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Science, Maths & Technology

Make a waterproof tent

Updated Tuesday 27th February 2007

How would you set about waterproofing a tent if you only had what was to hand, your wits, and science to help?

Mike Leahy and Mike Bullivant in front of the tent Copyrighted image Icon Copyright: Production team The Challenge
We were sent up a mountain with only a piece of sheeting and natural materials with which to make a waterproof tent. Being located on the rugged west coast of New Zealand’s South Island we were likely to come across freezing conditions or torrential rain.


What is the best way to waterproof cotton or other materials and what is the science behind this?

Waterproofing the tent
Most waterproofing agents work by virtue of their molecular structure, with the molecules on the surface of the material arranged in such a way that they will repel water, with their organic, non-polar, water repellent ends (if they have polar and non-polar ends that is) all facing outwards. Many liquids, oils and waxes would be expected to repel water. Some molecules interact with or attract water molecules so that they can mix freely with them. Others don’t, and they are the ones that we are looking for.

A tent would not be very waterproof if it had a flat roof because water would build up on top and eventually leak through. We soon settled on a simple ‘ridge tent’ design. This is a very traditional and well tried design. Because the roof is at a steep angle, water should run off easily, the triangular structure makes it strong without the need for high-tech materials and it is easy to build.

Modern tents are often constructed out of a range of advanced waterproof fabrics. Some tents are made of nylon with a PU coating. Others claim to be made of a rip-stop nylon with DWR water repellent. Modern coatings can make tents so waterproof that they can withstand a hydrostatic head of 1000mm or more. Put simply, this means you could place a column of water greater than 1m high on top of the fabric and it wouldn't leak.

Frames on modern tents are regularly made of strong lightweight high tech materials such as carbon fibre, glass fibre, kevlar or aluminium. We just have cotton (which does have some inherent water proofing qualities) and a few sticks. As it gets wet the fibres expand and close down the mesh of the weave. However, our sheet stands little chance against the west coast elements unless we do something to it.

What could we use to waterproof our tent?
We need to make the sheet waterproof by covering it or impregnating it with a substance that doesn’t mix with water.

We considered each of the following:

Lanolin - a mixture of non-polar (neutral) ester molecules that don’t mix readily with water
Coal tar - a complex neutral mixture of between 300 and 10,000 organic compounds. It is essentially the same as creosote, which is used to waterproof fences.
Ash - likely to make a mess
Flax - Ellen reckons it might work
Bees Wax - made up of neutral molecules which do not interact strongly with water molecules. Surfaces polished with bees wax certainly seem to repel water.

From this selection, we decided to try these out:

Lanolin - effective in keeping sheep dry and hopefully us as well
Coal tar - tars and oils don’t mix with water
Flax - this should work in principle
Bees Wax - waxes don’t mix with water and should be applied easily.

Obtaining suitable materials
We used the following simple methods to obtain each of our materials:

Lanolin was scraped off the top of water in which sheep’s wool had been boiled.
Coal tar was obtained by heating powdered coal over a fire in a metal container and collecting ‘fractions’ of the liquids which evaporated from the heated coal by way of a pipe leading from the container into a series of bottles where it condensed. This is called destructive distillation. The liquid we obtained was not a pure chemical but was a complex mixture. It looked and smelt enough like creosote that I was convinced!
We found some bees wax in our kit box.
The sticky plant flax was taken from the fleshy flax leaves, underneath the tough skin.

We tested small samples of cotton soaked in each treatment. They all seemed to be partially waterproof.

But what about the fire risk?

We experimented with the material samples and tested the flammability of each by lighting the material with matches. The results weren’t good:

Lanolin - flammable
Coal tar - very flammable
Flax - not bad but will burn
Bees wax - quite flammable

Despite the disappointing fire safety results, we decided to continue with our experiment. We constructed our tent with panels of cotton covered in the different materials to see which would be most effective in keeping us dry overnight.

Insulating the tent
Next challenge was to insulate the tent, to prevent our body heat being lost too quickly to the cold environment. We started by covering a simple wooden frame with heather and leaves, which was piled on in several overlapping layers. We then took some flax leaves and arranged them in a fan shape to make doors. The size and shape fitted the end of our bush tent’s triangular entrances exactly. Finally, Ellen made us a couple of flax leaf woven mats to insulate ourselves from the cold ground and keep drafts out where the tent meets the ground.

We also hoped to heat the tent. Mikey B. suggested the idea of heating up large granite stones in the fire and placing them in the tent overnight. Granite doesn’t split in the fire, and because the rocks we used were large they had a small surface area to volume ratio (the area of an object increases as a square, the volume increases by a cube. This means that as an object becomes larger its volume increases to a far greater extent than its surface). As heat is stored within the volume of an object but lost from the surface this means that large rocks take a lot longer to cool than smaller ones. The rocks were very hot when we went to bed and still warmish at dawn.

Positioning the tent
There is little point in making a tent that is well insulated from the cold if we camp in a exposed area with lots of wind. Likewise we don’t want to be soaked by rivulettes of water if it rains and ant nests are an absolute no no if we want to sleep well.

We didn’t have much choice when it came to the site for our tent, because we had to put it on the only piece of clear ground available - a tiny landing site and the only place for a helicopter to land for miles. We also needed to be near a gold bearing ‘reef’ and close to water. The hollow that we were in only saw the sun for an hour or so a day making the place very cold, with thick frost lying on the ground all day. However, we were sheltered, and had camped well away from potential flash floods or land slips. Not a text book perfect camping place but with no wind we shouldn’t feel too draughty.

How did the tent fare?
Luckily for us it didn’t rain in the night, but the treatments still needed to be tested.

None of them did really well, and Kate got very wet helping us test them . Our results were as follows:
Lanolin - poor
Coal tar - not so good
Flax - in the end this worked best (the best not meaning much in this company)
Bees wax - worked well on the areas that we managed to treat, but leaked like a sieve elsewhere.

The areas that leaked the worst were the seams. We did try to make the material overlap, and the seams did run vertically to help water run off but water still came in. Just goes to show that I'm no good at sewing. Perhaps we should have thought about glue!

Note about filming this challenge
The tent sequence was filmed on private land (not government land administered by NZ DOC) with the full knowledge and co-operation of the owners. The production team were advised by a member of the Westland District Council and accompanied by an experienced bushman.

The area where the vegetation was cut down is a landing site for helicopters for the owners to access their land (the vegetation is cut back regularly in this area to allow safe landing for helicopters).

The production team worked closely with the Department of Conservation in New Zealand to minimise any impact from filming.



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