Swain BS_36 Stability curve

Discussion in 'Stability' started by junk2lee, Mar 9, 2011.

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  1. RAraujo
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    RAraujo Senior Member - Naval Architect

    Sorry Ad Hoc but I didn't got your point...

    Of course if you move transversally a weight the GZ curve will vary and eventually you get a GZ value smaller than zero at a given angle (if that angle is smaller than the new equilibrium angle after shifting)...
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    The point is, numbers are just that, we can all cherry pick figures to say what we want. Anyone can do that...

    Mike summed it up very well:

    So, just posting numbers and a graph as "fact", is pure fantasy. The numbers must be verified as being correct...yet no data is presented to provide anyone with the means to independently say... this is correct. There is insufficient data...and that is the point. The point of the post was to get people to "accept" blindly, just as the poster did, this give the impression, all is ok.

    Especially when the person posting them just "accepts" them as facts, but then says he doesn't understand them anyway.
     
  3. junk2lee
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    junk2lee Junior Member

    Wiki leaks.
    I better look out for a warrant from Sweden...
     
  4. Tad
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    Tad Boat Designer

    As it happens I'll be speaking on the subject of "Vessel Stability: Myths and Reality" on March 19th.....http://maritime-seminars.shipyardraid.ca/

    The information presented in the first post in this thread fits my subject perfectly.....so I think I'll work up a model (from Brent's drawings) and some realistic numbers over the next week or so........should be interesting......
     
  5. pdwiley
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    pdwiley Senior Member

    Slight difference. Wikileaks (mostly) posts facts with data to confirm them.

    What you're doing is more akin to photoshopping someone's head onto someone else's body then posting the image on line.

    I actually think that Brent's 36' boat is pretty clever for someone with zero formal training but tactics like these just make it, him and you look worse. It makes me suspect that there really is something badly wrong with the stability curve, not that all is sweet. Congratulations.

    PDW
     
  6. pdwiley
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    pdwiley Senior Member

    Please do. As someone who seriously considered building one of these boats, I'd love to see the theoretical stability curve. There are posts on the Yahoo origami forum that say the as-built weight is over 20,000 lbs however not Brent's calculated displacement, not sure how this would affect anything.

    PDW
     
  7. wheels
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    wheels Junior Member

    as built weight is EVERYTHING. you can run all the theoretical numbers you want through a program to get your curve. But what you end up with is what really matters. I know I don't have a true grasp on how exactly it works but I do know that if I designed a roof to withstand a 3 foot snow load and then dump 5 feet on it that it may not perform as calculated. boat weight and stability are the same concept. If the mass is not in the right place you can toss the curve you have on paper out the window.

    I don't really know anything about brent's boats other than what I've read on forums like these and yes he does have innovative designs, but before I would build one I would want to be absolutely sure that what I build is going to match the numbers on paper. Either learning how to run the numbers myself or having them gone over by someone with the training to evaluate it properly.
     
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  8. Tad
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    Tad Boat Designer

    In the absence of an inclining experiment and flotation measurements on an actual boat, the curves I create will indeed be theoretical......But they will be run at various displacements (load conditions) and VCG heights thus give a general outline of the design's stability characteristics, if not exact data.

    So I'll do a reasonable weight study, collate it in a spreadsheet, and publish that here for all to comment on and change if they wish. Hopefully someone will actually measure a real boat and publish believable and understandable info.

    In the first post in this thread we are presented with a stability curve indicating displacement of 8.256 tonnes (18,200 pounds) and a CoG of .88m......Is this above waterline, above baseline, above keel? We're not told what the load condition is (tanks full, empty, crew on or off, stores?) or where the VCG is, or how the displacement was arrived at.....so the basis data can't be examined at all........

    Wheels......
    Your comments address the exact question I'm interested in addressing and trying to educate the boat using public on. There is always a difference between what the designers published curves show and the reality of the boat you built....For instance I have seen Swain designs with wooden (various species, some hollow some solid) aluminum, and steel spars....this alone makes a huge difference in the ultimate stability of the boat.....so lets look at what the real effect is......
     
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  9. Ad Hoc
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    Ad Hoc Naval Architect

    Exactly.

    All those designs of "...you can buy what you like and place it where you like, I buy mine from a junk yard not expensive suppliers...etc etc"...this mantra means that without doing an inclining expt after finish and launch, one has no idea, NO IDEA where the LCG, VCG is nor the final lightship displacement. Without these data, it is impossible to say with 100% certianty what the stability of the vessel is like. As such, going to sea in a vessel with no realiable stability data is dangerous, extremely dangerous.
     
  10. Tad
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    Tad Boat Designer

    ad hoc,

    Like every human endeavor there is some risk in all we do, sailing a small vessel offshore on a long passage is indeed somewhat risky......personally I think it's safer (lower risk) than driving a car to the corner store.......it's probably safer than flying in a passenger jet airplane........There is no epidemic of small steel cruising boats turning over and not righting themselves.....so we conclude the risk is minimal....until a freak storm catches a bunch of boats unawares and a major disaster (in the small world of cruising boats) develops.....the trick is not to be complacent or unaware of the reality....This is what I'm trying to do by creating a public study of a particular boat's real stability........

    I did the same thing on a passagemaker (power) list a while back with a Diesel Duck and a Nordhavn.....both are touted as the ultimate offshore vessel, I found they both ran out of transverse stability at something over 80 degrees heel......reality........

    To the project at hand.....a puzzle.......I have a collection of drawings of the Swain 36, reported to be 35'6" overall, various waterline lengths are given in various spots, one drawing states 31'9", scaling the same drawing gives me 30'6", another drawing states 30'0"......Beam is given in several places as 10'6" and draft as 5'10" so we'll take those as golden.....? Obviously draft will change with loading......

    I have the construction drawings and recently received two drawings (plan and profile on one sheet and a separate body plan) purported to be the hull lines drawings for the Swain 36', they do not have the designers name on them or a single dimension......the lines bear little resemblance to the shapes presented in the construction drawing and do not match in the three views.........The lines also bear only passing resemblance to the completed boats I have seen and have pictures of.........a forensic puzzle....
     
  11. tazmann
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    tazmann Senior Member

    Tad
    Brent had me post some drawings from his book on here
    From the book on the 36'
    LOA 35'5"
    Beam 10'6"
    Displacement 17,280 lbs
    Keel weight 5700 lbs
    LWL 30'0"
    Draft 5'10" assuming that is the single keel

    I can't verify this but it is in the book
    Tom
     
  12. tazmann
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    tazmann Senior Member

  13. Ad Hoc
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    Ad Hoc Naval Architect

    Tad

    I agree 100%. Mitigating such risks allows us to feel more comfortable and "safer" in the endeavour one is doing. Wherein lies the problem...how can one mitigate such risks in the absence of any real data?

    I come from the commercial world of design, not leisure. In the commercial world, verification/validation/corroboration of data is everything. No matter how obvious something may be, we are driven to support such data by other means and have it checked independently.

    Small steel bath tub of boats are "probably" ok....but i would not put my name to it and state categorically it is 100% safe until I run the numbers and provide the range of safe loading conditions, from known data.

    Good luck on your efforts. Unless you are close to someone with a BS 36" who is willing to allow you to perform a proper Inc. Expt. and take measurements of the final hull shape (since all are pulled in various directions), i fear you'll never be able to categorically state the boat is XXX. But very well worth doing :)
     
  14. Jack Hickson
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    Jack Hickson New Member

    I saw a very similar stability curve for a Contessa 32, a very similar deadrise, and hull shape. I suspect most boats of that era have a very similar stability curve.
    Its a huge distance from a plank on edge, to a super beamy flush decker, with a lot of possibilities and stability curves between them. Its not either -or.
    I hear Dudly Dix is now giving different stability curves for boats with pilot houses and those without, taking into account the huge effect of the buoyancy in a pilot house on stability
     

  15. Jack Hickson
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    Jack Hickson New Member

    There is a huge difference in what any two owners will put aboard a boat, something that no designer can predict, which makes micro calculations,and measurements , meaningless, in the real world.
    A steel mast, sealed, will float which makes it a positive asset in ultimate stability. A steel scuba tank, empty, with far thicker wall will float .
     
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