displacement question

Discussion in 'Boat Design' started by C-mack, Jan 24, 2013.

  1. C-mack
    Joined: Jan 2010
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    Location: Dallas, Pa.

    C-mack Boat Dreamer

    How much draft???
    If the boat is 12' long, with a beam of 3' . The 3' beam will be from the stern to with the in 4'of the bow. If the boat has a weight of 100 pounds and a person of 300 pounds. Just wanted to know if I'm thinking correctly about the draft of a boat like this...
    Thanks in advance
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It is about 64.4 lbs per cubic feet of water. You need to figure out the volume equivalent to 400 lbs. That is about 6.2 cubic feet. Divide that by the surface area of the bottom-36 sq.ft.- and you get .18 feet or about 2 1/4". That is if the boat has no rocker. If it does it is a bit more complicated.
  3. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    The density of sea water is 64 lbs/cu.ft., and the density of fresh water is 62.4 lbs/cu.ft. So if you have a purely rectangular shape 12' long, 3' at the stern, and 4' at the bow, the submerged volume, V, will be

    V = 12' x ((3+4)/2) x draft = 42 x draft in cubic feet

    That volume has to weigh 400 lbs with whatever water the boat is floating in.

    Weight = V x 64 lbs/cu.ft. = 42 x draft x 64 = 400 lbs. in sea water

    Solve for draft:

    draft = 400/(64x42) = 0.149' = 1.786", about 1-3/4".

    For fresh water, substitute 62.4 for the 64, and the answer will be 0.152' = 1.832", about 1-7/8", a little deeper.

    If your boat has a bit of rocker and deadrise in it, then you have to be more accurate in calculating the submerged volume. The numbers above are for a purely rectangular volume.

  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    A 12 foot boat with a 36 inch bottom, along with a 300 pound occupant is likely to be more than a little bit skittish. When the big guy steps into or out of the boat, careful foot position will be prudent.

    Is this one of the ubiquitous aluminum Jon boats? For a big person I would urge a bigger boat that has a wider bottom. I notice that the Jon Boat makers label them with numbers like 1028, 1236, 1442 and so on. That is the length and the chine width I presume. That has been the case on the ones that I have measured.

    You need not be too concerned about the draft of the boat but do be concerned about the stability numbers. The only humans that might be safe while standing up in a narrow chined boat would be olympic gymnasts who are good swimmers.

  5. C-mack
    Joined: Jan 2010
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    Location: Dallas, Pa.

    C-mack Boat Dreamer

    Thanks guys... all great info... i think you all gave we lots to think about...
    Now if I can get my PDR finished in this cold winter i could start the 12' pond boat.
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