Why catenary cut on bridge hammock?

[pdxhanger]

This is a really basic and probably stupid question to you engineer types, but why are the sides of bridge hammocks given catenary cuts? I don't mean the engineering reasons, just the practical results reasons. I'm assuming that without the cat cut the hammock doesn't lay flat, but how does it not lay flat?

I thought I read somewhere that without the curved sides the hammock will tend to bow up in the middle. (Seems counter-intuitive to me, so I wonder if I am misremembering it.) But if that were the case I would think you could counter it with a ridgeline.

Thanks!

[old4hats]

Perhaps not the answer you are looking for, but without them you would be laying in a trough.

[scrope]

The general idea is that the hammock will have to conform to a rectangle. The long sides are straight lines from one end of the spreader bar to the other because of the suspension. However, because the material isn't rectangular, but the outside *must* be rectangular, something has to give. In this case, material is "stolen" from the center of the hammock -- raising the center of the hammock and making it less of a trough and flatter.

[pdxhanger]

The general idea is that the hammock will have to conform to a rectangle. The long sides are straight lines from one end of the spreader bar to the other because of the suspension. However, because the material isn't rectangular, but the outside *must* be rectangular, something has to give. In this case, material is "stolen" from the center of the hammock -- raising the center of the hammock and making it less of a trough and flatter.

So without the cat cut the center will sag? That makes sense. Much more sense than what I thought I read about the hammock bowing up in the center without the cat cut.

Also an elegantly simple explanation. Thank you!

[ChrisJHC]

So why the need for a catenary cut? Wouldn't straight edges (i.e. a hex tarp) work as well?


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[TrailSlug]

So why the need for a catenary cut? Wouldn't straight edges (i.e. a hex tarp) work as well?

Did you see the posts above/below yours with the explanation?

[WV]

Excellent question. In my experiments with bridge hammocks, I found that I could make them more comfortable by varying the width of the hammock from the smooth catenary curve to produce crosswise ridges that provided support under the small of the back, the knees, and the neck. At first I did this by simply folding the edge at those places and sewing it . The problem with that is that it only works for one sized person. A hammock that's tailored to my body length won't be comfortable for someone taller or shorter than me. That led me to add adjustable lines along the sides of a bridge hammock so I could choose the location and tension of those transverse (crosswise) ridges. That's the most comfortable hammock design I have found, thus far. It's also complicated to build and adjust, and the lines get in the way of bugnet attachment. They also complicate getting into and out of the hammock a bit. In search of simplicity, I created the People's Bridge Hammock, which substitutes an interior angle and two straight side sections for the catenary curve (forgive me, Grizz!) This bridge design concentrates on the ridge under the knees that lets the feet hang a bit lower, thus preventing hyperextension of the knees (the biggest problem with any flat sleeping surface, IMO). As long as the hammock is long enough, it fits anyone, because they orient themselves by placing their knees over the ridge. Note: with this design, as with almost all bridge hammocks, I think, a pillow under the neck helps a lot, and the thickness and exact location of the pillow will be a matter of individual preference. To back up a bit, the PBH acknowledges that a comfortable sleeping position is achieved by distorting the catenary curve, so I tossed it out entirely to see what I could get away with, thereby making the hammock easier to build. It is also the best hammock in which to use a ccf pad for bottom insulation, though when your budget allows, you'll appreciate an underquilt. I have designs for fitted underquilts and insulated hammocks, but that's back in the land of the complicated.

[GrizzlyAdams]

The first warning here is that 'catenary curve' may mean something more general to you than it does to an engineer. To the engineer it means a curve whose equation has a particular form. You have already an answer of 'why the cut in the side of the hammock', pretty much equivalent to the one I give when a general explanation is needed.

However, the question did ask 'why the need for a catenary curve', and from the engineer's point of view of catenary, there is no need. One way or the other you bring lift to the middle of the hammock by narrowing the body there, with a curve that is smooth enough not to bunch up force on your fabric and tear it. WV has raised the design and construction of this curve design to an art form (modern art, with funny angles <grin>) they aren't even differentiable. The original zen master of DIY bridge hammocks, TeeDee, used a method for inscribing the curve on fabric which involved bending a thin fiberglass pole. Whatever curve that was you won't find it in an undergraduate engineering textbook. Me, I like the parabola for some engineering aesthetic reasons related to suspension bridges and how they move forces uniformly along the cable to the vertical supports. HYOH, and Cut Your Own Curve (but make sure your life insurance is paid up first.)

[WV]

Good point about the uneven distribution of forces, Grizz. I am aware that my angular eccentricities put more stress at the corners of the hammock and the bend at the knees, so I've taken care to reinforce those points. Also I favor 1.6 oz fabric or 1.9 though curiosity may eventually tempt me to try 1.1 just to see how it feels. So far I see no signs of stretched stitching holes or tearing. Modern art, eh? A "cubist hammock" would go nicely with my "Cubenist Manifesto". I see one of Picasso's sunbathers in a PBH ....

[Redfish]

All of this technical jargon causes dizziness and acute mental distress
How about a picture?