Catenary Shelters
Tarps, Tarp-Tents, and Tents
— Described —

The word "catenary" is latin derived, and originally referred to the curved shape of a length of chain suspended from both ends. The term, and the mathematics describing the curve, are nothing new. Both date back to the 1650s.

Regarding outdoor shelters, the term refers to the droop of fabric between its supports.

All fabric droops between its supports, to a greater or lesser extent. The amount of droop depends mainly on the tension in the fabric. Nonetheless, all shelters made of fabric are catenary, whether they are tents, tarp-tents, or tarps.

The covered wagons (prairie schooners) of pioneer days were catenary shelters. Their iron bows supported canvass bonnets. Between each pair of bows, the bonnet drooped in a catenary curve.

Army pup tents used in the civil war were catenary shelters. Of course, this general type of shelter dates back thousands of years. The design originated with prehistoric peoples, who might have slept beneath animal skins supported by sticks and secured with cordage. The only thing they lacked was the trendy term "catenary." And by the way, they thrived quite nicely - for perhaps millions of years - without our modern gear which we consider so essential.

The Pup Tent

An obvious improvement was the addition of a triangular piece of canvass sewn to each end of the two rectangles, forming a headwall and footwall. With a few more buttons and button holes, the ends could be closed off during inclement weather. The pup tent show here has internal poles, no floor, and in fine weather can be pitched a little higher, leaving a gap all around the bottom, for a little more air.

This version of the pup tent is still in use today, mainly by the military. It is the type that I first used in the early 1950s, on family backpacking outings in the high Colorado Rockies. Like all tarps and tents, it is the catenary type, as evidenced by the curve of its walls. Even the ridge droops slightly, depending on the flexibility of the pole. This pole does not assume a genuine catenary shape, but the fabric walls angling down certainly do.

Let's talk about the advantages and disadvantages of the pup tent.

You can find these for sale on the Internet under the names "tarp-tent" and so forth. Obviously they are patterned after the ray-way tarp which preceded them. But buyer beware! They have significant disadvantages.

To improve ventilation you might open the door, except that rain falling from the sky and splashes rebounding from the ground would then enter your shelter. Even if you pitched this shelter higher to create a "ventilating" gap around the lower perimeter, you would not, in fact, ventilate the interior. The moisture inside is warmer and therefore lighter, and will not sink to escape from the lower perimeter gap. Hot air balloons work on the same principle - no matter how large the hole in the bottom, it does not ventilate the balloon's interior and allow the warmer and more buoyant air to escape.

Those old cotton canvass pup tents did not act like hot-air balloons because the canvass material breathed. But today's silicone and urethane nylons do not. A pup tent made of these new materials can be deadly to anyone using a stove inside. Stove fumes are odorless and extremely poisonous. They are lighter than air, so will not escape out the lower perimeter gap. Even with the door wide open, cooking inside this type of shelter is very risky. In stormy weather a hot meal cooked beneath a tarp can make all the difference, and in prolonged stormy weather those hot meals could go a long ways to insure survival.

Because the pup tent is fairly small, it has little space for your gear. And due to its inadequate ventilation, in wet weather your clothes, shoes and rainwear will not even begin to dry, as they would beneath an open tarp.

Catenary hooped shelters

Hooped shelters date back thousands of years. In one method, small and flexable live trees were bent over and their tops staked to the ground, or lashed to the base of other trees. Over these curved support poles, animal skins were draped for shelter from the elements.

The covered wagons of Pioneer days were of course hooped shelters, with their iron bows supporting canvass bonnets. Which is only to suggest that the design is nothing new.

Most modern hooped shelters - whether they are called tarps, tarp-tents or tents - consist of flexible poles supporting nylon fabric. The most basic type, shown here, has no footwall, headwall, or floor. But as the drawing depicts, each pole has a line connecting its ends, to hold its curved shape. The shelter also has guylines extending from each end, (not shown) to support the hoops upright.

Some models have floors. Some have footwalls. Some have headwalls which slope in or out. Some have one or both ends open, but partially covered with beaks. But all share the same disadvantages.

The proper way to pitch a hooped shelter - behind a windbreak. We spent five days at this Alaskan village waiting for a better tent.

Design aspects of the hoop

Here are a few important considerations that are usually overlooked.

Imagine bending a stick, or a length of aluminum or graphite tubing, into such a severe shape. If it does not break, it will be very close to it.

All hoops used in tarps, tarp-tents and tents, which come out of their stowbags straight, and which the user then bends into the shape of a hoop, are very close to the break point. In other words, the supports in a hoop shelter are very weak. You cannot bend them very much further before they snap. What would bend them further? A sudden gust of wind.

Here is that same hoop deformed to one side by a crosswind (fabric not shown).

Because the pole is close to breaking in no-wind conditions, it is extremely vulnerable in any kind of wind. If there is a weaker configuration than a curved pole in compression, I could not imagine it.

For anyone who doubts the weakness of the hoop, here is a simple test. Pitch the hooped tent, tarp, tarp-tent - or whatever the manufacturer chooses to call it, and pretend that your hand is the wind, by pressing the hoop gently to one side. Push too hard and you will break the pole. But even if you do not push very hard, you will still get a feel for the pole's lack of strength, by how easily it gives.

This discussion pertains to hoops that come out of their stowbags straight. Some companies supply pre-bent hoops with their shelters, claiming they are much stronger. Go ahead and give them the wind test. And think about how on earth you would ever repair a pre-bent hoop should it snap in a sudden gust of wind. Think also about the former coziness of that shelter minus its hoop, and how this might impact your journey during a storm.

Some manufactures, thinking they are wise, build in various supports such as panels and side-angling guylines. Why are these not wise? Because when the wind blows they pull the hoop downward, placing even more strain on it. Lateral supports may help stabilize the structure in gentle conditions, but in stormy ones they only lead to an earlier demise.

And by the way, even the normal guylines running fore and aft, pull the hoops downward and bring them closer to failure. So does any line attached to the hooped shelter and staked to the ground.

The only way for the manufacturer to ameliorate the hoop's intrinsic weakness is to eliminate the hoop from the shelter's design.

Meanwhile, here are a few more disadvantages of the hooped design.

The structure shown at left is identical to one in current production. However to simplify the illustration I have omitted its beaks. And don't we wonder where they got the idea of beaks, if not from my books; and in fact I even coined the term.

To the human eye, the structure has a certain appeal, with its curved poles and fabric surfaces. In strong wind it would have much less appeal, as those poles wobbled and twisted, quite possibly to the break point. Nicely curved though they are, these poles are very weak. The only way for the manufacturer to make them stronger, would be to make them larger and heavier still.

To keep its weight down, this structure is made rather small. This leaves its occupant, or occupants, lying close to the edges - particularly to the sides in the foot area. As a result, the rain and splash can, and will, soak the sleeping bags. This does not happen with a ray-way tarp because it is so much larger, and its edges are quite far from its its occupants. The maker of this hooped tarp cannot make it as large as a ray-way tarp because the poles would be that much heavier, and weaker still.

The ray-way tarp works on the principle of ample coverage. This is how it provides a wonderful sense of openness while still protecting from the rain.

Shown is a ray-way 2-person tarp from above. Beneath it are two foam pads each 20" wide and 72" long. Hooped tarp-tents cannot even begin to offer this much coverage, even though they are considerably heavier.

Note also the two trees, which is the tarp's standard pitching mode. Compare the strength of these trees to a hooped tarps feeble poles. It is no contest.

In the basic pitching mode the tarp uses only 4 stakes. Hooped shelters normally require 5 stakes - three in the front and two in the rear. Add the fuss of its poles, and there goes the claim that the hooped shelter is easier and less complicated to pitch.

Why are the hooped tarp-tent's poles so weak while the dome tent's poles are not? Because the hooped shelter's poles are parallel to each-other and isolated from each-other. A dome tent's poles cross each-other at various angles, adding greatly to strength and stability - while also adding considerable weight. The ray-way tarp has no poles, so it avoids their weakness and weight.