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  1. #1
    Member I Splice's Avatar
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    Down Hammock Design

    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?
    Last edited by I Splice; 01-04-2009 at 23:47. Reason: wordsmithed a sentence to make it clearer

  2. #2
    jeffjenn's Avatar
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    I like the idea you have, but I have read many times that sewing through the body of the hammock can give problems. Sewing the baffles through the body will create a lot of points for possible failure IMO.
    Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways body thoroughly used up, totally worn out, and screaming "WOO-HOO, what a ride!!"

  3. #3
    Member I Splice's Avatar
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    Quote Originally Posted by jeffjenn View Post
    I like the idea you have, but I have read many times that sewing through the body of the hammock can give problems. Sewing the baffles through the body will create a lot of points for possible failure IMO.
    I've thought about the baffles weakening the fabric. It seems like transverse baffles wouldn't weaken a bridge hammock. I think the main stress on the fabric on a bridge hammock is from side-to-side, so baffles that go in that direction shouldn't weaken the hammock noticeably. I'm also using a pretty heavy fabric, which should give me a little more leeway.

    I plan on over filling enough so that I don't have to worry about the down shifting down to the bottom.

  4. #4
    Senior Member angrysparrow's Avatar
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    I'm sure you will have seen it already, but if not be sure to have a look at Jeff's previous efforts with down hammocks.
    I think that when the lies are all told and forgot the truth will be there yet. It dont move about from place to place and it dont change from time to time. You cant corrupt it any more than you can salt salt. - Cormac McCarthy

  5. #5
    Member I Splice's Avatar
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    Quote Originally Posted by angrysparrow View Post
    I'm sure you will have seen it already, but if not be sure to have a look at Jeff's previous efforts with down hammocks.
    I'd seen his first version but not his latest. Thanks for the link. He gets by with no baffles!? One of the reasons for making an insulated bridge hammock is that the baffles are much easier to design. Fitting a boat-shaped hammock looks really hard.

    I suppose with no baffles but some quilting-loops you could get good control over the distribution of the down...

  6. #6
    GrizzlyAdams's Avatar
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    Reading at the beginning of the thread I thought to make a point made later, that transverse baffles ought to be OK, structurally. Making the baffles with a differential shape will be a bit more work that way, but doable.

    While this is interesting and different and in no way would I discourage you from trying it out if you like, I'll just point out that extra effort doesn't save you a great deal of weight. With a bridge hammock a full length UQ needn't be more than 36-38 inches wide. Let's say 72 inches long. That's 2.11 sq yards. If you used DWR 30 denier ripstop, at say a real 1.2 oz/sq yd, the piece of fabric you're saving is 2.5 oz. Half that if you were to go the 1/2 UQ route.

    The advantages of a built-in quilt include no shifting around...but hanging a separated UQ from the corners has been quite stable in my experience. In fact, in one model I suspended the quilt from the sides of the hammock from a seam; this actually is the preferred method for tensioning the UQ (and is something you get for free from the built-in approach).

    You know the drill around here..if you build it, you gotta show us pictures! Good luck.

    Grizz

  7. #7
    Member I Splice's Avatar
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    Quote Originally Posted by GrizzlyAdams View Post

    ...

    While this is interesting and different and in no way would I discourage you from trying it out if you like, I'll just point out that extra effort doesn't save you a great deal of weight. With a bridge hammock a full length UQ needn't be more than 36-38 inches wide. Let's say 72 inches long. That's 2.11 sq yards. If you used DWR 30 denier ripstop, at say a real 1.2 oz/sq yd, the piece of fabric you're saving is 2.5 oz. Half that if you were to go the 1/2 UQ route.

    The advantages of a built-in quilt include no shifting around...but hanging a separated UQ from the corners has been quite stable in my experience. In fact, in one model I suspended the quilt from the sides of the hammock from a seam; this actually is the preferred method for tensioning the UQ (and is something you get for free from the built-in approach).

    You know the drill around here..if you build it, you gotta show us pictures! Good luck.

    Grizz
    If this ever gets built, I'll definitely provide pictures. That's a good point about the weight savings. Though saving 2.5 ounces is still 71 grams that I don't have to carry. A gram here and a gram there a pretty soon you're carrying an extra pound.

    I suppose that I could use momentum or even noseeum mesh to trim the weight difference even more. (Has anyone built a down-filled article with exposed noseeum mesh?)

    I was surprised by your suggestion of only needing 36 to 38 inches of width for an underquilt but I measured my shoulders and 36 inches will go around my back and will stick up about 2 inches on each side. I'll have to think about the implications of that.

    I think I'll want at least my height plus 2 inches for the length, that gives me at least 76 inches for the length. The partial underquilts just look cold to me. If I were to add down filled bulkheads (end caps), I'd probably want a little more length.

    BTW, most of my camping is during the spring and fall with just a little during summer and winter. It doesn't get very cold for my spring and fall trips, 30 F is cold for us.

    The hopefully reduced fiddle factor in setting up an insulated hammock would be a plus. OTOH, the lack of flexibility would be a negative.

  8. #8
    Senior Member dblhmmck's Avatar
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    I really like the idea of a dowm hammock. Your drawings are very high quality and well thought out. The savings in weight over the extra layer of fabric could be 5 or 6 ounces. {Edit:I didn't see Grizz's post above. I believe his weight savings analysis to be more accurate than the estimates which I listed}

    On the other hand, that extra layer of fabric when used on the hammock body would allow a lighter fabric to be used on the under-quilt (if that quilt were sepparate). Then our net weight savings may be closer to 4 ounces. The extra layer also gives the option of using different insulations in different weather conditions.

    That layer also adds strength. I would have also some concerns about forces when entering or exiting the hammock causing tears in the baffle material, if the hammock body was also the quilt shell. But this is only a guess.

    I can tell you what has worked for me. Bridge hammocks with inflated pads can virtually eliminate contact with the hammock walls above the pad. They do it by widening the bottom and lifting the occupant, and giving the most comfortable hammock experience imaginable. At least this is true with my 26" wide WarmLite DAM. My POE AO-Lite (at 19 ounces and 20" width) gives the flattest lay with pad fully inflated. Although, in cold weather, a CCF pad needs to be added to it wide enough to insulate the shoulders. Anyway, that is how I see the alternative to a down quilt, for bottom insulation on a bridge hammock.

    But to answer your question, in my own humble opinion, I would say make the hammock and quilt sepparate.
    Last edited by dblhmmck; 01-05-2009 at 01:03.
    "Better living through Hammockry"

  9. #9
    BillyBob58's Avatar
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    Quote Originally Posted by dblhmmck View Post
    I really like the idea of a dowm hammock. Your drawings are very high quality and well thought out. The savings in weight over the extra layer of fabric could be 5 or 6 ounces. {Edit:I didn't see Grizz's post above. I believe his weight savings analysis to be more accurate than the estimates which I listed}

    On the other hand, that extra layer of fabric when used on the hammock body would allow a lighter fabric to be used on the under-quilt (if that quilt were sepparate). Then our net weight savings may be closer to 4 ounces. The extra layer also gives the option of using different insulations in different weather conditions.

    That layer also adds strength. I would have also some concerns about forces when entering or exiting the hammock causing tears in the baffle material, if the hammock body was also the quilt shell. But this is only a guess.

    I can tell you what has worked for me. Bridge hammocks with inflated pads can virtually eliminate contact with the hammock walls above the pad. They do it by widening the bottom and lifting the occupant, and giving the most comfortable hammock experience imaginable. At least this is true with my 26" wide WarmLite DAM. My POE AO-Lite (at 19 ounces and 20" width) gives the flattest lay with pad fully inflated. Although, in cold weather, a CCF pad needs to be added to it wide enough to insulate the shoulders. Anyway, that is how I see the alternative to a down quilt, for bottom insulation on a bridge hammock.

    But to answer your question, in my own humble opinion, I would say make the hammock and quilt sepparate.
    Hey DH, that is very interesting. But when looking at pads vs UQs in a BH ( maybe I need a new thread?), When I have put my wide and thick Thermarest Camprest ( which is too heavy for packing anyway) in my BMBH, it has helped with the sides as you described above. But, I have found that this does not seem as comfortable overall- I think maybe my back did not feel as good (And a lesser concern was it felt a little less stable). But your experience was "most comfortable imaginable". Was the wide DAM fully inflated? Have you used the DAM in the hammock a lot with no durability problems? Mine is too heavy but I guess a DAM might be more competitive with a quilt weight wise.

    But when looking at a separate UQ vs. a down hammock, another factor might be versatility. You would save several oz of weight for one layer of fabric. But then- at least with that hammock, you would always be stuck with the full weight of an UQ, and maybe a winter weight one. Though I suppose that would only be a concern in the actual summer heat, since many people carry an UQ at least most of the year, if not always.
    For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us....that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. For we know that the whole creation groans and suffers the pains of childbirth together until now.
    Romans 8:18,21-22

  10. #10
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    Quote Originally Posted by BillyBob58 View Post
    But when looking at a separate UQ vs. a down hammock, another factor might be versatility. You would save several oz of weight for one layer of fabric. But then- at least with that hammock, you would always be stuck with the full weight of an UQ, and maybe a winter weight one. Though I suppose that would only be a concern in the actual summer heat, since many people carry an UQ at least most of the year, if not always.
    If you assume all things are equal and all the design issues are resolved, then there are 2 sides to this. A truly insulated hammock isn't capable of being vented during times where you might want to do that... but it won't vent on you during times where you don't want it to do that. There are no issues of sealing it around the edges.
    Youngblood AT2000

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