the “inside” of the quilt.
Now we need to calculate the stitch spacing on the “outside” of the quilt. That is pi/2 time the “inside” width that we just calculated above. In other words, the “inside” spacing multiplied by 1.57 to get the “outside” spacing of stitches.
Now divide the desired minimum width of your quilt by the “inside” spacing and round UP to the nearest whole number. This is how many chambers you need. So, the final width of quilt will be the number of chambers times the “inside” width.
How much down do you need? Just multiply the quilt width by the length by the average thickness and then divide all that by the fill power. That is how many ounces you will need.
When you cut your fabric, don't forget to add width and length to make big enough hems that you can run you suspension shock cords through (hint: go bigger than you think you will need!).
I run a 5' length of 1/8” shock cord through each of the two side channels, tie that to another 5' of 1.75mm Zing-it which loops over the end knot of the hammock and back to the other side of the quilt where it's tied to that shock cord. I do this at both ends. However, some folks like to attach shock cord to the corners of the quilt (no side channels) and run that around the end knot of the hammock. I like my way because I can then slide the underquilt toward the head or foot as needed, once I'm in the hammock.
I run 3” of 3/32” shock cord through the head and then the foot end, with a cord lock on each, to cinch up the head and foot ends of the underquilt, as necessary.
Some folks like to incorporate the “triangle thingies” in their suspension, but I have not found it to be necessary. YMMV.
Here are the pics to prove I didn't just make this up