Rocker in straight sections

Discussion in 'Multihulls' started by nzdavo, Feb 1, 2015.

  1. nzdavo
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    nzdavo Junior Member

    Hello all

    I'd like to ask a question but need to start with a disclaimer!
    I'm an industrial designer - so keen on all things design but completely new to the idea of boat design!

    I was sketching some hulls recently and wondered why the minimal rocker on a crusing catamaran can't be made in straight sections? Similar to how round bilge is approximated with flat panels.

    If you were home building this could be a fast/easy way to build with flat panels. (I do understand that building the hull is only a small percentage of total build time.)

    I downloaded Delftship and modified a cat hull in the library to test it out. I know it is terribly bad to use a tool you don't understand to do a job that you also don't understand but I was suprised by the result.

    Please see images attached:
    I modified the hull so that the rocker is approximated with a sharp joint in the middle where the panels meet. This made virtually no difference to the resistance report.

    I'm pretty sure there is a catch but I wonder if straight sections will have a very negative effect on a cruiser? (Not a racer)

    Thanks :)
    David
     

    Attached Files:

  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course you can build a boat that consists of nothing but flat sections, like the facets on a diamond, but the discontinuities created may not be optimal functionally, or aesthetically, or even make for an easier build, with more joins than might otherwise be needed
     
  3. redreuben
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    redreuben redreuben

    Flat panels usually don't have any trouble taking the shape of the long curve that is rocker so I don't see why you would bother ?
     
  4. Milehog
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    Milehog Clever Quip

    Also a curved panel is stronger than a flat one.
     
  5. upchurchmr
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    upchurchmr Senior Member

    You always need to know if the program you are using accurately predicts real world results.

    Most will have specific weaknesses, your question is not something that would normally be assumed as desirable. The program may not accurately predict actual drag.

    I have no data on the programs you used.
     
  6. rapscallion
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    rapscallion Senior Member

    There are "stepped" hulls very similar to the one you describe, which work well when the hull is traveling at a high speed, like a power boat going full throttle for example. However, "steps" like the one you describe create extra drag at lower speeds, and often "rounded" hulls are chosen as a result.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    As others have said already, don't put too much faith in those resistance curves.

    No, don't build a boat like that! again for the reasons mentioned. I'd also say that the creased hull you show has much less buoyancy aft than the fair hull. That will be bad for loading and, guessing, your Cp will be lower, which means relatively more drag at higher speeds

    Stepped hulls are only used for very high Fn, like on floatplane floats for example. So unsuitable for most sailing boats.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. rcnesneg
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    rcnesneg Senior Member

    I don't like the second one. I say go with the first one that is rounded. The bottom one reminds me of the weird racing canoes.(see pointy sides on the canoe on the left)

    [​IMG]
     
  9. nzdavo
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    nzdavo Junior Member

    Thank you all for your comments.

    I realise the frustration of a newbie trying to solve problems with little knowledge!

    I'm tempted to ask more about approximating rocker with straight sections but I'll resist and let the idea sink :)

    Thanks again
    David
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    All flat panels seems to excite military thinkers wanting stealth capability in naval vessels, otherwise it seems not to have much application.
     
  11. champ0815
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    champ0815 Senior Member

    As for the weird racing canoes: Iirc, their design is based on complying to a minimum width rule, so they built boats with a minimal waterline width and a width to the rule at a position (way above waterline) where it is least cumbersome for biomechanics.
     
  12. upchurchmr
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    upchurchmr Senior Member

    Flat panels on military naval vessels are on the top, not the bottom.
    They simplify the calculation of Radar Cross Section or how much radar is reflected back to an enemy. I don't think you need that.

    Nzdavo,

    You might as well ask. You really need to get it out of your system or it will slow you down in understanding good boats.

    Besides, as others say it will be harder to make the shape you showed than a long plywood panel just bent to shape. Take a look at the W17 tri or Woods Strike 18 as an example. You are just breaking the panels in the wrong direction.
     
  13. nzdavo
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    nzdavo Junior Member

    Hi again

    I think Upchurchmr is right - I probably need to get it out of my system!

    Here was my reasoning:
    My son and I were folding paper boats and I thought I invented the KSS system. (Bit of internet searching showed I was a few decades late!)
    But it got me thinking about 'prefinished' panels. The problem, like paper, is getting compound form from them. I found that there are plenty of outfits that supply finished fiberglass panel 3meters wide in rolls up to 200 meters long. (Gel coat applied and up to 6mm thick panel)

    Rolling this up like a U produces the round bilge but then you have straight sections to contend with. I wondered if you could still get reasonable performace using straight sections.
    But you do have a joining issue at the end of each section - so perhaps these areas could be subjected to some fairing when glassed together. (see pink areas on image attached) This would get rid of the sharp joints.

    Would an approximated rocker with faired joints be reasonable?

    The thought would be that you set up formers/bulkheads and drape a long section of panel over it up to each 'joint' where you connect it to the next long section. A foam or similar core is then glued to the panel. Now an outer panel is glued to the foam.

    There would be minimal finishing to do - only at the faired joints.

    Hope these pictures explain it!

    Cheers
    David
     

    Attached Files:

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, for starters, bending a prefabricated fibreglass panel to such a tight radius will be a tough job, unless it is very thin material.
     

  15. rapscallion
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    rapscallion Senior Member

    Harry Proa builds boat hulls similar to the way the OP proposed; Rob simply bends a flat panel to a semi circle and adds ends. His hulls have zero rocker however.

    Basically, what is proposed is possible, but isn't done due to the fact that stepped hulls are slow on displacement boats. If you are going to use a chine, you are better off having the chine parallel to the flow of water vs perpendicular to it.
     
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