How do I calculate displacement from weight?

Discussion in 'Stability' started by Valk, Jul 3, 2007.

  1. Valk
    Joined: Jun 2007
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    Valk Junior Member

    Is there a way to get a rough idea of what a small craft would displace? Like if a small boat weighs in a 50 lbs and the user weighs 200 lbs, is there a formula to find out about how much water the craft with the user in it would displace?

    I would think you could then fill the boat with that much water and see about where it will sit the water, kinda... Makes sense to me :rolleyes: Roughly of coarse, Not taking into consideration the thickness of the hull etc.
     
  2. Hank Rosendal
    Joined: Oct 2006
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    Hank Rosendal not old enough for some things, too old for others

    Hi Valk
    Filling your boat with water, would give you a figure in liters: the volume of the boat.

    You need to find the weight of the boat. A small dinghy you might be able to weigh directly on 2 scales or park the trailer with boat on 2 or 3 scales and weigh the trailer again empty.

    If your boat is too big for the scales, nearly all cranes and travel lifts can give you give an indication of weight and then you have a figure of displacement.

    For more reading and formulas go to:

    http://www.uscgboating.org/safety/boatbuilder/safeloading/subc-4a.htm

    Or check this forum:
    http://www.physicsforums.com/archive/index.php/t-33706.html

    If you really scientifically inclined go to:
    http://www.ceanet.com.au/Products/Mathcad/tabid/136/Default.aspx?gclid=COffmY_CjI0CFQbIbgodcR4Zpw
     
  3. SamSam
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    SamSam Senior Member

    Fresh water weighs apprx. 62# a cubic foot and saltwater apprx. 64#. 4 cubic ft of tapwater would weigh apprx. 248#. Hanks formulas might tell you what water weighs per gallon. 8+# I think. What you say makes sense to me too.
     
  4. Valk
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    Valk Junior Member

    Great, thanks for the help gents!
     
  5. alexlebrit
    Joined: Aug 2006
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    alexlebrit Senior Member

    If you can cope with metric it's a lot easier though. I litre of water weighs one kilogram, and is 1000 cubic centimetres. Everything's based on this and it's all tens, hundreds thousands etc, so much much easier to work out.

    So in your case 50lb boat + 200lb captain = 250lbs = 113.4 kilograms

    So you'll displace 113.4 litres of fresh water

    or

    29.96 US Gallons.

    I think Google will convert these things back and forth from metric to imperial to us if you want though.
     
  6. Verytricky
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    Verytricky Large Member

    Bloody Hell - Imperial is so complicated!!

    Discover METRIC. It works....


    One litre of water weighs one kilogram....

    No math involved.

    90kg man with a 25kg boat will displace 115 litres of water....
     
  7. Frosty

    Frosty Previous Member

    The question was 'how do i calculate displacement from weight"

    The answer is weight is displacement.
     
  8. SamSam
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    SamSam Senior Member

    The question in the end was if he could take the water displaced by weight and put it inside the boat to get a rough idea of where the waterline would be. It seems like that would work. Kind of sort of. When I get to thinking about it though, if you actually have the boat to put the water in, why not put the boat in some water, climb in and see exactly where the waterline will be. That would also take into account the distribution of weight and show where the trim will actually be, which putting water in the boat won't.

    Imperial measurements. Is that the system that gives you wrench sizes like 19/32 or 27/64?
     
  9. colinstone
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    colinstone Junior Member

    1 ltr of fresh water weighs 1 kg, 1 litre of sea water weighs 1.025 kg. In salt water a boat will displace less, and so have less draft than in fresh.
     
  10. Trevlyns
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    Trevlyns Senior Citizen/Member

    Frosty has it right – weight and displacement is the same animal.

    You have the weight of the empty boat. The person or anything else you put into it is the payload. The total weight equals the displacement at that specific moment. It may not necessarily be the maximum or designed displacement. This would be the empty weight plus the maximum payload as determined by the designer and drawn as the Lwl (Length of waterline) – and often referred to as the DWL (Designed waterline). This is the line at which the craft is designed to sit with its maximum payload.

    Displacement is determined by the internal volume of the hull (Volume displacement) which is multiplied by the density of the water in which the boat floats. As pointed out already, that’s why there is a slight difference between fresh and salt water.
     
  11. SamSam
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    SamSam Senior Member

    External?
     
  12. Trevlyns
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    Trevlyns Senior Citizen/Member

    Beg your humble pardon, SamSam! :D
     
  13. Verytricky
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    Verytricky Large Member

    So you float higher in seawater but your propellors are slightly less efficient....
     
  14. SamSam
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    SamSam Senior Member

    Yes, well, we can't let these small discrepensies get out of hand to the point where the planets won't align correctly and the resultant misbalance of the universe allows the earth to tip over. It's just too much bother to clean the workshop after such events. ;)
     

  15. Trevlyns
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    Trevlyns Senior Citizen/Member

    Touche! :D
     
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