Pontoon vessel Calcs

Discussion in 'Boat Design' started by kavin mommsen, Sep 23, 2019.

  1. kavin mommsen
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    kavin mommsen New Member

    Hi All
    Please could someone assist me with calc's for stability of the vessel i intend on making
    i would also like to work out the draft lines i have a complete scaled model with all the mass data and COG

    Vessel.jpg
     
    Last edited by a moderator: Sep 23, 2019
  2. messabout
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    messabout Senior Member

    Kavin; you will need to furnish dimensions of the pontoons and the overall weight that you anticipate. The weight or displacement of the boat include the boat, engines, fuel, passengers, furniture, and any and all other items that have weight. In addition provide information as to the expected speed that you contemplate as well as the power output of the engines.
     
  3. Ike
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    Ike Senior Member

  4. kavin mommsen
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    kavin mommsen New Member

    Thanks for the reply

    Hope all this extra data is what is needed
    I really take my hat off to you all. DIM.jpg
     
  5. bajansailor
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    bajansailor Marine Surveyor

    Kavin, what sort of sea (or lake) conditions will this vessel be operating in?
    I would hope that it is flat calm, as she is not going to cope well with even small waves.
    Some rough calculations suggest that she will be floating with the pontoons immersed approx halfway for a displacement of 5,500 kg.
    Can you buy the pontoons 'ready made' in South Africa?
    Re stability, do you need to submit details to the Authorities or is this for your own personal interest?
     
  6. DCockey
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    DCockey Senior Member

    How is that information used in stability calculations?
     
  7. messabout
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    messabout Senior Member

    David, neither of those questions are of interest for the purpose of determining static stability. They may very well be of use to determine suitability when the boat is put into service.
     
  8. kavin mommsen
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    kavin mommsen New Member

    Thanks for all the replies.

    The vessel is going to be used on a river with almost zero current or waves.
    I don't need to submit it to any authorities but would like to have all the data if needed i will feel a lot better about it
    all the calculations i suppose are out there but i am very interested to be able to do them myself i just cant find any good enough examples especially on pontoon boats
    and i was hoping someone could assist me with this one as an example i understand a lot better with an example.

    Any help would be much appreciated.
     
  9. messabout
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    messabout Senior Member

    Kavin, forgive me if I am being too basic in the following. I/we do not know how familiar you might be with physics, in this case Archimedes principal. A boat or other floating object has to push water out of the way in proportion to the weight of the boat...or floating object. Imagine that you have a cylindrical container that is capable of holding 4 liters of water. Fill the container so that it has only two liters of water inside. (in this case you must let us assume that the container has no weight) place the container in the lake and it will float half way down the height of the container. The weight of the water is pushing an equivalent weight of water out of the way. ....so far so good. One liter/one kilogram

    Calculate the total volume of the pontoon and you will know how much weight it will support when it is just barely sinking. In your case you do not want the pontoon to sink so assume that maximum load you can tolerate is half the total volume. Best practice is to have the pontoon less than half submerged. The displacement (how much water it is pushing out of the way) of the pontoon varies with the depth that it is in the water. At one quarter immersion it is not displacing the proportion of one fourth of the total displacement. It displaces much less than that. The calculation of circular sections gets a little bit sticky but it is no too hard to figure. If you are fairly good at maths then I, or one of the other of us, will provide you with the equations to figure it out.

    It is far easier to calculate the displacement of a square or rectangular pontoon. Rectangular as opposed to circular also has some distinct advantages.
     
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  10. kavin mommsen
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    kavin mommsen New Member

    I have been through just about every video and detail on the web my physics i think is reasonably good at this point.
    Unfortunately there is nothing on round objects ,Meta center, righting arm, center of gravity ,center of buoyancy,angle of heel etc
    I have been through i just don't get any examples of pontoons.

    It is so frustrating that you would like to know but its just not there.
    if anybody has any examples of something similar would be great.

    Thanks
     
  11. TANSL
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    TANSL Senior Member

    I have made some calculations about your boat and I enclose the most interesting results, I suppose, for you.
    As I do not know the weight of the ship, I have assumed that it was 5.6 tons at full load : 4200 kilos of the boat fully equipped but without passage and 20 people (1600 kilos) on board. This results in a freeboard of about 340 mm. Since the ship does not have to comply with any regulations (which I don't think is possible), only you can know if that freeboard is adequate or not.
    In hydrostatics file you can see the displacement of the boat to the various drafts. Several things can be pointed out:
    - the center of buoyancy of the hulls, for the displacement of full load is 4.98 m from the stern end of the cylinders. This value changes very little with the draft.
    - the center of gravity of the boat should be at this same distance so that excessive boat trim is not produced. You must redistribute the weights to get it as it currently seems that the weight is very concentrated at the stern.
    - When fully loaded, the ship has 183 kg * m as a moment to trim 1 cm. That means that 1 people located in the bow would produce a trim of about 2 cm.
    Transversal stability curves:
    - The boat is stable due to the trimaran hulls.
    - If we placed the entire passage to a band, the ship would heel around 4.4 degrees. That is an acceptable value but the ship, in general, is very sensitive to the movements of the passage.
    If you need any further clarification, do not hesitate to request it. I hope I have helped.
     

    Attached Files:

    Last edited: Sep 27, 2019
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  12. bajansailor
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    bajansailor Marine Surveyor

    Following on from Tansl's excellent stability analysis above, here are a few more thoughts.

    Re the aluminium cylinders, can you buy them 'off the shelf' as a standard diameter of 0.688 m., or will you have to specially fabricate them?
    If you have to fabricate the cylinders, you could consider building a catamaran instead if you can roll the plate to make a larger diameter cylinder.
    You would have approximately the same total buoyancy if you had two hulls each 0.843 m. wide rather than three hulls each 0.688 m. wide.
    (this is only approximate - in the calculation I have not considered the conical bows, just the main tubes).
    Yet the aluminium weight of the two larger hulls would be approx 80% of the weight of the three hulls. Hence you will have extra carrying capacity.
    And the frictional resistance while underway would be much less with a catamaran.
    You could also consider putting cones on the transoms as well, in similar fashion to those on the bows. As otherwise you will be getting some turbulence behind the transoms, especially when loaded, which means extra drag / resistance.
     
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