Introduction: How to De-salinate Seawater When Stranded on a Deserted Island

About: "Better to have it and not need it than to need it and not have it."
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Makeshift Castaway

There are few fears more primal than being cast away on a deserted island in the middle of who-knows-where. Maybe it's a throwback to our eviction from the Garden of Eden, or the nagging realization that Earth itself is just such an island in the endless ocean of space. Whatever the reason, it has been and remains a mainstay of the collective imagination.

The Scenario:
You are on a small sailboat in the South Pacific when a freak wave of Biblical proportions swallows your craft. You awake to find yourself on the rocky, sandy beach of--what else?--a tiny deserted tropical island. As the fierce equatorial sun beats down on you, you realize that the boat is gone, but a large section of the white, waterproof nylon sail has washed up on thet beach.

Relieved that you are alive and have sustained no major injuries, you quickly scout out the island. There is a cave for shelter, an abundance of vines and vegetation, but no trees to speak of. You see enough sea birds and marine life to provide a subsistence diet, but there is no source of fresh water! And the rainy season is still months away.

The Challenge:
Come up with a reliable way to produce potable water until you are rescued--or the meaning of life is revealed to you, and being rescued no longer matters.

Your items:
You have only the nylon sailcloth and what you were wearing when you washed ashore: a dark, waterproof windbreaker; a T-shirt; and shorts, in which you find your Swiss Army knife (or Leatherman tool) and a pack of waterproof matches. If it provides additional motivation, feel free to be cast away with the fantasy celebrity of your choice--but this person is still counting on you to provide drinkable water. And if you're looking for extra points here, forget the pack of matches.

Good luck, and rest assured that we're all out there looking for you.

Step 1: Gather Materials

According to the gauntlet that was thrown down we have:

A white waterproof sail
A black windbreaker
A Leatherman
Waterproof matches (we don't need no stinkin' matches)
The beach
The ocean
Local foliage and raw materials

I was having a difficult time trying to figure out what materials to use in place of the items above. I used a white grocery bag for the sail (cutr in a triangle), and a black bag cut in the shape of widbreaker. The big pink lump is not my appendix, but my son's modeling clay (This will be my "grains of beach sand").

We don't need the Leatherman, or matches for this endeavor.

Step 2: Make a Berm Out of Beach Sand

Survey the beach. You will notice a definite difference between the sand that gets pounded by waves, and the dunes that don't get wet no matter what time of day.

1) Pick a low spot at the edge of the dune/wet sand border.
2) Lay the sail down on the sand and spread it out flat.
3) Trace the size of the sail on the sand.
4) Dig a trench approx. 2 ft. inside the traced triangle, and place the excavated sand within the 2ft border (inside the trace), and place some sand in a pile in the middle of the triangle.
5) Keep digging until there is about 2" of water in the trench.

Step 3: Create the Cachement "system"

1)After creating a suitably deep divot in the mound at the center of the trnch lay the waterproof windbreaker in the mound to create a deep bowl.
2)the top of the "bowl" must be considerably lower than the top of the berm (1/2 the height is probably a good average - it's all eyeballed at this point).
3) Stretch the white waterproof sail over the trench and bowl.
4) Carefully place heavy rocks on the perimeter of the sail to hold it down.
5) Cover the remaining perimeter with sand, effectively sealing the edges of the sail to the beach.
6) Place a stone in the center of the sail, directly above the "bowl".

The stone must be big and heavy enough to pull the sail down to a point above the bowl.

Step 4: Explanation of the Evaporation Cachement Idea.

As the sun beats down on the beach the water within the berm will try to evaporate. Since the sail is waterproof the water will collect underneath the sail in little droplets. As the droplets get bigger and heavier throughout the day they will gravitate towards the middle where the center weight was placed (heavy stone). This water will dribble down into the bowl made out of a waterproof windbreaker.

The quantity of fresh water that will be collected depends upon the size of the sail, the heat of the sun, and a host of other factors.

If you had a really large sail, you could use the Leatherman and make multiple, smaller, freshwater stills. This might maximize your fresh water collection.

As far as the matches go, save them for the signal fire.