A Dual Mode Bridge Hammock
The Dual Mode Bridge (DMB) Hammock
After experience with my DIY bridge hammocks, I came to see that the most significant limitation was that I cannot close up a tarp
close to the hammock in heavy weather---the spreader bars are in the way. Natural solutions to this problem involve a larger tarp,
with longer sides than are usual. The idea of a bridge hammock that can also be suspended without spreader bars suggested itself
to me as a way of solving this problem. When the weather is nice, use the hammock in bridge mode. When more protection is needed,
shift the suspension to gathered end mode and bring in the tarp close. Considerable interest was expressed in the Hammock Forums
when I announced a prototype of this hammock, and correspondingly I've put together this article that describes key steps to making
it. Thanks to AngrySparrow, a pdf version of this article is available on-line, with the figures embedded in the document. Figures in this article are all obtained by following embedded URLs that reference Figures. This is a side-effect of it being relatively easy for me to transform the article written in the Latex format to one written in vBulletin format by turning Latex references to figures into vBulletin links to URLs.
The DMB described here is a double-bodied hammock, with an entry point between bodies to slide in a pad. This design gives one a great deal of flexibility in use of the hammock, and by using two layers of 40 denier nylon, the double hammock body is as strong---if not stronger---than a single layer of 70 denier nylon. However, going the double-body route led to some challenges in design and assembly of the hammock.
The DMB prototype weighs 24 oz., excluding the spreader bars, the straps around the tree, and the suspension line from tree strap to biner. As such it is considerably heavier than the light-weight bridge hammock I described in the Hammock Forums (which weighs 20 oz., excluding only the poles, and includes a carrying bag). The added weight is due primarily to two carbiners (about 1 oz each), more fabric used---the DMB body in the prototype is cut 11' feet long, while the lightweight hammock body is less than 7' long, and also due to use of a heavier grade bugnet material. The lightweight version uses no-seeum mesh from Thruhiker, that comes in at 0.7 oz/sq yd, while the material in the DMB is from Speers, advertised at 2 oz. / sq yard. I went with the heavier grade netting largely because I already had a bunch of it on hand, and in gathered end mode I could easily be sticking my feet on it!
Cutting Matching Rectangles
The inner body is cut at 57" wide, the outer body at 57.5", the length of both 132". This is before hemming. My prototype loses 1.375" each end because of the way I hemmed it, and 0.75" on each side due to a double rolled hem, reducing the dimensions when hemmed to 51" x 129.5" for the inner body and 51.5" x 129.5" for the outer body. I used uncoated 1.05 oz nylon for bodies, and this has held up my 195 pounds without incident. I have some concerns though about this material stretching, and will be keeping my eyes out for evidence of that.
Cutting two quality rectangles that have to align posed enough of a challenge for me to invest in some hardware help. At Home Depot I found a square that shoots laser beams at right angles, with accuracy of a quarter inch over 12'. See Figure 1(a) and Figure 1(b). With the laser laying down a straight line, I mark the straight line with masking tape, and then make the cut using the tape as a guide. I am happy with the results of this technique.
Construction of the Suspension Arc
The DMB bridge suspension puts a 6 inch, 72 inch long deep parabolic cut into a piece of 10.5 inch deep, 73 inch long 1.9 oz ripstop. Along the parabolic cut I create a channel. A length of 3mm Vectran 12 cord passes through the channel, and bears the weight of the occupant. This suspension piece is sewn together with the inner and outer bodies in a manner to be described later.
Table 1 gives coordinates in inches for 6" deep cuts along a 72" length. The view of the table is from the top edge of the fabric to be cut; the depth at the left and right endpoints is 0, the depth at the center is 6". You can measure and mark the depths along the top edge, connect adjacent points with a straight line, and cut along the curve so defined.
I cut the curve into the fabric to exclude the extra at the ends for hemming. Here I used very narrow double rolled hems at the ends of the piece, allowing just 1" hem allowance.
The first step is to hem the sides, and suspension curve. On the suspension curve I use a double-rolled hem, 3/8" wide (Figure 2(a)).
I wanted the option to use either webbing or suspension cord for the support, so I created a channel that was sure to be wide enough for the (0.5") webbing, by pinning the hem where it would create a channel just wide enough for the webbing (Figure 2(b)). This channel will be carrying load, so I put in three stitches. (Figure 2(c)). To support the use of suspension cord, I sewed in short webbing tabs at each end of the channel (Figure 2(d)).
The suspension cord on each side is cut at 144", with bowline loops tied at both ends, so that the finished length is 136". Thus, on each side, one cord will go from the apex of the suspension triangle, form the side of the triangle, pass through the suspension channel, emerge on the other side to form a side of the opposite suspension triangle, and terminate at that triangle's apex. Next I tie another line, 9" long, onto the long suspension cord, tautly, using bowline knots. Between these knots is positioned an overhand knot. See Figure 2(e). The double line is looped through the tab, and tightened at the knot where the lines meet. Then the suspension line is pulled tautly along with the suspension piece, and the line is tied off on the other tab, to hold the tension. This keeps the suspension piece from sliding towards the middle when under load.
The DMB uses what I call a "structural seam", i.e., a seam that bears significant load. I first saw this idea used in the JRB BMBH where it creates a 26" wide pad pocket. I thought to make the distance between seams wider, so that I could put attachment points on the seam for things like an under-quilt, weather shield, or over-cover; it was also key in my lightweight bridge hammock for reducing the amount of material used to create a double body. Experience with past bridge hammocks suggested to me that an inter-seam distance on the order of 36" would be good, as I'd built hammocks where the minimum hammock width was 36", and that was enough for under-quilt coverage.
The seam brings together three pieces of fabric: the suspension curve, an inner body, and an outer body. After sewing there are three pieces of fabric above the seam, and two below (because the suspension piece does not go below the seam. Above the seam the suspension piece is sandwiched between the outer body and inner body pieces---this is why you cannot see most of it in the DMB.
Steps in assembling the seam are illustrated in Figure 3(a), Figure 3(b), and Figure 3(c). In viewing the figures keep in mind that the width dimensions are not to scale with the height dimensions---all those layers are compressed together in the seam. The figure is useful for showing the folded structure, and for analyzing how much fabric for each
piece is embedded in the seam.
- shows how the three layers are aligned prior to any stitching. The inner and outer bodies have folds; after creating the seam, the short side of the fold will be pulled above the seam, and the long side of the fold will drop below the seam. The double body is effectively the long sides of the inner and outer body. The orientation of the folds are important in the assembly---the long sides are brought together. The short side of the fold on the inner body is 8 inches, the short side of the fold on the outer body is 7.5 inches. The difference is due to the folds in the structural seam, and is needed for the top edges of the short sides to match in height above the structural seam.
- To create the fold I'd suggest measuring and folding one edge of body over the length of its short edge, iron the fold to crease it, and even run a stitch to hold the crease in place. In my DMB prototype I actually cut the short edge off the main body and reattached using a flat-felled seam, but in retrospect that was unnecessary. However it is done though, the stitching should be only in the middle 72" of the body, the same 72" that will be incorporated into the seam.
- Just as an aid to reduce the number of free edges flapping about, sew together the suspension piece and outer body as shown. The edge folded over should be 0.5"---this is the basic width of the whole seam, and every fold we do in assembling the seam will be likewise 0.5".
- Now we have really only two separate pieces to deal with, and the rest of the seam construction is a double-rolled flat felled seam. We consequently align the edge of the outside body 0.5" above the edge of the inside body, in preparation for the flat-felled seam.
- Fold the extended edge of the outer body 0.5" over the edge of the inner body.
- Roll the top edge over 0.5", creating a flat-fell seam fold.
- Roll the top edge over 0.5" again, for a double-rolled flat-fell seam fold.
- Before sewing stitches we move the suspension piece and the two short sides of the body into position. The short edge of the inner body is rolled up from under the seam, the suspension edge and the short edge of the outer body are pulled away from the top edge of the rolled seam. Once in position, three stitches are applied to hold the whole thing together.
The suspension piece loses 2 inches in height in this seam, measured from the seam's top. The maximum height of the suspension piece loses 3 x 0.375" + 0.5" due to the double-rolled hem, and 0.5" channel. So we subtract 3.625 inches from our starting height of 10.5 inches, and see that at the maximum height the suspension piece is 6.875 inches above the structural seam, and 0.875 inches above the upper portions of the inner and outer bodies.
Figure 4(a) illustrates the real layers laid out to show there are three of them. You can see that the short edge of the inner body (shiny brown) is on top, that the outer body (camo) is in the middle, and the suspension piece (dull brown) is at the back. Figure 4(b) depicts a finished seam.
On my prototype the seams are 35" apart (measured from the mid-points of the seams); I lost an inch of width of the inner and outer bodies by separating the shorter and longer sides, and then re-attaching with a flat-felled seam. If you create the short side by a simple fold, the hammock will be an inch wider, with the added inch appearing between the seams.
Connecting the Bodies
After the structural seam is created on both sides of the hammock we have still to sew together the inner and outer bodies that lay above the structural seam. We need to do this selectively, to leave gaps for the suspension ends, and to leave a large gap for inserting a pad.
The scheme is described by Figure 5(a). We leave an 8" gap at each end of the structural seam to allow the suspension to come out. At three of the four sections past the end of the structural seam we join the bodies again, for 12". On the fourth side we do not, leaving a space to insert or extract a pad. Measured on the fully assembled hammock, I have a 25" wide gap from the end of the structural seam to the whipping.
To tidy things up a bit I sewed through all three pieces above the seam, along the short edges of the suspension piece, so in this gap there is a closure at the seam. I did not sew the full 6" length, just the bottom 4", to leave the suspension piece free to extend out under load without putting tension on the top portions of the inner and outer bodies. See Figure 5(a) for a diagram of joining the bodies, and Figure 5(b) which illustrates on the prototype where the structural seam ends and the outer body continues to the left as one integrated piece, and Figure 5(c) where a pad is inserted between the bodies.
The bugnet is made up of one large center piece, and two end pieces. The center is cut as a rectangle, 39" wide and 72" long. I edged it with bias tape; in doing so did not lose any width or length. I edged three sides of the center piece with a number 3 coil zipper (roughly 150" long), with two double sided zipper pulls. To "make the corner" on the front side, I cut an arc at the corners of the center piece on the front side, edged both sides of the cut with bias tape, and reattached the cut corners to the main body, with the zipper in between.
The bugnet end pieces narrow from 39" where they are sewn to the center piece, to 9"; an end piece is a trapezoid whose parallel sides are 39" and 9" in length, and whose angled sides are 19" in length. The bugnet is sewn to the edges of both inner body and outer body in those places where the inner and outer bodies are joined, and is sewn only to the inner body in regions where the two bodies are separated. There is one important exception, however. In order to permit passage of a spreader bar through the hammock, the four inches of an end piece closest to the center piece are NOT sewn directly to the inner body. Instead, opposing strips of velcro (in my case, FreeMagic) are sewn to the bugnet, and the inner body. We will see illustrations of this in use when we talk about the suspension.
To complete the coverage, a 9" wide rectangle of inner body material is sewn from the narrow base of the bugnet end piece to the end of hammock. This rectangle then becomes part of the hammock end that is whipped---the hammock body at the whipping point is now a tube.
This design is illustrated in Figure 6(a). Figure 6(b) illustrates where the center piece meets an end piece, and Figure 6(c) illustrates the end piece in the prototype.
When sewing the bugnet to the body, put a few long straps in the long zipper-free side , leaving one end of the strap on the inside of the hammock and another on the outside. This enables you to roll up and tie down the bugnet when you don't need to use it.
Notice the three tabs on the border where the center and end pieces come together. These are used to hold up the bugnet. I have found a good way to use these is to put a trail stick through the loops, attach a short piece of cord at the center, pull the cord up at about 45 degrees towards the tree, tie a couple of half-hitches around the ridge-line, carry the cord to the support biner and tie it off. It should not be too tight, just enough to lift the bugnet up. One cord to the center tab with no stick works well too, in both modes.
The ends of the inner body are whipped, conventionally. A single strand of suspension rope is tied at the whipping, and is passed through the folds of the outer body. Then the outer body is whipped, and a suspension cord tied to it. The ends of the suspension cords ought to end in small loops, with the loops being at the same height under tension, and being as close as possible to the end of the hammock.
Figure 7(a) shows the end of a gathered end suspension, while Figure 7(b) shows the end of a bridge suspension. The orange line in these photos is the ridge-line. I have mine set at 115"; the length of the hammock from biner to biner is 125". This gives good sag to the hammock in gathered end mode, and leaves the ridge-line with some tautness when in bridge mode. Tautness in the ridge-line provides a convenient vehicle for pulling up the bugnet. The red cord has bowline loops at each end,
and is 10" long from loop top to loop top. In gathered end mode the loop through which the hammock suspension loops pass is clipped to the biner (Figure 7(a)). We have to relieve the tension on the spine through in bridge mode, and this is accomplished by exchanging which end of the red cord is clipped to the biner. In bridge mode the gathered ends hang somewhat loosely from the red cord (Figure 7(b)). Here we also see the suspension cords coming up from the tips of the spreader bars. Figure 7(c) shows how the spreader bar passes through the enclosed body of the hammock. In Figure 7(d) we see a close up of the spreader bar end, with the tip of the spreader bar slipping into the slot we created by tying an added length of suspension cord to the main cord. Note also how the velcro strips placed there are used to close up the gap around the pole.
The spreader bars shown in these photos are replacement tent poles, 0.625" diameter aluminum. Each pole is made of two 18" segments that interlock; the finished width is 36".
After dozens of hangs I have never had the poles slip through the cord when under tension. I have been careful while getting into the hammock not to jostle the arrangement. Under tension, the two lines close up tight around the pole's nub. That said, the poles can fall out of position when the hammock is unoccupied. Figures 7(e), 7(f), 7(g), and 7(h) illustrate how a small bit of lightweight but strong cord can couple the tip to the cords. A loop is tied around the pair of cords, the pole tip is slipped into the loop as it is inserted between the cords, and that's all there is to it. The cords can't separate enough to let the pole jostle through.
I will be looking into more proactive ways of holding the ends in place, such as webbing caps as have been suggested by others.
I can also use my Pacer Pole hiking poles as spreader bars, building on ideas explored in other of my bridge hammocks. Figure 8(a) shows use of the wirenut and washer, on the spreader bar tip. Figure 8(b), Figure 8(c), and Figure 8(d) show the steps in using a securing device for the handle end. I use 22" of webbing, sew a steel ring at one end, and attach a length of spyderline to the other. The webbing wraps around the handle, but I take care to ensure that only webbing (not the ring, and not the spyderline) touch the handle---this to spread out any compression stress that might be carried on the sides. The spyderline wraps around the suspension cord, and is tied off on the steel ring. Admittedly this is a solution peculiar to the Pacer Pole, but is worth illustrating as similar solutions that would attach a web cap for a handle would easily be devised.
On my lightweight bridge hammock I learned I could use the structural seam to attach a small underquilt. I intended for the DMB to follow, and to be able to use the UQ I built for the the lightweight bridge.
On account of the difference width between structural seams on the two hammocks, I couldn't just put more hooks on the seam and clip the UQ on the way I did with the lightweight bridge. Instead I sewed in small rings along the structural seam, on both sides. I made small cord loops and attached them in matching positions along the UQ. I tied a bit of cord to the head corners of the UQ. Each cord loop passes up through its corresponding ring, and the cord from the corner on that side passes through the loop. The end of the long cord is tied to a bit of shock cord, which is itself tied to a clip, and the clip is attached to a utility ring at the corner of the hammock. See Figure 9(a), Figure 9(b), and Figure 9(c).
The idea is that the shock cord allows some "give" if needed when the hammock is loaded and the UQ were stressed to pull farther down. This would happen only if there was a lot of stretch in the hammock body; time will tell. It is a bit tedious to thread the UQ onto the hammock though; I'll be thinking about faster ways to do the attachment.
I modified the UQ slightly to put cord on the ends so that I can cinch up the ends to better conform to the shape of the hammock when in gathered end mode. Experience has shown that using the sides attachment only in gathered end mode leaves air gaps---the hammock body is not spread apart fully as it is in bridge mode. Traditional shock cords from the corners up the the suspension would appear to be the solution if one uses this UQ in gathered-end mode.
I can transform the DMB from bridge mode to gathered end mode in about 1 minute (at least with the tent poles as spreader bars). All that is needed is to
- remove the spreader bars, close up gaps through which they passed into the hammock body,
- pull up the gathered ends by changing which end of the 9" cord pulls the ends up,
- adjust the tension on the bugnet support.
It is not necessary to un-attach the cords making up the sides of the suspension triangle, as there is no tension on them without the spreader bars. It may not be necessary to change the length of the ridge-line, although if one wants to, it can be -as simple as un-clipping the ridge-line on one end where there is a sort of daisy chain of loops, and re-clip at a length suited for the mode.
Transforming the DMB from gathered end to bridge mode takes a little longer, only because putting spreader bars in takes longer than taking them out.
Note again the observation made in the underquilt section, that for the underquilt to be effective in gathered end mode, adding a more traditional suspension is necessary in order to pull the UQ up to be flush with the hammock body. I still have some experimenting to do to make this part as seamless as is changing the suspension.
I'll conclude with some views of the DMB in both modes. Figure 10(a) is an exterior view in gathered end mode. Figure 10(b) is an interior view in gathered end mode.
Figure 11(a) is an exterior view in bridge mode. Figure 11(b) is an interior view in bridge mode.
Figure 12(a) is a view of the DMB in gathered end mode, at the Velvet Rocks shelter on the AT. Figure 12(b) is a view of the DMB in bridge mode, at the Moose Mountain shelter on the AT.
Finally the author in his native element
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