Reduced buoyancy in the Bow V (Hydrostatics)

Discussion in 'Hydrodynamics and Aerodynamics' started by Hobbyboatbuilder, Sep 18, 2017.

  1. Hobbyboatbuilder
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    Hobbyboatbuilder Junior Member

    Hi,

    I'm looking for a calculation to provide an equal amount of buoyancy from bow to stern?

    The bow area reduces to a point providing less buoyancy. I've noticed to solve this the stem isn't completely horizontal. Is there a way of working out this angle?

    Many thanks
     
  2. TANSL
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    TANSL Senior Member

    Do you mean the same buoyancy per linear meter over the whole length?
    o_O It is never horizontal, it is usually almost vertical.
    I think some complementary explanation would be enlightening.
     
  3. Heimfried
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    Heimfried Senior Member

    Assuming you meant keel instead of stem, there would be probably a geometrical solution to go deeper with the keel to counterbalance the loss of buoyancy, which is in the case even keel caused by the decreasing of the area of the immersed V-part of the cross section of the hull.

    But I don't think it would be viable, because the large increase of the draft near the stem, given a usual shape of hull.
     
  4. markmal
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    markmal Junior Member

    You can calculate hydrostatics in free programs like FreeShip. Create your hull there, set mass, and you can analyze buoyancy from different perspectives.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Easy, design you boat along the lines of a shoe-box, that way every section will be the same. One end will be as buoyant as the other. But I'd suggest it may not be a desirable boat. I think more information needs to be given to establish what problem requires the solution being sought.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Agreed about the shoebox approuch. There's no good reason to intentionally design your sailboat with a similar bow/stern buoyancy rate.
     
  7. Hobbyboatbuilder
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    Hobbyboatbuilder Junior Member

    I'm attempting to design a planning, tri-hull tender with out board engine (20hp/51kg) for 3 persons. It has generous amount of dead rise (for the moment) and low displacement.

    V Hull, split in half.

    Bow to stern.

    Thanks
     
    Last edited: Sep 19, 2017
  8. SamSam
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    SamSam Senior Member

    That sounds like the opposite of a low power, planing boat.
    I'm confused...is it a tender of some kind meant for a tri-hull, or is it a tender that is tri-hulled...?
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Confusion is contagious here, how does a V-hull split along the length become a tri-hull ?
     
  10. PAR
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    PAR Yacht Designer/Builder

    I think he's referring to a cathedral hull form, sometimes called a trihull. It's a pretty draggy full plane mode powerboat form. There are several versions, all pretty draggy.
    [​IMG]
     
  11. messabout
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    messabout Senior Member

    Generous dead rise ? Three people ? 20 HP ? Add a lot more power or decrease dead rise a plenty if you wish to have some degree of efficient performance with low power.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    OP another poster who provides scant information, and expects answers.
     
  13. Hobbyboatbuilder
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    Hobbyboatbuilder Junior Member

    I'm just starting out. Thank you for your help. I have many questions and they will be improving.

    I have an idea of how I'm hoping the vessel will look although getting something in 3d is another matter, which is why there is a deep v than probably practical. I've started by calculating buoyancy using Simpsons rule which has driven towards the question on this title.

    Thanks again
     
    Last edited: Sep 25, 2017
  14. messabout
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    messabout Senior Member

    ^ HBB one of the most common amateur mistakes is to begin with a notion of what your boat should look like. Physics does not work for the convenience of the stylist.

    Your first order of business, if you presume to design a boat, is to make a carefully considered system of requirements ...an SOR. After that you can design the boat within the limitations of your SOR and within the limitations of what is acknowledged to be proven design characteristics. Appearance can be, and should be addressed, but as one of the last of design considerations.
     

  15. Hobbyboatbuilder
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    Hobbyboatbuilder Junior Member

    In design they say you look at the customer/markets requirements/needs then consider the design (similar to what your saying). I've taken the American approach of hell yeah! thrown all that out the window and selfishly building something to the best of my abilities.

    Starting by seeing if I can make things, accurately (fibreglass/plugs/moulds), at practical costs (design costs, engineering, naval architecture) and using things that are available (space large enough to build/cheep materials/cheep methods).

    So far its looking pretty cool and has ticked off most of the points set myself. I don't have much space so the thing is pretty small. Though this fits as I'm hoping to hang it of the davits of a 40ft flyridge boat.

    I checked it would float first, checked I can build stuff close to professional trades and now looking at the design.
     
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