Heptagonal barge

Discussion in 'Boat Design' started by mtht110, Oct 31, 2020.

  1. mtht110
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    mtht110 Junior Member

    I was working on a small heptagonal barge (5m dia & 0.5m depth) which will run by an outboard engine. I was trying to determine the required power by Maxsurf Resistance. But it's not showing any result above 5 knots speed. Don't know why. Need help.
     

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  2. bajansailor
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    bajansailor Marine Surveyor

    Do you want it to be able to travel at more than 5 knots?
    If yes, then you should improve the hull shape - even at 5 knots the resistance of that barge will be enormous (relatively).
    What do you want this barge to do, and why does it have to be this shape?
     
  3. alan craig
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    alan craig Senior Member

    I don't believe the numbers: It apparently travels at 5kts using just 2.5hp while displacing 4.5 tons and being as wide as it is long.
     
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  4. mtht110
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    mtht110 Junior Member

    Cannot change the shape. It's the builder's requirement. It's a floating tiki bar. My question is why Maxsurf Resistance doesn't show results above 5 knots?
     
  5. TANSL
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    TANSL Senior Member

    If you provide us with the Maxsurf file we could analyze the problem.
     
  6. mtht110
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    mtht110 Junior Member

    Here is my file.
     

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  7. TANSL
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    TANSL Senior Member

    The Holtrop-Mennen method has the limitations indicated in the accompanying figure. Perhaps that is why Maxsurf does not calculate more than up to a Froude number of 0.4.
    Try the "KR Barges" method which seems to work very well.
     

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  8. mtht110
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    mtht110 Junior Member

    Yes sir, the KR Barges method is working. But KR Barges methods result is almost three times higher than the Holtrop method. Confused about it.
     
  9. TANSL
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    TANSL Senior Member

    Please, analyze the coefficients in the table of my previous post. Check that, for example, your L / B = 1 ratio is very far from any of those listed for any of the ship types to which the method can be applied. Your vessel cannot be calculated with this method. That was probably why someone set out to create a method for barges.
    If you analyze other methods you will also get strange results. Savistky planing gives results for a boat that we know for a fact will never plan. These are not puzzling results, they are invalid because these methods simply are not applicable to any type of ship. Even the barge method is doubtful whether it is reliable for a barge of these dimensions. Perhaps if your barge had 6 or 8 sides, the results could be more reliable.
     

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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    What speed is required, might be a good start.
     
  11. mtht110
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    mtht110 Junior Member

    I understand sir, Thank you very much.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Neat, I've swam out to these many times in the past. :D

    If the objective is to be a floating bar... whatever speed it may or may not do...is not relevant...
    Thus why obsess or be fixated on speed .. and above 5knots at that?...are you asking your barflys to be swimming while attempting to order and drink at the same time?? o_Oo_O
     
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  13. bajansailor
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    bajansailor Marine Surveyor

    Mtht, are you looking to build something like this?

    Bar with outboard.jpg

    I do not know where this one is - I found it online some time ago, and saved it to my floating bar folder.

    It might be easier to find an old catamaran, and convert it into a Tiki bar, like the one below (this was taken in Carriacou in the West Indies).

    Floating Bar at Carriacou.jpg

    Or use a few simple pontoons to build a trimaran Tiki -

    Floating-Bar at Hansen Bay USVI.jpg
     

  14. Robert Biegler
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    Robert Biegler New Member

    If the 0.5 metres is a draft, that implies a displacement of nearly 10 cubic metres. You could give the software a more conventional hull of that displacement, as a better than your best case scenario, and see whether a small outboard can push it at over 5 knots. If not, then your shape won't make it, either.
    If the dimensions and shape define what is most visible above the water, you can do what you like underneath, and you can have far less displacement. Then you have a chance of pushing the bar to reasonable speed on modest power. You could put a trimaran under the platform, and hide the hulls by letting some cloth hang down to the waterline, or wooden slats that can be pulled up when moving, to get out of the way of waves.
     
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