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

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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."

MAKE Magazine Challenge Link:

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.

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    60 Comments

    I saw them do this on an educational video called "Voyage of the Mimi" in 6th grade. I hoped I'd never need to know this >

    2 replies

    Voyage of the Mimi FTW. I think I saw the second one, where they take on looters and talk about archaeology.

    Ha! Ben Affleck is in that Episode 10B of Voyage of the Mimi!! Also explains why he grew up to be a pacifist.

    you could do the same to retract water out of your urine, with a pot instead of the pit and on a much smaller scale..

    1 reply

    Hmmmm that's a toss up, Desalinated sea water or deurinated pee water!

    I know I'm a little late here, but my fish tank evaporates about a 1/2-to 1 gallon of water per day and that only has an exposed area of about 3 square feet, maybe even less. According to my admittedly non-scientific anecdotal evidence, it could be quite a small still needed to produce enough drinking water for a day. Plus, how would all those castaways survive if they needed acres to make a still?

    this is cool but is there any way to desalinate water on a sail boat or no im looking at you to get extra points as the know it all person.

    5 replies

    a urine still should work fine, just put salt watter in it to, you might want to make it tall though because you dont want the liquid your putting in it getting into the fresh water container, maybe put in on a cheap stabilizer if you have the time/money, if you dont know what a urine still is its a container with a smaller container in the middle with a water proof material on top and a weight over the smaller container, the water evaporates and collects on the waterproof material and then drips down into the smaller container. you can use almost any liquid to get water that way, and you can modify it to distill some beer :P and work over a fire...

    You know, they actually make a water desalination device for boats. They cost about 1 to 2 thousand dollars but provide high volume potable water....

    I read about that a few moths ago. Great idea, but a little too big to fit in a small sailboat. It is about the size of a small chest freezer or dorm refrigerator. He plans on it being used by small communities. I don't know if he has a portable unit.

    Portable Desalinator

    The contest has been over for quite some time, and someone else won.

    The desalinator mentioned in the link above is a portable device that is recommended for "abandon ship" situations. If you're looking for a homebrew answer that will fit in the confines of a sailboat I'm sorry to say that I don't think that exists, or is practical due to the amount of room it would take up.

    I'm not sure if I answered your question.

    ~S~ G'day all, Speaking of Australia LOL how ya goin cobbers ? Just to let ya know it wasn't me CT, but that is a fact, In shore from the beach you would surely find some greenery, and if you had some bags or waterproof type things to make bag like things tie them around the green stuff with a bit of wieght to bend it down alittle, the condensation will form. I have done it and is very drinkable and with a bit of flavour sometimes depending, If indeed you did have a sail handy, where the boat was no longer usable, or asccessable, what you would do is keep it whole,(much better for shelter later) You would still use the gully and mounds/bowls but in little island sets, so you would have a few under the one roof, do this far enough away from the high tide mark and make a channel to bginf the sea water into your man made little valleys.Next go to where the green stuff is and break of sticks and green leaves, lay these also in the water you have let in, the extra condensation will help as well as help remove any saltiness. The bigger you can keep things like a sail the better !!! 1 -- as I said shelter after collecting some water and can be re used. 2-- makes a much much better signal so aircraft can see it. . I have used the bush/bag method myself while camping with mates on deserted beaches during some surfing safaris along the east coast of Oz, summer and winter, and it works fine, just dont expect barrels full, you can also if you are lucky dig up some pipis/cockles/clams on the waters edge as well as beach worms(great bait) and crabs, Oh yeh, dont forget to block off your channel before tide get too high and lets too much water in.. ALL the best , glad ta meet yas and keep on surviving. It does a person good to get out of the comfort zone and rough it for a bit and live on your inginuity and smarts. Us humans in civilisation today have it tooo easy and we need to keep those bare survival skills alive, AND it also good for us that are slightly how you say overweight, LOL you can't but help but lose it when left to your own devises like that... Bout time more of the younguns left the ipods at home and roughed it up for a spell... ~S~ Regards from Lozza from Australia PS I reckon instructables is a bloody good web site with a ton of great stuff to share. Take care now .......

    Let me first say: We are going to make it!!! I am a Scout leader and survivalist. Your solar still works but, we may have a problem with the amount of fresh water it makes. A little known fact is that if we could go about 75 yards inland from the shore we could dig a well where fresh water will collect. Mother has already done the hard work for us. Side note; I was once asked, if stranded on a deserted island who would I want with me? My answer, My best Eagle Scout; How long do you think we will be stranded?

    3 replies

    Someone pointed out that a live plant in a garbage bag actually produces more drinkable water than this solar still. By "Mother" do you mean mother nature? I've never known, or experienced the 75 yard rule. I grew up around beaches and dug holes on the beach all the time. I don't think I ever found fresh water, but I'll take your word for it. If you are a woman, and look like Christy Brinkley, I'll stay on the island with you for as long as you want, and even wave the rescuers away. ;) Let me know how your little expedition turns out.

    Cement truck said: "Someone pointed out that a live plant in a garbage bag actually produces more drinkable water than this solar still. " How would one use a live plant and a garbage bag to produce drinkable water?

    Someone from Australia commented on this post a while back that if you wrap a plastic bag around a shrub and make sure that the bag's opening is shut tight the plant's respiration will cause condensation to form inside the bag and will eventually yield some drinkable water.

    As a Canadian Venture i say this. using these you would need at least 6 of these to stay thirsty. best idea is to keep yourself out of the situation. Be Prepared!