Houseboat pontoon placement

Discussion in 'Boatbuilding' started by cj88, Nov 6, 2018.

  1. cj88
    Joined: Mar 2017
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    Location: Australia

    cj88 New Member

    I’m in the very early stages of planning to build a houseboat. It will be 12m/40’ in length and between 5m/16’8” and 6m/20’ wide. I’m looking at making the structure (9m/30' x 5m/16'8" to 6m/20') out of composite panels with a Styrofoam core and fibreglass outer skins. This will be mounted to the pontoons via a frame of either fibreglass pultrusions or aluminium. For the pontoons I’m looking at using 4 lengths of 12m/40’ long x 750mm/30” dia. polyethylene pipe. I want to put some bulkheads in 2 of the lengths to create some water storage. Would it be better to have a pair of pontoons on each side with the ones containing the water storage being the ones closest to the centre or have the pair with the water storage in the centre and an empty pontoon on each side?
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    With all the space at your disposal there, why not have the water storage above deck ?
     
  3. cj88
    Joined: Mar 2017
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    cj88 New Member

    I want to keep the weight as low as possible as it will be in a coastal area (fairly sheltered) and fully off the grid so I will have a large solar panel array set up like a bimini above the upper deck which raises the centre of gravity. One water storage tank will be filled using a watermaker and the other will be filled with collected rainwater.
     
  4. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    I don't know, but that sounds like the same thing said two different ways. Maybe you're saying have 4 pontoons spaced equally, with the two inner ones with the water storage, or the two inner ones (with the water storage) close together or touching, in the center of the boat. Offhand I'd wonder if two empty tanks close together in the center might create problems with the structure above, as far as uneven support because of uneven flotation in wave situations. On the other hand, it might work better that way when one tank is full and the other is empty.
     
  5. cj88
    Joined: Mar 2017
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    cj88 New Member

    To clarify, I was thinking of either option 1. On each side a pair of pontoons that are touching, with the water storage pontoons being the pontoons closest to the centre or option 2. One pontoon out on each side and a pair in the centre that are touching and that have the water storage. I hadn’t considered equally spacing them. Hope this is clearer.
     
  6. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    With the two touching in the middle it seems there might be different loading issues as I said before, but it's just a guess and I can only guess what the loads might be. With the outside ones touching you more or less have a catamaran, with the two in the center touching you're more like a trimaran.

    In the catamaran style loads would be applied at the sides so strain on the structure above would be compressive with a possibility of the end walls buckling. In a trimaran mode, it seems the center would have more flotation than the sides and so in general there would be a tensile load on the structure above, with a possibility of the end walls splitting apart in the center. In the trimaran mode you might actually get both kinds of loads, depending on whether the waves are coming from the ends of the boat or the sides.

    If that was the case (if building it trimaran like gave extra flotation in the center) if you built it on a flat surface like a parking lot, when it was put in the water it would acquire a whole bunch of different loads compared to being evenly supported by the parking lot.

    I'm probably overthinking it, but it is a rather large contraption and polyethylene is somewhat flexible so whatever frame holds it all together will probably be the most important part.
     

  7. Mike Inman
    Joined: Oct 2018
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    Location: Jacksonville, Florida

    Mike Inman Junior Member

    With the mass of the water tanks in the middle, they won't add as much polar moment on the roll axis as they would on the outside - means the boat will be quicker to rock side to side, but also quicker to stop after it starts rocking, with the mass in the middle. I think the net effect of a larger polar moment would be less movement from boat passing wakes and similar disturbances, but maybe also a tendency to rock for longer once it has started.

    As noted by SamSam, if you put the mass in the middle, you may need stiffer (deeper) stringers to support it - 20' side to side is a pretty wide span and if you're going to fill those water tanks with a few thousand pounds above the waterline, that could start to add to the flex in your floor.
     
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