How much weight can my pontoons hold at half emersion?

Discussion in 'Stability' started by christoph, Jan 31, 2010.

  1. christoph
    Joined: Jan 2010
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Houma Louisiana

    christoph New Member

    I have a calculation that I need answered by you guys.

    I have 3 pontoons that are 33 feet long with a 24 inch diameter. How much weight will these pontoons hold at half emersion in fresh water? The pontoons are aluminium.

    If some one could post the actual equation that would be awsome to! I have found no place on the net that has this equation.

    Thanks for the help
     
  2. sharpii2
    Joined: May 2004
    Posts: 1,863
    Likes: 86, Points: 48, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    What you are really asking for is the displacement at the half diameter.

    I'll assume the pontoons are not just straight tubes with a flat bulkhead at the ends.

    You need a separate calculation for the ends which is based on their shapes.

    For the straight tube portion, the formula is:

    (Diameter in ft squared)*0.785*Length*Number of pontoons*62.2 pounds/2

    This will give you the displacement of just the straight sections at half diameter immersion. The weight of the pontoons must be subtracted from this to give you the weight they will actually support.
     
  3. christoph
    Joined: Jan 2010
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Houma Louisiana

    christoph New Member

    is this correct?

    Thanks for the quick reply .I do have some questions though. I was looking on a web site called u-fab pontoons and according to there spread sheet it says that a pontoon of 24 inch diameter at 30 foot long will hold 5500 lbs . why is this so different than the calculation that you gave me?
     
  4. sharpii2
    Joined: May 2004
    Posts: 1,863
    Likes: 86, Points: 48, Legacy Rep: 611
    Location: Michigan, USA

    sharpii2 Senior Member

    It looks like they are either loading the pontoon down to almost all its diameter or they are assuming you are actually using two, as most pontoon boats do, and using the full volume of one to calculate the half volume of two.

    3.14*30*62.4=5878

    which is pretty close to what you got on their spread sheet. They may have allowed for the lesser displacement per foot of the ends.
     
  5. Submarine Tom

    Submarine Tom Previous Member

    Each cubic foot equals 62.4 pounds of water.

    You can figure out the rest.

    -Tom
     
  6. pistnbroke
    Joined: Jan 2009
    Posts: 1,405
    Likes: 33, Points: 48, Legacy Rep: 404
    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    feet inches pounds ...god help america
     
    1 person likes this.
  7. Submarine Tom

    Submarine Tom Previous Member

    I get ~8500 pounds of buoyancy half submerged. That's assuming tapered or rounded ends and circular cross-section.

    Your total vessel weight should not exceed ~6000 pounds as this is the total buoyancy available from one pontoon.

    -Tom
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Did you mean "feet, inches, pounds..."?

    I agree with you on the second part!

    -Tom
     
  9. froudedude
    Joined: Feb 2010
    Posts: 10
    Likes: 1, Points: 0, Legacy Rep: 10
    Location: Annapolis

    froudedude Junior Member

    Really, unless we get grammar and punctuation correct, America is done for. Oh, did I end a sustenance with at preposition?
     
  10. JFdez
    Joined: Feb 2010
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Spain

    JFdez New Member

    weights pontoon

    Weight that the pontoon will support=Weight of the pontoon submerged volume at the desired draft- total weight of the pontoon
     
  11. JFdez
    Joined: Feb 2010
    Posts: 4
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Spain

    JFdez New Member

    ......-total weight of the pontoon (of course, without considering the added weight)
     
  12. netjaws
    Joined: Jan 2004
    Posts: 75
    Likes: 1, Points: 8, Legacy Rep: 22
    Location: None

    netjaws Junior Member

    cristoph, be careful with using 1/2 pontoon depth as your design draft. beyond the pontoons being 1/2 submerged, your TPI decreases exponentially with each slight weight addition, leading to a very precarious stability situation
     
  13. conceptia
    Joined: Nov 2009
    Posts: 202
    Likes: 7, Points: 0, Legacy Rep: 97
    Location: Houston

    conceptia Naval Architect

    netjaws has made a good point.
     
  14. hoytedow
    Joined: Sep 2009
    Posts: 5,402
    Likes: 194, Points: 73, Legacy Rep: 2489
    Location: North of Cuba

    hoytedow Wood Butcher

    Never use a preposition to end a sentence with.
     

  15. ddrdan
    Joined: Mar 2009
    Posts: 67
    Likes: 2, Points: 0, Legacy Rep: 37
    Location: Raleigh NC

    ddrdan Junior Member

    I received conflicting info on this same topic. How will the pontoon stability fair in the info below?

    (used 62#/ft3)
    Hull Shape: Square
    Pontoon Size (ft): 2.0625'(bottom)* 2.02083' * 14.7' * (2)
    Area (ft2): 61.25
    Displacement (ft3): 126.3
    Max Displacement (#): 7892
    Loaded Weight (#): 3420
    Volume Displaced (ft3): 53.45
    Draft (in): 10.47
    Freeboard (in): 13.78
    Load Ratio: 42.3%
    Safety factor (Base=50% of Max): 7.7% or 620#
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.