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

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

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

Picture of 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

Picture of 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"

Picture of 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.

Picture of 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.


Abigail Lee (author)2016-10-22

This is very helpful

purple evil minion (author)2015-08-23


Eureka (author)2007-05-13

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 >

Wolf Seril (author)Eureka2009-06-11

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

msanabria (author)Wolf Seril2014-11-13

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

ridecruz88 (author)2008-10-14

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..

Drakekay (author)ridecruz882011-04-13

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

darcitananda (author)2010-06-29

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?

HOMEPIE64 (author)2007-07-13

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.

minipancho94 (author)HOMEPIE642010-06-13

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...

freema0720 (author)HOMEPIE642009-07-27

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....

senacharim (author)HOMEPIE642008-08-21

Dean Kamen came up with a marvelous water desalination tool... it's the Vapor Water Distiller.

CementTruck (author)senacharim2008-08-21

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.

CementTruck (author)HOMEPIE642007-07-14

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.

Sharkzz (author)2009-08-13

~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 .......

shepard1 (author)2008-02-29

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?

CementTruck (author)shepard12008-02-29

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.

Bryan Smith (author)CementTruck2009-02-01

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?

CementTruck (author)Bryan Smith2009-02-02

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.

pickford78 (author)2008-08-18

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!

Benho (author)2007-05-20

Although this is a sound theory, to provide enough drinking water for even one person would require acres of these little set ups. I too saw voyage of the mimi and was eager to try this out. I have also spent a month on a deserted island ;)

CementTruck (author)Benho2007-05-22

If I may ask, What island were you "stranded" on, and was it voluntary? I Googled "Voyage of the Mimi" as I have never heard of it. It looks like it was an educational show produced in the mid eighties, and Making Dew / Expedition 10: Water, Water Everywhere was the episode where this subject was broached. I probably would have enjoyed the show.

Ben Affleck was on that show... it's what made me a fan.

Benho (author)CementTruck2007-05-22

I spent a month living off of Coconuts and Wild Pigs on Cocos Island, now I just stay with the Rangers at the Ranger station when I go and eat Beans and Rice. I've also been shipwrecked in a remote part of Honduras.

0.775volts (author)2007-02-22

You'll need many of these to make a viable source of water. you may want to add as a second possiblity a "respiration bag" or something like that (I can't remember what it's called). but you take cut plants (the leafier the better) and put them in a large trash bag (black is best). then tie the bag closed, place it so that there is a corner lower than the vegetation that water can drip into. you can harvest a decent bit of water this way if you have several bags, but even one will yeild more than a solar still.

jtobako (author)0.775volts2007-02-23

where on the desert island do you get the black plastic bag? how much vegitation are you going to have to clear to support yourself (and posibly your mythic other?)? it's a make magazine chalange-you don't get to add random (or usefull) items to your list.

Weissensteinburg (author)jtobako2007-02-23

If youre on a boat, there are probably trash bags there. Another method is to add vegetation to the salt water, so that extra water is drawn out by the salt water as well. I saw this done on Survivorman.

Didn't see that episode but catch it when I can. I was just going with the materials the original contest mentioned.

Survivorman - I've got respect for this guy.

Man vs Wild - This ASS will get somebody killed for ratings. He does the most inappropriate stuff.

0.775volts (author)CementTruck2007-02-24

Yeah, no kidding. I saw the rainforest episode where he drank unpurified creek water. The best part was when he woke up in the middle of the night puking his guts out. I also saw one where he threw himself into the water to let the current take him down stream. has anyone ever noticed that every time he hits a waterfall, he climbs down IN the water, not next to it? F-. but Survivorman was awesome.

CementTruck (author)0.775volts2007-02-24

I've also noticed that he wears a life vest under his windbreaker/jacket when he jumps in the water. I get to see his inane antics when I channel surf, but I've never sat and watched a whole episode. I cannot believe Discovery has stooped so low.

John Smith (author)CementTruck2007-04-07

I noticed the life vest too. And, in one episode, he was going to repell down a cliff, and used ropes that appeared out of nowhere. That seems like cheating to me.

0.775volts (author)CementTruck2007-02-25

Yeah, that's true, I forgot about that, but you're right. He also places too much emphasis on finding food. here's a synopsis of the two shows: Survivorman: there's a bear over there, I'm going to keep going so I don't startle him, and I can make camp a safe distance away. Man Vs Wild: Look, a grizzly! Now, you should never attack a grizzly bear if you see one, but if for some reason you're in a fight for your life, you should attack it like this...

Drackar (author)CementTruck2007-03-02

Mm...I would have to seriously disagree with you here, as someone who actually has, on occasion, spent time alone in the wild. I've not put myself into positions nearly as bad as either of these...I always go out properly equipped. The person in "survivor man" makes major mistakes, so many i stopped watching his show. His methods WOULD have you die of starvation, dehydration, and so on. He teaches you how to walk on rocks, and not to sleep in cracks. Nothing about doing things that you need to do to survive in the wild, which is finding steady sources of food, water, and fire. From what I saw, by the end of his few days in the wild, he's usually near starved, half dehydrated, sick, and often somewhat injured. While Man Vs wild does have a ...over-zelus approach on occasion, in every episode i've seen he's shown actual necessary skills and advice for surviving In the end, sometimes you DO need to take some risks, in order to make it out alive in decent shape...though again, many people don't have the skills necessary to take some of those risks.

CementTruck (author)Drackar2007-03-02

I respect your opinion, but would like to point a couple of things out.

Survivorman - Carrys a heavy load of video equipment. Something I would discard as useless during a real survival situation. Stands tripods up for shots, then walks away towards his goal. After a few hundred yards, he has to come back and retrieve his gear, and make the trek over again, this time with a heavy load. I understand why he's tired, and battered, and bruised. Most of his time is spent getting the shot, rather than getting the food. He's only got X amount of time to get to the rendezvous point. (I agree with what you say about the starvation part, but I think the network execs are the ones to blame for that. It's just not that interesting to the general viewer.).

Man vs. Wild - comes with a camera crew (so he's never really alone), does not carry any equipment. He always travels light. He gets himself wet at the drop of a hat (jumping in rivers, lakes to get to the other side quicker) withought giving the slightest thought to hypothermia, a guaranteed killer in the higher elevations, and colder regions. I believe his show is usually over after an overnight campout, while Survivorman does a whole week (I think).

When I first heard about the now popular Survivor TV series, I was exited. I thought that they would test peoples determination, and will to survive. Whoever didn't bow out would end up the winner. Alas, it turned out to be a total drama fueled show, exposing the flaws and pettiness of all who were cast. I was really looking forward to see some ingenuity at work.

Thanks for commenting, and your post regarding the hot stones. It gave me something to think about.

Drackar (author)CementTruck2007-03-02

I do agree that survivorman would do better without the camera gear, I as actually thinking about mentioning my view that it's a bad idea, but that would have made a already long post even longer. It puts an additional strain on him that people would not usually have to worry about.

And while in man Vs wild he's never truly alone, and you can argue that there's a chance that this could lead to cheating... If it's done without interference, it makes more sense. After all, you WONT have the extra load.

As for the constant exposure to water...It's dangerous. Yes. There are examples where he does it for demonstration, however, which make a decent amount of sense. Knowing how to pull yourself out of a hole in ice, how to get yourself out of quicksand, and so on ...really good things to know. When he climbed down a waterfall? I wouldn't have done that.

And man Vs Wild tends to very. It's usually a shorter period of time, but he concentrates on useful information over those two days, as apposed to useless exposure shot after useless exposure shot. Usually the camera crew or the camera he has with him is showing him talking about some survival tip or other. Sometimes it's fairly useless, or even admittedly stupid unless a last case resort, but at least your learning SOMETHING.

Oh, and as a side-bit...someone mentioned that Man Vs Wild showed him drinking raw water and vomiting, as I recall, In survivormans Desert episode, *one he almost killed himself on* He drank repeatedly from a stream...

It's the episode that hes going to survive on a raft in the midle of the ocean. I agree on both regards

0.775volts (author)jtobako2007-02-23

Hey man, no need to get snarky. Did I know this was a make magazine challenge? No. This instructable says nothing about being a response to a challenge, I thought he was just posting a method for getting water in a creative (and pleasant to read) fashion. that being said, the dark, waterproof windbreaker would also work. You'd want to fill the bag/windbreaker as full of vegetation as you could, provided you leave space for a corner to be lower and free from branches/leaves. Fill it, but don't pack it in there, you want the plants to be able to breathe. the water yield would vary with vegetation, as much as a cup or two, or as little as a couple ounces.

CementTruck (author)0.775volts2007-02-24

I scoured my instructable looking for the mention of a Make Magazine challenge, and apparently all I had on the top was a small link to their site. Sorry for the confusion. I have corrected the oversight. Thanks for commenting.

CementTruck (author)jtobako2007-02-23

Speaking of mythic other(s), I would probably not want to have the responsibility to provide for these people, however.... If I really had to, I think that the Olsen twins would be the best "celebrity" to be with on the island for many reasons. Let me enumerate: 1) They don't eat much! 2) They look identical, so you can have a spat with one and still have pleasant relations with the other. 3) If neighboring savages come by to barter it would not be difficult to figure out which one to give up (eeny, meany, miney....) 4) They're pretty young, so they probably won't die on you any time soon. (Unless they starved themselves to death.) Soooo many more - use your imagination to fill in the blanks. (I consider the twins "a" celebrity because I don't think they've ever been in a movie "solo". They're only reason for fame is each other.)

CementTruck (author)0.775volts2007-02-23

The respiration method works best with a plant variety called succulents. These plants include cacti (cactus), who have meaty, juicy centers. The digging of the trench until water appears can be substituted by spliting the cactus and dumping it in the trench until full. Even then, this respiration method will only work until the vegetation "dies", and no longer pumps out oxygen. Once that happens, and the plants dry up, there goes your humidity to produce condensation. Besides, I was not given any black trash bags.

CementTruck (author)CementTruck2007-02-23

Actually, the Cannabis Sativa herb would probably be the best for the respiration method since they breathe faster than any other plant out there. ;)

chuckr44 (author)2007-04-03

At first I didn't see what you were trying to do exactly, but now I do. Great idea. A hose going from the central water catcher/depression to outside the sail means you don't have to take apart your still each time you want a sip. Keep in mind that bamboo is not hollow all the way down the length of itself. At each "joing" or swelling, it is sealed up. So that's great for making cups, not great for making pipes. Hollow grass might work as a straw.

CementTruck (author)chuckr442007-04-03

I grew up around bamboo and am quite familiar with the culm "dividers" you speak of. They can be knocked out with a smaller diameter bamboo shaft. We used to make canons out of the large shafts by poking out all but the last divider this way. I have personally seen a network of bamboo tubes carrying water from a water source into peoples homes.

tanda333 (author)2007-02-23

i wonder if its possible to rig a setup where you can use a fire to increase the rate of production, you would have to make it increadible high to protect the plastic, and with local foliage you could construct any structure you desire. also, i learned this in the scouts, but we just were told to dig a hole, put a pot or something in it, put the sail overtop, wieght it down then weught down the center, we were taught that theoretically, the bigger we could make the hole, the better. however your plan of using an actual pool of water and not just the dampness in the ground sounds like it would be faster. (our plan would work even in the desert though (theoretically)) though we were told that our method would easily produce enough water to keep us alive, so i wonder if any advanced methods are needed? (sorry about the long post but i figure since i was taught this i should share)

Drackar (author)tanda3332007-03-02

You could have the benefit of the heat without the risk of melting and the problems with fire and ash in your wet area by heating stones.

Linux_Galore (author)2007-02-24

A common survival trick here in Australia is to put a leaf covered branch into a plastic bag then tie of the end so its pretty air tight (bark will work). Then sit back and rest and let the sun do the work for you. You should get about 2-3 cups of clean water a day per plastic bag.

Aeshir (author)2007-02-22

Suggestion: add a little river/pool leading downhill a wee bit (and outside of the whole contraption) of the fresh water, it'll let you hold more water. There's some totally obvious flaw in my thought, but i'm to lazy to find it out.

Weissensteinburg (author)Aeshir2007-02-22

Well it needs to be sealed decently so as much of the water is evaporated as possible. Because it doesnt happen very quickly, or bountifully. You would also need lining to keep water from going into the sand,

"Well it needs to be sealed decently so as much of the water is evaporated as possible. " Step 3 substep 5 - Cover the remaining perimeter with sand, effectively sealing the edges of the sail to the beach.

I was responding to Aeshir's comment: "Suggestion: add a little river/pool leading downhill a wee bit (and outside of the whole contraption) of the fresh water, it'll let you hold more water. There's some totally obvious flaw in my thought, but i'm to lazy to find it out."

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