Newibe with posting question.

Discussion in 'All Things Boats & Boating' started by AndyBorris, Sep 29, 2013.

  1. AndyBorris
    Joined: Sep 2013
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    AndyBorris New Member

    Hi everyone.

    As the title suggests, I'm a newbie and unsure on the best section to post a question about pontoon/raft design.

    Not very exciting I know, but still something I've got a question about, at least it floats!

    Thanks in advance.

    Andy
     
  2. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Raft and pontoon threads are usually in this 'open discussion' forum, the one you posted in. So you can post here. If you go to where it says 'search this forum' and enter 'raft' it will show other threads about rafts. Then scroll down to the bottom of the page and you will find 4 'similar threads' listed also.
     
  3. AndyBorris
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    AndyBorris New Member

    Thanks, I'll have a search, maybe my question won't need posting!
     
  4. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Actually, go to the main sub-forum 'Open Discussion: All Things Boats & Boating' and there is where you'll find the 'search this forum' option. There are 3 options, 'search this thread', 'search this forum' or just plain 'search' which searches the whole site.

    What's your question anyways?
     
  5. AndyBorris
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    AndyBorris New Member

    Just realized, a UK pontoon is somewhat different to an USA one!

    In the UK, pontoon tends to describe a floating platform, not a slow "party" boat!

    My question is around the maths of flotation, I have been asked to build a flat platform raft which can carry 6 tons.

    I'm a welder, not a boat builder, so I'm hoping someone will look at my design/maths and say, "yes, it'll float" or "it'll sink"!

    This is a charity project, so I'm hoping for free advice....basically, the design is a flat platform with 3 triangularly shaped pontoons under it, the platform is 41 ft long and 19 ft wide.

    3 pontoons (2m x 2m x 1m) along each of the longer side, equally spaced. 1000 liters volume in each, 6000 liters in total, supports 6 tons?

    The peak of the triangle shaped pontoon, will be expected to settle into mud at low tide.

    Must warn you, I'm a welder, not a boat builder and unsure how I've ended up doing this for nothing!
    Oh, I remember, the wife volunteered me!

    Thanks
    Andy
     
  6. philSweet
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    philSweet Senior Member

    I suggest you post in the main Boat Design subforum for maximum exposure, or you could post in Stability for a more calculated response.
     
  7. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    The volume of water displaced tells you how much load you can put on something. Fresh water weighs 1000 kg a cubic meter (salt water 1025 kg).

    So if you had a steel cube 1 meter square, when it is halfway submerged, it is displacing 500 kg of water.

    So, if the steel cube itself weighed 100kg, you could put 400 kg on it and it would submerge half way.

    I think submerging half of the displacement is about right for most things, safety wise.

    That is not necessarily half the height of a pontoon, as it depends on the actual shape. With a triangular pontoon, with a pointy end down, more than half the height would be submerged to displace half the water. The other way around, with a flat side submerged, it would go less than half way. Displacement of water is directly tied in to the volume of the shape displacing it.
     
  8. AndyBorris
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    AndyBorris New Member

    Thanks guys.

    I understand now, a 1 meter cube will support 1 ton at a .5 meter floating (?) line, AKA will float half in and half out of the water.

    A 1 meter triangle with equal angles will support half the weight and sit deeper in the water, to where the halfway volume line is, guess I can find this float line by filling a pod with 250 liters of water? The internal level will show me the float line on the outside, if had a weight of 500 kgs on it?

    Because a triangle is half a rectangle, the 1m x 1m x1m floats will carry .5 ton minus it's own weight. So approximately 12 1m triangle floats for 6 tons, or lengthen the floats and use less.

    Thanks for the above everyone.

    And now my next question, 60 degree angle point on the sharp end of the float, it needs to sit into mud because it's on a tidal river.

    I'm hoping that it'll settle level, am I deluded? I figure that the weight will allow the floats to "bite" into the mud, as the tide withdraws.
     
  9. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    No, a 1 meter cube will support .5 ton at the .5 meter waterline. It will support 3/4 ton and be 3/4 submerged. At 1 ton, it would be submerged to water level.


    I don't think what you describe is the same volume. A triangle with 3 sides of 1 meter each is less than 1/2 square meter. To divide a square meter in half and make two triangles, you would draw a line from one corner to the other. The triangles would have two sides 1 meter long, but the side from corner to corner would be longer.

    http://www.mathsisfun.com/area.html

    Over all I'm confused. are you talking 12 long floats with 3 sides and 2 ends, or 12 triangular floats like pyramids?
     

  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Andy, you'd be best advised to find a student of yacht design or hungry designer, who can pen up a simple structure to meet your needs. The general and basic nature of your questions suggests, you may need much more understanding than you currently have. You can study up and eventually get something or just bite the bullet and have the basics worked through, so the math isn't something you're wondering about come launch day.

    Can you post a sketch of what you propose?
     
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