Origami steel yacht construction

Discussion in 'Metal Boat Building' started by origamiboats, Nov 30, 2001.

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  1. Tad
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    Tad Boat Designer

    One of the reasons this site exists is to try and dispel marketing hype and inject some reality into design discussions. Designers have experience with real boats, we know they are typically overloaded, we also know the VCG is too high to have positive stability at anything like 175 degrees. Thus we are saying, do an inclining and establish an actual VCG......With "hundreds" of Swain boats out sailing surely one can spare a few hours for a test?

    I assure you stability is not moot. I recently wrote a stability booklet for a active ocean cruising 54' steel Bruce Roberts cutter. The owner was under the impression that he had positive stability to 115 degrees. Actually, under certain load conditions, he runs out of positive stability at 87 degrees. He does not have that 28 degree cushion he thought he had.....so he'll be reefing a bit earlier and paying more attention to squalls.......rather than moot, this hard data is valuable information aiding in the safe operation of a large (passenger carrying) sailing vessel.

    Ignoring stability and displacement just because home builders overbuild and overload is irresponsible.......again, with boats launched a NA can easily do an inclining and establish real VCG and displacement, and from them calculate real stability........"positive stability to 175 degrees" is not real, and it is dangerous to perpetuate this myth.....
     
  2. Wynand N
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    Wynand N Retired Steelboatbuilder

    and here are some example of that - two pages from a few I posted on a mast design post elsewhere.
    The stability curve shown is actually a very high righting moment boat and still very far from the mystic 175 degree BS rant.

    BTW, the first page actually shows the dynamics of a mast and one of the reasons why sufficient scantlings has to added to a "frameless" boat. The pull on the chainplates are massive and the mast compression can exceed the displacement of the boat easily under certain conditions and the keel support has to be adequate at least - and so the structure at the chainplates.
     

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  3. junk2lee
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    junk2lee Junior Member

    the picture in Wynand's post is what I was thinking...I guess Tad's plumbbob would have to be from the masthead to measure a mere 3 degrees!or some kind of laser/digital unit...man,it'd have to be calm alright!

    Still,given a rough sketch I did with a moderate beam and a trunk cabin sealed up and finally a lead ballasted keel(or 2?),it still seems to me an angle of 185 or 195 might right them as Brent described in his modelling experiment-Brent's contention being that the trunk cabin becomes part of the buoyancy.The sketch in Wynands picture (is only illustrative sketch,I know)has no cabin and likely rings Brent's bell just looking at it....Anyways,roughly speaking,a cg above cb is not stable either.The only thing that KEEPS the boat upside down is the centre buoyancy moving outboard on the inverted boat....

    Hopefully,people button up their hatches before heeling to 90 becomes the norm!
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    The trunk won't have any effect before ~90 deg heel and thats about the same as downflooding angle and where the rightening moment of the ballast starts to decline..
    And as stated before "seems" and "might" aren't excactly hard facts ;)
    The "good news" considering the forces (from the rigging down to the hull) aren't so great after all if the concern about the stability is true :p
     
  5. MikeJohns
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    MikeJohns Senior Member


    The CoG of Brent’s boats will always be above the CoB whether it’s upside down or the right way up.

    The static GZ curve is a good guide to the resistance to being rolled over by a large beam breaking wave. It’s been shown by people like Cloughton and Renilson from work in wave tanks to be most effected by the ‘reserve’ stability past 90 degrees. Most significantly the area under the curve from 90 to the limit of positive stability. It’s a dynamic event and relates to the energy or the area bounded by the curve from 90 degrees heel to LPS. The height of a ‘significant’ wave is relative to the boat size.

    The Roberts Tad was referring to is a very unsafe boat in heavy weather. This is the sort of stability problem you see when the decks and deck houses are made of thicker material than the designer specified. It can be done but needs to be accompanied with a deeper keel.

    Downflooding isn’t really significant in a survival situation since it should be fully battened down. The deck house, coach house, wheelhouse are all allowed in the stability curve if properly constructed. They do make a big difference.
     
  6. Brent Swain
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    Brent Swain Member

    The compression loads on the mast support can never exceed the tensile strength of the rigging. Once that point is reached, the rigging breaks,, and the load disappears, period.
    A trunk cabin starts to add to stability as soon as the side decks go under and the cabin sides start to submerge.
    I have put my spreaders in the water , as have some 36 footers . All came back up quickly.
    Calculations based on a hull ,without the buoyancy of the cabin and wheelhouse, are a deliberate attempt to deceive.
    I just found that quote from Herreschoff I mentions .

    " However, as time went on and the mathematician began to measure displacement he evolved very complicated rules , for as I have said before he was never a man of much common sense, but rather a perfectionist, so that such rules as Tchibyscheff's rule and Simpson's first and second rules began to appear as well as the so called trapezoidal rule . All these were duck soup to the college professors , for they are mostly exhibitionists and want to get up in front of the class with something real complicated . If someone asks them why they teach the most complicated and involved methods of doing things they will in variably answer, "Training for the mind , young man , training for the mind", as if discouragement instead of enthusiasm were the secret of teaching. I suppose that if these professors were to train a ballet dancer, they would first buckle the lead shoes of a diver to her and say "Now , young lady , train yourself." But they would only break her heart ,as they now do with much of their best material, such as those with an artistic instinct and a sense of proportion.
    Measuring of displacement is really a very simple problem and amounts to nothing but a little mensuration, addition and simple multiplication You simply measure the area of the sections which are below the water, add them together , and multiply by their spacing."
    Herreschoff. The Common Sense of Yacht Design.
     
  7. Brent Swain
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    Brent Swain Member

    It’s been shown by people like Cloughton and Renilson from work in wave tanks to be most effected by the ‘reserve’ stability past 90 degrees.



    Downflooding isn’t really significant in a survival situation since it should be fully battened down. The deck house, coach house, wheelhouse are all allowed in the stability curve if properly constructed. They do make a big difference.[/QUOTE]

    Despite the math, it took a model in a wave tank to tell the real story. That is why I prefer a model for ultimate stability conclusions.
    Down flooding is why I prefer watertight aluminium doors, to the sliding hatch and drop board arrangements, which are impossible to get completely watertight. They seal like the lid on a pressure cooker, something that is impossible with sliding hatches. They are also easier to get in and out of, as long as they have adequate slope to them..
     
  8. Brent Swain
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    Brent Swain Member

    4 successful origami boats up to 55 feet have been built and cruised for many years and tend of thousands of open ocean miles in all weathers.
    5 , the weight would be about the same , but with the weight in skin thickness rather than frames, you have wider margin for corrosion , pounding on sharp rocks and far less potential for distortion.
    6 Math is important , but doesn't over rule experience and common sense. When math disagrees with experience , the math is wrong.
    I can get a shell tacked together and lockable in a week or two, then lock the tools inside, less time than it takes to build a shed.. I use black plastic poly pipe for covered wagon hoops ,taped to the stanchions, with a poly tarp over it, once the lifelines are on. I take one side out to some tie points to make a sheltered place alongside for on the ground projects.
    Be careful, some not too deeply coloured tarps can have a greenhouse effect in sun. The darker tarps are sometimes better. A white, epoxy compatible primer on the hull, in the sun is like adding air conditioning on a steel hull. The difference is huge. If you plan to sandblast then any free white paint from the recyclers will do.
    Working in rainy BC mostly in winter, I have never been held up for more than a few days , when I really needed the rest.
    Why spend a lot on a Walker Bay, when we are awash with solid used dinghies, for a fraction the price.
     
  9. Brent Swain
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    Brent Swain Member

    Buy a set of plans and find out
     
  10. Brent Swain
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    Brent Swain Member

    When people ask me how much a boat will cost I always ask them "How much do you have? That is what she will cost you. If you have little money and are prepared to scrounge and be resourceful, you will probably get her sailing for little. If you have a lot of money , you will probably a spend a lot."
    I have never told anyone that they can hire others to do all the work send money in the mail, for hours alleged by someone a thousand miles away, buy all new gear and materials, etc etc and end up with a reasonably priced boat.
    I tell people how to build some of the most reliable blocks available any where ,for any price, for under $2 each, as well as anchor winch , roller furlers , windvane, etc . When they say they are looking at new , expensive, manufactured blocks , I can conclude they are doing the same for the rest of the boat, and the result will be anything but cheap.
    I can only advise them . What they do is something I have no control over, and I'm thus not responsible for. If all else fails, then follow the directions. Ski out of bounds and you are on your own.
    While some have criticized my structural details, no one has suggested an alternative which won't involve raising the main water tank several feet, or make cleaning it impossible.
    What I've drawn , works, and is structurally sound, as many collisions with rocks has proven over the years. When math, and decades, and the combined experience of hundreds of thousands of miles of offshore cruising, severe groundings, etc, disagree, then the math is wrong.
    No bigger waste of time trying to prove that which has worked well for decades "Won't work" That is Bernie Madeoff style creative accounting, which the sea doesn't recognize.

    You are 30 years to late to make that argument.
     
  11. MikeJohns
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    MikeJohns Senior Member

    Don't get confused Brent
    The mathematicians are quite separate to the Naval Architects. NA's have evolved simpler ways of doing things that the mathematicians don't hold to.

    A pure mathematician want's everything defined and predictable, but the reality of hydrodynamics is that it's not fully predictable. If you want an analogy look at the difference between chemistry and physics. A pure physicist argues that chemistry is just applied physics, but the maths does not predict the compound formed.

    Displacment is the click of a mouse button these days.

    Models in wave tanks are necessary since the dynamics of breaking wave and hull interactions are only able to be modeled on such observations.

    Stability with models are valid if you meet the criteria that we outlined before, something you haven't achieved.

    It's very important that you fully and properly investigate stability. It has a very serious bearing on the suitability of vessels for theaters of operation and survival tactics.

    Above all your boats cannot have stability to 175 degrees that's clearly not possible and you mislead yourself with your own model. Don't continue to mislead others.

    So how about we talk you through determining the CoG for your boat, then I'll give you a GZ curve if you'll give me the lines.
     
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  12. junk2lee
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    junk2lee Junior Member

    (quote is partial)

    Really!how do you figure?
     
  13. bearflag
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    bearflag Inventor/Fabricator

    Last edited: Aug 13, 2010
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    It's something obvious for anyone ever calculated the CG.. Take a moment and test it yourself in a simple manner. Draw a midship section and place all the main components ; keel, engine, tanks, interior, gear, decking, portlights, stanchions, dinghy, rigging, sails etc (and hull of course) their weights and place them accordingly on a vertical segmented line on the side of the drawing. You can now aproximate the CG either geometrically (determing the common cg's like with sailplan ce's) or count momentum for each component separately and take their avarage..
     

  15. welder/fitter
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    welder/fitter Senior Member

    :rolleyes: I really should know better, by now.:rolleyes:
     
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