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Will a Jerrycan with oil sink?

  1. Jan 4, 2015 #1
    Hi

    I am writing a movie script and have a question. I you fasten a metal chain to a 20 Liter Jerrycan (pressed steel) and fill it to the brim wit oil, and you then fasten the chain to the ancle of a man (fully clothed) and throw him over board..

    Will he sink rapidly? Or will the Jerrycan actually buoy him? If the chain is quite thick also.

    What else could you fill it with to make him quite rapidly sink? Water? Sand?

    Or even preferably something you would keep on a boat, so maybe not sand.

    and I promise, it is for a movie script :)

    -pidah
     
  2. jcsd
  3. Jan 4, 2015 #2
    Without doing any calculations: He will sink the faster the denser the material in the jerrycan is. But you have to find out if you actually drag him down, or give him a lifesaver. Due to the archimedean principle the upward force of the empty jerrycan is equal to the weight of the water that would fit in.

    Lets ignore the walls of the jerrycan for a while. In this case an empty jerrycan will be pushed upwards under water, with the a force equal to the weight of the water that would fit in. A jerrycan filled with water would exactly feel no force at all under water (if you ignore the walls). This is because the upward force is equal to the weight on the water which would fit into the jerrycan, and the downward force (gravity) is equal to the water in the jerrycan with water as well. A jerrycan filled with oil will feel a total upward force, because the weight of oil in the jerrycan is less than the weight of the water which would fit into the jerrycan - if you assume the oil is less dense than water. In all these cases the downward force would be equal or less than if you only attached the chain (or only slightly larger if you take the walls into account).

    So you should at least put something more dense than water into the jerrycan to drag something down. Sand is denser than water, so this should work best of the mentioned possibilities.
     
  4. Jan 4, 2015 #3
    In every ship (not a yacht or recreational craft) that I sailed on there were sand buckets as fire fighting gear, so if it's a ship a supply of sand would be easy to obtain.
     
  5. Jan 4, 2015 #4

    sophiecentaur

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    Unless the can of oil is a vital part of the plot, why not just use a short length of anchor chain on its own? Most commonly used oils are less dense than water (certainly the grades of oil that you can 'pour' into a can. On its own (thin steel walls and 20l of 'oil') the can will float. Sand would be good - like the 'concrete overshoes' that the gangsters used to use in the Hudson River. Sand is around 1.6 the density of water so 20l would give a downward force of about 120N (the equivalent weight of a 12kg block of lead, say) - which would be hard to support when swimming. Even worse if your ankle is tied to this chain!
     
  6. Jan 4, 2015 #5
    Why not replace the can by the spare anchor ?
    It is good practice to have a spare anchor on board.
     
  7. Jan 4, 2015 #6

    SteamKing

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    That's why crooks use cement. A guy wearing cement overshoes isn't coming back once he's tossed overboard.
     
  8. Jan 4, 2015 #7

    Wes Tausend

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    Physics should be fun. I guess we can play with imaginary death.

    You could possibly make the movie script more interesting by tying the man to the bottom of the boat for a half hour or so. When his lungs finally fill with water, he will likely sink on his own when released. Also, I believe that even a man with air in his lungs will sink further if one can force him deep enough that the water pressure can compress the air reserve in his lungs below the minimum buoyancy needed to float a body. Hold-their-breath record deep divers find they must actively swim to rise, even with practiced amazing lung capacity.

    The thing is, the body is mostly made of water which is neutral in water floatation. Parts are also made of body fat which does float quite well. But other portions are made of bone which is heavier than water. So the body barely floats when the lungs are inflated, especially when one has not much body fat. When I was a skinny child I didn't realise this and tried to float without holding my breath. I still floated, but not high enough to keep my nose out of water without a bit of paddling. I also have exceptionally large bones, good for not-breaking, but not floatation.

    After a few days, some of the heavy body liquids in a corpse will react and become expanded gas through bacterial action. The deceased body will again rise to the surface unless it is weighted enough. "What else could you fill it with to make him quite rapidly sink?" Lead fishing sinkers are kind of boring. Maybe you could make the victim swallow just enough lead bullets to sink him. When he floats to the surface because of gas bloating, another boat could hit him and the bullets could explode. Ugh.

    Disclaimer: I am only helping you because it is just a movie. And rather macabre, I might add.

    Wes
    ...
     
  9. Jan 4, 2015 #8
    If you puncture an initially empty jerrycan it will sink. It weighs about 12 libs, so once filled with water it will pull down your man by a force of about 5 kg.
    Together with the chain that will finish him off. However the body may float back to the surface after a day or so.
    Sand filling is far better, but your sand filled jerrycan weighs 38 kg's on land, so is a little awkward to throw overboard.
     
    Last edited: Jan 4, 2015
  10. Jan 4, 2015 #9

    sophiecentaur

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    A couple of years ago, I bought a new, bigger anchor; not cheap. Sand is definitely cheaper and also harder to trace. The ships chandlers doesn't sell a lot of anchors and he'd remember your face (for the benefit of the plot). Old car wheels and engines are easy to get hold of too, anonymously.
    I think a chain round the waste would be better than one on the ankle because the foot could get detached after a few weeks. OMG - I'm actually thinking seriously about this!!
     
  11. Jan 4, 2015 #10
    Still trying to work myself into the script and start a Hollywood career ;-) : A missing spare anchor would surely be spotted by Columbo, you are right.
    A jerrycan takes time to fill with sand, during which the murderer (I am assuming murder by now) can be discovered.
    So it has to be prepared beforehand, which can also led to discovery. Better bring a spare battery !
     
  12. Jan 4, 2015 #11

    sophiecentaur

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    Most boats of any size will have more than one or two anchors for 'kedging off' and as spares because boat owners can't ever bring themselves to chuck anything out. Where would his chain come from? IF off the main anchor, it will be well shackled on and he'd have to saw off a few metres (bolt croppers are on most sailing boats). The criminal would have to come well prepared or be very familiar with the boat. Even I can't find half of the things I can on board. A small outboard engine (for the tender) would make a good sinker- but an oil slick could attract attention.
     
  13. Jan 4, 2015 #12
    Most anchors bear a unique serial number. This might be a problem if you have a nice, new yacht.
     
  14. Jan 5, 2015 #13
    Only if the density of the oil is less than 800 kg/m².

    Water would definitely be better than oil.
     
  15. Jan 5, 2015 #14
    I hope that all the people who've given advice on this subject won't end up in court as accessories.
     
  16. Jan 16, 2015 #15
    haha :) thank you guys for advice. The murder itself is not a major plot point, but the film is about oil so that's why. I wanted this guy to get drowned buy oil. (no need to advice on other creative ways to kill with oil :)

    thank you though. I think I got it

    -pidah
     
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