After reading the epic thread on bridge hammock construction, I'm
getting ready to try building a bridge hammock again. I did some initial
experiments with bridge hammocks after seeing the Aussie page back around
2005 but gave up due to lack of appropriate fabric and troubles with
the suspension.
This thread is jut to document what I'm doing and to solicit feedback.
I'm trying to design a feasible down hammock. I've got some down and
some heavy fabric (2.5 oz/ yd*yd) from Ed Speer. I need the heavier
fabric because I'm weigh too much for Ed's standard 1.9 oz/yd*yd
. I don't no whether or not the 2.5 oz/yd*yd fabric is down
proof.
If an down-insulated hammock turns out to be too complicated or a
separate hammock and underquilt might be more efficient. If it does,
then I'll make an underquilt instead
I'm thinking that I could save a little weight by building a down
hammock instead of a hammock and separate underquilt. It seems like I
should be able to save one layer of fabric by making the body of the
hammock the top of the quilt. I'll supplement the down hammock with a
Peapod and/or a down sleeping bag as a top quilt when the weather
requires it.
The first thing to think about is what is the best distribution of
down? The same thickness everywhere or should it be a little thicker
on the bottom? If I were to hang a 2nd layer below the hammock, then
a cross section at the head would look something like this, using
parabolas for both the inner and outer shells and estimating the other
dimensions from the epic bridge hammock thread.
Because of the side cut, the top edge would be about a half inch wide
at the center of the hammock (~5 inch side cut). In this example, the
area between the parabolas is 64.4 sq. in., so for a 76 inch long
insulated area, we have 4,894 cubic inches.
Note that I'm assuming straight sides, i.e., ignoring the side cut.
This example give a pretty extreme bias of down towards the bottom, i.e., 3
inches on the bottom and zero inches at the top.
What does an even distribution of down look like?
Note that the thickness in this drawing in 2.75 inches. With a 76 inch
long insulated area, it holds 10,716 cubic inches of down, roughly
twice as much as the first drawing. Looking at the drawing, it looks
like the down at the upper edge can't be doing much. So, let's cut it off.
That removes about 1,000 cubic inches reducing the volume to 9,720 cubic inches.
OK, so what if we go farther and put 3 inches of down at the bottom and reduce the thickness as we go up the sides? This reduces the volume to 9,427 cu inches.
Looking at that picture, it looks like my shoulders would get cold. My shoulders measure about 26 inches from mid-deltoid to mid-deltoid around the back. So, if I move some down from the bottom to the sides, what happens?
That has an area of 117 sq inches, which gives a volume of 8,892 cubic inches.
Of all the shapes, I think the last one has the best hope of being successful.
Since I like that distribution, what would the hammock look like?
Looking at this picture, it looks like the area up in the points where the suspension will attach won't be providing any useful insulation.
OK, that's enough for now.
Are their any suggestions or feedback? Have I done something stupid?
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