XX Tarp Tent – A Work in Progress
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The name, Doublecross (XX), is for the two points where the poles cross. An older design, with some new twists. It came about from thinking about sagging fabric and its supposed cure, catenary seams; and how both limit space inside a shelter. Why couldn't the same fabric surface be convex, rather than concave, and allow more room inside?
The design is intended to:
> Make use of silnylon's elasticity to create a partial dome shape without seams.
> Use crossing poles in a manner that allows the fabric to add rigidity to the frame.
> Optimize use of stiff carbon poles, by:
– Limiting the bend of the arcs with elbows
– Crossing the poles twice to reinforce each other
> Simplify MYOG construction by allowing the dome canopy to be made from a rectangle.
> Eliminate pole sleeves and/or clips by allowing the poles, 2 at each end, to be slipped under the canopy from outside, and then be joined by reaching under the canopy peaks to connect the poles to the elbows.
The two elbows at the peaks were made from 3/16" Ti rod tent pegs, weigh 1/4 oz each, were warmed to room temp, and bent with a large radius to 120°, an angle determined by experiments with scale models.If left alone, the poles would cross at a lower point on the canopy, but the crossing points have been raised to improve stability and make the ridge seam more taut. Below are the pole set, and examples of Ti elbows, pole tips, a ferrule and a pole receptor made by taking apart a swivel hook:
The weight penalty for the carbon pole set is 9.5 oz. It is offset by:
> Increased head and shoulder room
> Entry and exit unobstructed by poles
> 'Freestanding' design, with 4 pegs – 2 at front and rear, and 2 at the sides to eliminate sway.
The main canopy fabric piece is a rectangle 3' wide by 10.66' long, plus seam allowances. The peaks are 45" high, the depth is 36" overall, the floor will be 36" wide by 90" long, with 6" walls. The pole tips make a rectangle 40" wide by 102" long. The canopy ends are 4" above the deck, and will extend beyond the floor by 4".An oval patch will be bonded to the canopy at the pole crossing points. For both ventilation and protection from condensation, the rectangular areas of the canopy below the crossing points will be covered inside by insect netting suspended inside the poles. At the crossing point, the poles will slip through a twill tape reinforced break in the seam between the netting and canopy, with a guy-out loop sewn to the outside.
The insect netting will be large triangular pieces at front and rear, probably with a slight catenary cut when pinned up to the canopy. Net zippers will be YKK #3, and can be inverted T-shaped or L-shaped, with zips at the rear to allow easier access to the rear vestibule for storage.
Although the reinforcement patches were bonded with SilNet, they will still need sewing with light thread along their edges. Also, it was a mistake to bond the lap felled ridge seam, as this reduced the elasticity of the nylon at the ridge, limiting the tautness of the canopy. The seam was removed and redone with Mettler Metrosene polyester thread at low tension, and heavier DMC poly/cotton thread (no longer made)was used to attach to the seam the twill tape loops that hold the elbows in place. The Mettler thread was great for multiple layers of silnylon, but unsuitable for thicker materials.
Because the front and rear canopy openings are simple triangles, there are a number of possibilities for end covers, depending on how much protection is wanted. Although the occupant is protected on all sides by netting, the tent is 'single wall' and will be for three season use; so the emphasis will be as much on ventilation as protection.
On completion, there should be an update, with a link to a video of the pitch and strike.I'm pleased with how this canopy turned out, both in terms of tautness and rigidity. The toughest decision now will be choice of floor and end cover materials, and obtaining them. No sign of any WPB woven unobtainium yet.
[Edited 4/6/11 to correct to reflect use of twill, not grosgrain, tape]
[Edited 4/20/11 to correct length of main canopy fabric piece to 10.66', or 128"]"you will have to come up with some neat way to pitch the tent"
Roger,
Don't know how neat it is, but the plan is with the tent spread on the ground and a peg holding the rear awning if need be, then at each canopy end two poles will slide under the canopy edges and over the netting, through the break in the netting at the cross point, and then be pointed and pushed to the general vicinity of the internal elbows. Ditto, the other side. There will be zip sliders at the tops, as well as the bottoms of the net doors, so they can be opened from the top, and one can reach inside. The end covers, or awnings, will also allow one to unzip up top and reach inside and underneath the peaks. So the plan is to reach inside, underneath the peaks, and attach the upper pole tips to the elbows. The tip inserts are drilled out to have a snug press fit over the Ti rod so everything will stay in place during pitching. With the elbows in place, the bottom ends of the poles will be inserted in the receptors (instead of the usual grommets). Once all four bottom ends are in the receptors, the tapes from the receptors to the ladderlocks sewn to the canopy corners are tightened up (not captured in the photos – lousy photographer). As the canopy rises, lines from the 4 bottom floor corners to the receptors will hold the pole bottom receptors in place so the poles cannot spread wider than the intended arc. That's a mouthful, but once the upper pole tips are fixed to the internal elbows, it won't look much different than any other dome of similar design going up. Guess that's why a video of the pitch is intended. Will the plan work? Pretty sure."sure you've thought of tons of options for the awnings already."
"Pertex Endurance or Epic fabric could be a solution …"
David,
Something like the GG One awning in the front, and at the rear, a fairly common triangular awning, such as the ones on the Hubba, or the Rainbow. Gave up on Epic Malibu because of the weight and almost total loss of water resistance when the surface tension is lost at around 1500mm HH. Also, polyester Malibu would probably not stretch enough for this design. From Alan Dixon's article on this site, the urethane coated WPB's appear to transmit vapor only when sufficient humidity pressure is built up; as with a jacket, for example. So, it will be just the best waterproof sil available for the canopy, and more sil or one of the Cubens for the awnings – not sure yet."fabric must be 6 ounces"
"I wonder how it would do in wind?"
"What happens if wind blows rain in the sides?"
Jerry,
The sil is ca. 2005 40 den 1.36 oz from Quest. Expect under 2# total, but have expected before. So it's a wait and see on the weight.This design was quite a bit more rigid and stable than the single cross domes I tried. Once full size frames were built, realized that none of the single cross two pole domes had much rigidity rotationally, no matter what gimmicks were tried, and strong winds could raise heck with them. Even strung with all the lines, this frame wobbled like the dashboard doll in the BPL store:
This XX frame, once the the canopy is stretched over it, is rock solid, both rotationally and front to back; but there is slight side-to-side sway when pushed from the sides; hence, the loops for the side tie-outs from the pole crossing points. Only concern about the wind is the need for good ventilation at the front; so if the wind shifts all the way to the front of the tent, the canopy will baloon. Front and rear guys from the awnings in addition to the side-to-side guys should keep the tent from blowing away, but yes, balooning can be a problem when camping in exposed locations. A taut canopy, as this one has, helps a lot with that, though.
The alternative is to run the canopy and the awning down to the ground, as TT did with the second version of the Scarp, and as with the superlight Terra Nova tents.
Have no desire to camp in a sweat lodge, so having none of that. Older tents, with no more coverage than planned for this one, have kept me dry and resisted wind blown rain in exposed areas so extreme that the PU fabric coatings failed before any water blew in from under the edges or awnings. So IMO, total coverage for 3-season shelters is over-rated, is more psychological than practical, and just makes folks uncomfortable in foul weather due to the lack of ventilation. Even in heavy snow, I'd be very worried about air quality with total coverage covered by a foot or two of snow.But please note: This design can readily be done with total coverage for those so inclined.
Hi Sam
Ah, OK: the elbow is retained inside the tent. That works – I did that myself in the late 1960s.
About 7' square, 70 d nylon, fibreglass poles – we hadn't even gone metric at that stage. It took the weather very well under some extreme conditions in SW Tassie – winds direct from the Antarctic. The big weakness was not the poles going sideways – they didn't, but the centre tended to pop downwards when pitched under a cliff when the downward vortices got too much. But a quick whack of the hand would push the top back up. I did add diagonal tension guys inside from the corners to the peak.
The thing had poor ventilation of course, but the steep roof and the separate groundsheet meant things were quite livable despite that. You can see the guy ropes on the poles and the sides: they were generally sufficient.
The notable thing about it was what it looked like when I got inside it to fix the last pole into the 4-way elbow at the peak. There were comments about wrestling matches inside …
The one thing which might give a problem will be where the fly presses on the poles. Rain hitting there will create a far higher pressure, and you might get some leakage along the pole lines.Changing from silnylon to EPIC would, in my opinion, be a total disaster there.
Interesting.
Cheers
Samuel,
I love your design! Very clever. Obviously a lot of thinking went into this. Good job of building to accomodate the characteristics of the materials used, particularly the carbon fiber poles.
You said "but there is slight side-to-side sway when pushed from the sides; hence, the loops for the side tie-outs from the pole crossing points".
Not quite sure which movement you are referring to given the two possible definitions of "side" with a side entry tent. I can assure you, however, that things really tighten up when the triangular vestibule/beak pieces are added to the long sides of the tent. Here's a photo of a similarly shaped tent showing one side vestibule:
When the vestibules are pulled out and tightened on both sides of the tent in the photo it really tightens things up. I've used it in 40mph winds without any other guy lines.
Your tent canopy is nice and wide (36") so the vestibules or beaks could be made of uncoated (non waterproof) 1.1 ounce nylon and weigh only 1 or two ounces each (no zippers). (The tent shown in the photo has a 24" wide top canopy.)
Thanks for the helpful comments.
"You said "but there is slight side-to-side sway when pushed from the sides; hence, the loops for the side tie-outs from the pole crossing points".
"Not quite sure which movement you are referring to given the two possible definitions of "side" with a side entry tent."
Daryl,
Sorry for the confusion. By "sides," I meant the surfaces the poles are under; so, when looking at the tent from the front, the sway is to your left and right. It is much less than with some single pole "side" entry tents, like the Hubba, but should be almost eliminated for use in high winds by running guy-lines from loops at the points where the poles cross out to pegs on the left and right. Hope that is clearer.Agree that the staked end covers will also help with reducing sway, not to mention keeping the tent from tipping over front-to-back. You have got that front end cover very taut, all things considered. I am thinking about bringing a similar front end cover only about half way down to the ground, and using WP fabric rather than netting for the lower part of the door. This should keep the rain out, but as mentioned earlier, will allow the wind in, and allow some balooning. Awnings with a little more coverage on the netting doors have worked well on my Bug Dome in keeping out wind blown rain:
So it is just the wind itself that worries me if the front awning does not come all the way down to the ground. The consensus in the many posts I've read on BPL seems to be that this is not a good idea. Joe Valesko added bottom beaks to his awning to address just this situation. I really like being able to cook in the rain under a front awning, though.
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