Weight distribution in design of boats

Discussion in 'Stability' started by JohnMarc, Oct 29, 2020.

  1. JohnMarc
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    JohnMarc <--- My ultimate goal

    Right I see, yes you understood the question many thanks, so does that mean the distance between the centre of the demi hull to the centre line is 320.... erupt another way the distance between the two demi hulls centres is 640?
     
  2. Heimfried
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    Heimfried Senior Member

    Yes
     
  3. JohnMarc
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    JohnMarc <--- My ultimate goal

    Okay so as a newbie to line drawings would this be correct.... before I make a fool of myself and post the entire line drawing
    1.png
     
  4. TANSL
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    TANSL Senior Member

    It looks very good. I think you can continue, sections of the stern, longitudinal profile and a plan of the deck and, if you want, the water lines although with all the above you can already calculate the center of buoyancy and the transverse stability curves.
     
  5. JohnMarc
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    JohnMarc <--- My ultimate goal

    Thanks Tansl.... once again dipping my toe in the water
    full drawing.png
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    how is it powered ?
     
  7. TANSL
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    TANSL Senior Member

    Very well. Now, we go step by step, calculate the longitudinal position, for the estimated displacement, of the center of buoyancy (geometric center of the submerged volume), which should be aligned with the CoG.
    If that does not happen, you will have to change the arrangement of the weights (working in the spreadsheet), until you achieve it, or admit that the boat, at rest, will have a certain trim.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    So, now we come to the operational side of the boat. What speeds do you want/expect from the boat?..and how do you anticipate the boat to be powered?

    Speed...this influences the shape of the bow and the lines accordingly. Currently, as drawn, it would suggest you are not interested in speed, other than just a hull to produce buoyancy for your house boat.
    However, if speed is an input...i.e. you may like to motor over to a different location and if you wish to do that in a reasonable amount of time, then the bow region needs to be changed. For example, here:

    upload_2020-11-2_9-31-8.png

    This is too bluff...or too blunt. The water flow is like hitting a brick wall.
    Thus, if speed is a consideration, then the lines need more shape to help "promote" the flow of the water aft - rather than - as it currently is - stopping it at the bow, so to speak, like so, for example:

    upload_2020-11-2_9-32-45.png

    And this then falls into the means of powering...if it is a simple outboard that is easy enough, you just hang it off the transom...
    However, if you decide on an inboard motor, of whatever type, then promoting the flow of the water into the prop, assist sin this regard, like so:

    upload_2020-11-2_9-34-55.png

    You slowly raise the keel line, so the prop can fit under the hull, but also if the prop extends below the keel line, you then need to think of the easiest/best way to protect the prop.

    So, in short, great start to your lines.
    BUT...other than providing buoyancy to carry you weights... you need to consider how the shape will be influenced by the speed you hope to get, if speed is an input.
    As this can greatly influence the shape of the hull, and hence the Lines.

    There is no point doing anything else, unless you decide whether speed shall be an input to your final design.

    So looking at the hull lines referenced by Heimfried, you can see the difference:

    upload_2020-11-2_9-41-38.png

    The lines above, are clearly designed for a speed.
     
  9. JohnMarc
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    JohnMarc <--- My ultimate goal

    90 Hp outboard
     
  10. JohnMarc
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    JohnMarc <--- My ultimate goal

    Wonderful, Thanks Ad Hoc, lots of things to consider, really grateful for the input I am getting, allowing me to have a far better understanding of where I am going and what I want to achieve..... Many thanks will do a redraw after rethinking my priorities... ie. speed or weight carrying capacity
     
  11. JohnMarc
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    JohnMarc <--- My ultimate goal

    Gentleman may I ask a question while in contemplation of your earlier comments regarding my hull, which of the two shapes are better for my pontoon boat and reasons why......

    hull shapes.png
     
  12. TANSL
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    TANSL Senior Member

    Both will be very similar from the point of view of stability, drag and maneuverability. So I would choose the one that is easiest to build, according to the mechanical and human means you have.
     
  13. JohnMarc
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    JohnMarc <--- My ultimate goal

    Thanks...yes easiest and the extra 20 Kg volume I get from the rounded shape seem to be the way to go.
     
  14. gonzo
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    gonzo Senior Member

    There is a fundamental mistake in the diagram if both boats weigh the same.
     

  15. TANSL
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    TANSL Senior Member

    @JohnMarc, the polygonal section can be much easier to build, in the fore and aft areas, than the rounded one. Analyze several cross sections and some longitudinal before deciding on one or the other.
    Don't worry about 20 kg more or less, the mistake you (or anyone) will make when calculating the weight is going to be greater than that number.
     
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