Cat pontoon safety margin

Discussion in 'Boat Design' started by JayBoy, Mar 31, 2017.

  1. JayBoy
    Joined: Mar 2017
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    JayBoy New Member

    First off, I will have pros design/build this houseboat but I am doing the early feasibility study. Designed for 'small, calm inland lake waters', 5 knot max (electric), 14'x30', tall single story. Fiberglass pontoons are 4' x 3' x 30' (22,320#lift each). My question is what safety margin should I allow? 10%? Vessel fully loaded would then support 20,088#. I built a scale model and 10% seemed stable under all conditions.

    Thanks
    Jay
     
  2. TANSL
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    TANSL Senior Member

    Welcome to the forum, Jay.
    What margin do you mean, the displacement, the freeboard, at the initial metacentric height? In general, from my point of view, it should not be designed looking for margins, but it is enough to meet all the criteria that, in terms of stability and safety, must comply the boat.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Instead of working on the details, such as safety margins, just focus on what you want in the design. The professional you select to "flesh out" the design will worry about the details and their appropriate margins.
     
  4. JayBoy
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    JayBoy New Member

    TANLS;Yes, the displacement safety margin. The freeboard is 24".
    PAR; The problem is, I might want what isn't possible!
     
  5. TANSL
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    TANSL Senior Member

    As I said before, I am not in favor of establishing such margins. However, if you want to speak on the margins, I think it is safer to set a margin on the freeboard rather than on the displacement.
    Let me give you an advice: never make a decision because something "seemed" sufficient, check that it is sufficient.
     
  6. messabout
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    messabout Senior Member

    It is certainly well to address safety margins. On the other hand you are approaching this design in a backward way. A suitable vessel is designed to satisfy a statement of requirements (SOR) that the builder/buyer will set out before the first line is even drawn.

    You have already designed your pontoons. They are 3 x 4 x 30 feet. First question is how is the section dimension, 3 x 4 feet, might be oriented. Is the bottom 4 feet wide or three feet wide. It matters. You want 24 inches of "freeboard" so whether the rectangle is at one or the other rotations will make a difference

    Have you done a careful assessment of the actual weight of the occupants, cargo, equipment, supplies, propulsion systems, nav gear, and all that stuff? That is the starting point. Then there is the capacity in terms of sea state, wind pressure, and a whole bunch of other things that need to be considered. There is a whole lot of difference in traveling around on a peaceful lake or stream than there is with navigating Cape Hatteras.

    Get back to us with some of the critical details and some of our our knowledgeable members can take it from there.
     

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

    Jay, there are many considerations involved in any design, safety concerns included. If the professional is much like me or others I know, most of your in hand "design work"
    will be looked at, but little else. This isn't an insult, but simply the designer or NA will have no choice but to do a full work up, specing scantlings and appropriate dimensions for your project and not assume the previous work on your part meets specific class and society requirements, were applicable. Naturally they'll take your "GA" sketches in mind, but reverse engineering these isn't likely, if only the additional time it would bring to the table.

    I say this because your fully loaded pontoon displacement figure isn't even close to what we'd employ in a 30' boat (with 4'x3' pontoons. In fact, you've just calculated the total volume of a single 4'x3'x30 'toon will displace at full emersion.

    Yep it's possible your goals aren't realistic, but this is why you develop a SOR with a professional, to establish the project goals and targets (you can't hit the goals without knowing what they are). Again, if it's me, I'll just tell you straight up, nope can't do that, but I'll also quickly follow up with what we can do. This is the reason you hire a professional in the first place.
     
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