Boat extension

Discussion in 'Boat Design' started by Albert Jr., Oct 29, 2013.

  1. TANSL
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    TANSL Senior Member

    When I speak of a "cylindrical body" I'm talking about the same issue that Me.Efficiency when he says: "continous section of the hull that is a prism". If this section exists, it is easy to elongation. Otherwise, you can do, but the result will not be as nice, especially below the waterline.
    Stability may, in some cases, increase, but in general, can be decreased. Whatever happens, it is wise to study this issue.
    Sorry to clarify here that, whatever my experience, I have the right to review and, I know, I did not say any outrageous but I have discussed things that from the technical point of view, are reasonable and prudent.
    Whatever my experience, no one should despise because he thinks it is scarce.
    Regards
     
  2. myark
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    myark Senior Member

    Tansl
    Take his insults like water on a ducks back, < reported post: personal comment removed >
     
    Last edited by a moderator: Oct 30, 2013
  3. DCockey
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    DCockey Senior Member

    If a boat is stretched linearly in length only by increasing the distance between stations, the waterline location stays the same (which means that the displacement scaled with the length), the longitudinal position of the CG remains at the same relative position/station, and the CG height remains the same then:

    1) The Metacenter height does not change

    2) The distance from the CG to Metacenter does not change

    3) The Righting Arm vs Heel Angle curve does not change

    4) The Righting Moment vs Heel Angle curve scales with displacement
     
  4. DCockey
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    DCockey Senior Member

    If a boat is stretched linearly in length only by inserting a prismatic shaped section at the maximum beam location, the displacement and longitudinal CG locations change such that the waterline on the bow and stern sections remain at the same location, and CG height remains the same then:

    1) The Metacenter height (at zero heel) increases

    2) The distance from the CG to Metacenter (at zero heel) increases

    3) Changes to the Righting Arm and Righting Moment curves will depend on the freeboard of the inserted portion but the height of the curves at small heel angles will be higher.
     
  5. kerosene
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    kerosene Senior Member

    I think David hits point with a mixup going on here. Extending by adding a section between TWO stations vs. evenly stretching the whole boat.
    I think the OP means the latter - which seems sensible enough.
     
  6. TANSL
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    TANSL Senior Member

    Only three comments, but could do more:
    - Displacement does not increase proportionally to the length. That is wrong.
    - The position of metacentre is function of the ratio I/D (I is the inertia of the flotation, D is the vessel displacement). I changes when boat is elongated and D changes to lengthen the boat but do not at the same rate. Thus I / D switches to lengthen the boat.
    - The C of G changes, surely. GM therefore changes.
     
  7. kerosene
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    kerosene Senior Member

    < The post this is in reply to has been removed > Even if it does, your choice to go for a personal attack with your own twist doesn't seem too classy.

    I have no affiliation with PAR but I do know this: he has designed quite a few boats that have been, and are being, built all over the place.
     
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  8. kerosene
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    kerosene Senior Member

    care to elaborate? I see you use cad program - looks like Rhino3d wires but not sure. When you have a model - of any shape - and you do 1D scale on that object,
    1) does the volume of the object increase proportionally to the scaling? (it does, naturally)
    2) does the center of mass stay in same place when that scaling takes place? (it does, in relation to the general shape)
     
  9. TANSL
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    TANSL Senior Member

    kerosene, I think you're partly right. When I scale a boat, displacement naturally increases at the same rate but when you increase the length by a certain amount, without changing the other dimensions, the displacement does not have to increase in proportion to the length.
    The vertical position of the vessel center of gravity may change without direct relation to the increased length.
    As the CAD software I use, I am very devoted to, AutoCAD with I do most of my calculations. MaxSurf is another program I use. I make my models with AutoCAD and export them to MaxSurf (which I find more difficult to create 3D models). I have a Rhinos copy, not as a working tool but as a mean of exchanging information with other professionals.
     
  10. DCockey
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    DCockey Senior Member

    The weight of a bare hull may or may not increase proportionally with length. The change in displacement will depend on the structure, propulsion system and other equipment, contents and payload of the vessel. How those items change with length depends on the particular boat, how it is lengthened and how it is used.

    Edit: But if the length is stretched linearly and the weight and CG location of the resulting boat are such that the stretched boat floats on the same waterline as the original boat, the displacement increases proportionally with length.

    Itransverse scales with length if hull shape is scaled by length only, assuming the waterline location stays at the same location. The displacement scales with length if the waterline location stays the same. The ratio of Itransverse/D stays constant if the hull shape is scaled by length only with the waterline location staying the same.

    You may want to consult a text on the basics of hydrostatics and stability and/or work through the math yourself if this is not clear to you. Also, as kerosene suggested this can be verified with numerical experiments using software such as Rhino.

    The CG height may or may not change depending on how the structure, propulsion system and other equipment, contents and payload of the vessel change with length.
     
    Last edited: Oct 30, 2013
  11. TANSL
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    TANSL Senior Member

    DCockey, you are saying that IF certain things happen, the c of g, displacement, metacentre, etc.. can not change.
    I say something like that, but I think they change, sure, except that certain things happen and do not change.
    In any case, before making a lengthening, I think we should check what happens. This is the origin of all this discussion and that is what I have tried to tell the creator of this thread.
    Perhaps the figure below, which shows what I would do, explains better the things I'm saying. There are other ways to lengthen the boat but, to me, this seems the most easy. I think this is an issue that also could be of interest to Albert Jr.
     

    Attached Files:

  12. DCockey
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    DCockey Senior Member

    I'm saying that if:
    - the boat is lengthened in one of two ways and the mass of the boat changes such that the waterline location and CG height do not change
    - then the metacenter height, righting arm curves and righting moment curves behave in certain ways and stability does not decrease.

    An increase in length and therefore a decrease in beam to length ratio does not always mean that the stability of the boat decreases. This may be contrary to "intuition" based on "experience" where long narrow boats are generally less stable than short wide boats of similar displacement, emphasis similar displacement. How stability changes depends on what else changes.

    For a typical commercial type of hull the easiest way to length a boat may be to insert a constant area section in the middle of the boat, particularly if the boat has a parallel mid-section. That is what I was describing in my second post, a boat is stretched linearly in length only by inserting a prismatic shaped section at the maximum beam location, which appears to be what your illustration shows. This method of lengthening a boat may be also desirable if the primary object is to increase cargo/passenger volume.

    For a new-build recreational boat which does not have a parallel mid-section lengthening the boat by increasing the space between stations is a commonly used method for wood and metal construction. A boat such as PAR showed in post 12 cannot be lengthened by inserting a constant area section in the middle and have the resulting hull be "fair" without changes to the bow and stern. Recreational boats built in molds are sometimes lengthened by adding to the stern. This is sometimes planned for when the boat is designed and the mold is built.
     
  13. TANSL
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    TANSL Senior Member

    What I am saying from the beginning is that the design ship's draft changes and , therefore, the surface of the flotation and its position (the draft) also change. Consequently, changes its shape and physical properties.
    I know what to do with the molds, I have worked a lot with that, but I think all that does not fit the case posed Albert Jr. He says he has some plans and wants to lengthen the ship by 12%. The molds do not exist and therefore does not have to extend aft of the mold or have to separate the frames of the mold uniformly .
    Albert Jr is talking about modifying a plane and therefore can do what he wants but for the resulting boat will have to check if it continues to meet all standards , regulations, etc. that primitive boat is supposed to meet . And , of course , scantlings drawings and calculations , would have to review them. Actually , if he had to ask permission to build (he does not have to do it, you´ll say ) , would have no choice but to make a new project and submit it to the competent authority approval .
     
  14. PAR
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    PAR Yacht Designer/Builder

    The station mold spacing on the boat Albert is looking at is around 36". If these are re-spaced at 39" - 40" (35' 9" - 36' 7") he'll have a boat that is ~12% longer. It's literally as simple as this. You can make it much more difficult than this is desired, but to what end. It's a private yacht, so certification is moot. If the designer supplies plans, he already has permission to build at least one, so why the discussion of complications where none exist?

    Yes, he'll have to re-fair and line off to the new length, but he'd have to do this anyway. He would also have to work out trim, but this isn't especially difficult either, particularly if he has a professional work out the stretch.
     

  15. nzboy
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    nzboy Senior Member

    In the 1970 s a New Zealand designer Frank Pelin designed some launches
    named Pelin Empress one model 21-26ft 2nd model which my father built 27-33ft all achieved by frame spacing Ive actually seen one extended to 36ft which is a bit silly because Frank had a model named shikaere 36-39ftwith a 12ft beam as opposed to 10ft on the 27-33ft empress. A problem that seemed to arise was motor placement. My father on franks advice moved the engine forward 2ft in the 29ft build to improve dynamics when planeing because a sister boat planed with a bow up syndrome overcome with quite a bit of weight under the front bunks Obviously there were a few problems. So stretching a boat 12% isnt a lot but may provide room for a toilet at rear of cabin in lobster boat .With smaller boats its only when they are in the water some weight adjustments can be made ie some extra water storage under front bunks or diesel tanks towards the stern A talk with the designer is normally the best way to go with computer designing weight placement is a little simpler
     
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