Bolger Bantam : Direct 50 % scaled up version........

Discussion in 'Boat Design' started by boat fan, Sep 7, 2009.

  1. boat fan
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    boat fan Senior Member

    Intended use : Sheltered estuaries , lakes , inlets , and rivers along Australia`s East coast.

    Scaled up 50 % , as is ....please comment , good , bad ......neither......
    or .......Stupid Idea..............:D .
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  2. boat fan
    Joined: Sep 2008
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    boat fan Senior Member

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    Edit...pictures not showing...
     
  3. lewisboats
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    lewisboats Obsessed Member

    bunch of additional displacement... but otherwise interestingly revised
     
  4. boat fan
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    boat fan Senior Member

    Yes ...Agreed.

    Is that necessarily bad though ? I`m thinking the additional buoyancy of the hulls would carry the extra materials ( weight ) of the larger boat.
     
  5. GAZZABO
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    GAZZABO Junior Member

    I seem to remember reading somewhere it was OK to plus or minus 20% on an existing design without altering the factors to much to make the design perform differently. I remember a friend spacing out the frames of a Roberts from 60 to 85 ft and was surprised that the angle of the stem raked a lot more!( there are frames in the bow area!)
     
  6. boat fan
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    boat fan Senior Member

    Yes I think you would have to be careful doing something like that.
    I would not have the nerve to alter a project of that magnitude.
    Better to choose an existing design , or get a N A to draw what works....

    Scaling a large and beamy monohull opens a pandora`s box when you consider the changes induced by heeling waterlines , wetted surface areas and reserve buoyancy etc..etc..
    Thank god there are people that know how to work those calculations .A job for the experts no doubt.:D

    What I`m particularly interested in with the Bantam is the behavior of the vessel if scaled up.It seems to me , given that the hulls are little more than elongated pointy boxes , the boat should function as before .

    I`m hoping some of the designers on this forum will give some insight ...

    I think most of them would consider this particular boat a little too "crude" to be worthy of comment , after all long elongated box hulls are not that " sexy " by most people`s standards.I still like it for its utalitarian simplicity and despite it`s scaled up size , I do believe it would not present any great difficulties for a dedicated home builder with tool skills.

    Best regards ..BF:)
     
  7. PAR
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    PAR Yacht Designer/Builder

    The laws of mechanical similitude will apply here and you'll be quite disappointed with the results.

    These laws also apply to the loads on the structure as well.

    What does all this mean, well simply, put you'll be converting this one ton boat to a 4 ton boat, so you can kiss off any idea of powering it with a little 25 HP outboard or easy trailering.

    It would be much easier to just find a 24' design that more closely satisfies your needs.
     
  8. boat fan
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    boat fan Senior Member

    Thank you PAR.

    This is what I need to know.

    Unfortunately there seems very little on offer as far as plans go ...
    some scow houseboats perhaps...

    When you say " you'll be quite disappointed with the results " ,
    are you referring to speed , handling , fuel economy or other , or all the above ?

    I must admit the increased loads worry me more than a little.

    So whats " out there " for a modest self build houseboat / pontoon boat 25 - 30 ft if anything....?
     
  9. liki
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    liki Senior Member

  10. boat fan
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    boat fan Senior Member

  11. Guest625101138

    Guest625101138 Previous Member

    There is nothing wrong with hard chines. A lot is written about the fundamental advantage of a half circle hull form to get the minimum wetted surface but this is only part of the story.

    When you start to get into wave making it is the slenderness that counts. So a section that produces large volume for beam is advantageous.

    My view is the best shape for low power at modest speed is a stabilised monohull. It takes this Bolger concept to the extreme. The outboard hulls normally sit above the water level and only come into play to provide stability. You can get very easily driven hulls.

    If you set some design criteria such as length, beam and target load carrying plus accommodation I can give you a hull shape to think about.

    Here are a couple along these lines:
    http://www.boatdesign.net/gallery/showphoto.php/photo/15019/size/big/cat//ppuser/18624
    http://www.boatdesign.net/gallery/showphoto.php/photo/14408/size/big/cat//ppuser/18624

    Rick W
     
  12. PAR
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    PAR Yacht Designer/Builder

    You may like my RYD-23.2 design. It's 26' on deck, 7' beam and does well with modest outboard power. You can see several of the configurations for this hull. It has twin skegs aft to permit her to take to ground bolt upright and the engine lives in an enclosed well out of sight, no smell and much reduced noise.

    Or you may consider a larger version of the same idea. RYD-29.6 is available in three beams 7', 8' and 9'. The 7' (a) and 9' (c) versions are shown. The 7' version is very efficient and requires little power to get to hull speed. The 9' version is a bit more decadent, but has considerably more internal volume (elbow room).

    All are hard chine, simply built and will make fine power cruisers.
     

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  13. wardd
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    wardd Senior Member

    par do you have a web site?
     
  14. messabout
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    messabout Senior Member

    Par; Nice presentation of a superbly practical boat. I reckon that the smaller one would prosper with modest (spell economical) motive force. It would be near perfect for cruising the Harris Chain, the St. Johns from Sanford to Jacksonville, and other liesurely cruising. She could do a mile in ten minutes with almost no fuss, wake, or noise. Ahh, Paradise. To hell with Donzis, metalflake bassers and jet skis.
     

  15. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Nope, I haven't gotten around to a web site. Hell, I'm lucky to get the grass cut before I get yelled at. Drop me an email if interested.

    The narrow version of 29.6 is an especially efficient shape. Long drawn out lines make for easy motion, no wakes and small fuel bills. It's a little big for most folks to feel comfortable with any more, but it's a size that was once very common for commuting and short trips. You do have a lack of elbow room for all the efficiency, but everything is a trade off. This would be a good hybrid or electric power craft.

    RYD 23.2 is a little more bang for the buck. It's a size most can be comfortable with, can tow with a mid size car and will not force you to get a second mortgage to pay off the building materials.

    The wide version of 29.6 is an attractive riverboat. There's a model of this one under construction by a fellow in North Carolina, that I can't wait to see. It would make a nice bachelor pad.
     
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