Can I add a second level?

Discussion in 'Stability' started by ubgrit, Jun 27, 2017.

  1. ubgrit
    Joined: Dec 2015
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    Location: USA

    ubgrit New Member

    I am rehabing a 62-ft Catamaran Cruiser houseboat.
    The boat will be installed in a live-aboard slip and never move.
    The slip has a 40-ft finger pier on one side only.
    There are other boats on three sides (think wind shadow).
    I want to ad a 21-ft long second story to the boat amidship.
    The boat is 14 feet wide along its entire length.
    It sits on two fiberglass pontoons which measure 40 inches deep and 30 inches wide with a flat bottom (no ballast).
    The current draft is 20.5 inches.
    These hulls have no thru-hulls and are sealed.
    This is basically a house trailer floating on two pontoons.
    The sides of the house are 91 inches high, plus the 20 inches of freeboard (?) for a total height of 111 inches above the waterline.
    If I add the partial second story, that height will add another 91 inches for a total of 202 inches.

    The boat will be berthed in Key West, Fl. so it is in an area that could experience hurricane conditions.

    The floating finger pier sits on the starboard side.
    There are four cleats along the finger pier.

    I am concerned that I am creating an unstable floating structure because I am increasing the weight and the "sail area" of the higher structure.
    Yet half of the vessels in this live-aboard community are full two-story boats or floating structures.

    Please help me help myself.
    What additional info should I develop or share?

    And if I modify it, I have to get it insured (liability only) as a boat.

    Thanks in advance.
     
  2. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Since she's not going to move much have you considered adding bustles to her sides to help boost her stability and somewhat increase buoyancy? Or would that make her too wide for the anchorage?
     
  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    The difference between the height of the pontoons and the current draft seems to indicate that there is sufficient freeboard to withstand a considerable increase in weight. Another issue is the resulting stability by increasing the height of the new center of gravity.
    All this could be easily studied to give a reliable answer to your question if you provided a general arrangement plan of the boat as it is today and how it will remain after the modification. That drawing with dimensions and data of the current weight (and its c of g) and of the final weight (and its c of g) would allow to make a simple calculations and to answer you properly.
     
  4. ubgrit
    Joined: Dec 2015
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    ubgrit New Member

    I dont have drawings.
    But I will post pics when I get back to my computer.
    Thanks
     
  5. ubgrit
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    ubgrit New Member

    BTW, what is a bustle???
     
  6. Rurudyne
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    Rurudyne Senior Member

    It's a term I've seen used to describe a bulging add onto the sides of a boat hull. The word seems to have been adopted from a now uncommon woman's clothing style that poofed out a skirt to make the butt look much fuller. Bustles on boats was also a term used to describe a stern bulge to prevent a sternwheel paddleboat's rudders from fouling.
     
  7. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    The real issues can be answered with the make, model and year of the original design. Lacking this, a set of accurate lines will be necessary, to work up rudimentary hydrostatics and a stability book. From the sound of things, as you've described (~16 tons displacement) you may be right at the max your vessel can tolerate. Construction details might offer a clue, but out of hand, I wouldn't recommend this approuch to get a bigger houseboat. Insurance companies will run quickly, without the appropriate paperwork and requirements being met, in spite of any survey they might also have. Some insurance companies will permit a "declared" value, which may be more lax on requirements. Lastly, though many of the houseboats you see have a second story, are this also floating on two, fairly scrawny 'toons?
     
    Last edited: Jun 27, 2017
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    That fashion certainly died in the rear end.
     
  9. Rurudyne
    Joined: Mar 2014
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    Rurudyne Senior Member

    Now very fat people wear spandex and thongs in Walmart. Progress?
     
  10. Ike
    Joined: Apr 2006
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    Ike Senior Member

    What he is referring to as a bustle is essentially a small pontoon outboard of the existing pontoons. To add to what PAR said, your boat may be near it's weight limit. If these are round pontoons and the water level is at half the pontoons depth, then he is right. Adding a bustle would increase the ability to carry weight and increase the stability. But again, without drawings and dimensions, all this is speculation. Before doing it someone should work the numbers. An insurance survey would probably say "no way" unless you had the numbers, and they may even demand an inclining test to determine if the numbers are right. (Especially if you decide to put a hot tub up there, like I have seen on some houseboats!)
     
  11. Windship277

    Windship277 Previous Member

    You can always add a third pontoon between the other 2. Do you get large wakes abeam?
     

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

    A third 'toon will help his displacement issue, but his static stability concerns will not be addressed.
     
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