Swain BS_36 Stability curve

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

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  1. Jack Hickson
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    Jack Hickson New Member

    How does heeling a boat a few degrees take into account the 10,000 lbs of buoyancy, and righting effect , of the cabin and wheelhouse?
    Tad knows full well that two BS 36 's have been moored in Silva bay most of the winter, as well as another in Degnen Bay, unless he has little to do with boats in the area..
    Still doesn't deal with the above point.
    Jim's stability curve in post one is still the most accurate. He did one without the cabin , minus the 5 inches of extra freeboard of newer 36's, which was still a higher AVS than Tad's 125 degrees.
    It's hard to picture the 36, with it's mast only 35 degrees below horizontal and its 5700 lb keel out of the water , its mast with over 4,000 foot pounds of righting moment , capsizing. One would have to be self delusional to believe that.
    While a single keeler of the same draft as a twin keeler will have the same righting moment , the spreading out athwartships of the ballast will give her a slower roll period, proving that roll period proves nothing.
    As the keel is the center of rotation, twin keels give a boat two centers of rotation, drastically reducing the roll period. This is why cruise ships find them so useful. They have no effect on stability, but do slow the roll period.

    Go check post 141. Tad's calculations. Check the debate leading up to it.
     
  2. pdwiley
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    pdwiley Senior Member

    Now that I'll agree with. If the boat I showed had even had the transverse frames properly welded to the hull plate then the denting would have been far less, as the frame distorted off the plate allowing the plate to upset. I have welded my frames to minimise the chance of this happening.

    I don't think the doubler plate is the most efficient use of extra steel, however. If one added the same amount of steel at the bisected angle of the chine then much greater resistance to distortion would be gained. It would also be a potential rust trap so some strategic holes would be needed to allow any water to drain to the bilges. I am still contemplating this one.

    PDW
     
  3. pdwiley
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    pdwiley Senior Member

    Except that the mast does not have over 4000 ft-lbs of righting moment as has been shown here. One would have to be self-delusional to believe that.

    If you think differently, 'Jack', post your calculations on the mast righting moment or shut up about the mast righting moment, pick one.

    PDW
     
    Last edited: Apr 18, 2011
  4. MikeJohns
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    MikeJohns Senior Member


    Brent, you are confused !
    An inclining test has nothing to do with a roll period test, it's an accurate way of determining the Vertical Center of Mass of the boat, AKA Vertical Center of Gravity.
    The stability curve is then derived from a 3D model. It's the most accurate way of generating a GZ curve. You have had this pointed out to you in detail numerous times by now and you continue to get it confused.

    Also a single or twin keeled boat doesn't roll about the keel.
    Twin keels don't generally increase the roll inertia except for the entrained water between the keels. The roll inertia is governed by the sum of the masses and the square of the distances from the roll axis.

    Tad's post here [http://www.boatdesign.net/forums/stability/swain-bs_36-stability-curve-37070-10.html#post453543]] is another set of issues. Conventionally you don't include the masts volume as beneficial in a GZ curve.
    You also need to look past the imagined extent of the limit of positive stability and realise that the curve is the righting arm (GZ) curve. To get the righting moment multiply it by the displacement. And that righting moment is low whether the mast is included or not.

    The stability curve in post 1 is simply rubbish. It's not at all accurate. Poor Jim has also muddled the units, maybe you can say what units was he trying to work with? presumably we should read m as ft ? That's both the righting arm and the metacentric height .........meters or feet?
     
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  5. Tad
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    Tad Boat Designer

    There names please? Can you point them out in this picture of Silva Bay taken from my office window? As far as I know the boat on the mud in Degnen Bay is incomplete and thus useless for inclining purposes.....

    [​IMG]
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    Maybe.. but it's for a wrong (=imaginary) boat, not BS 36. Seriously, no boat with about the same dimensions as BS 36 has never GZ at any heeling angle of 1m, not even close..
     
  7. Lister

    Lister Previous Member

    You know Tad if I was wealthy (and younger) I will hire you to design my next ship, to be able to spend time in your office looking through the window at the flying boat moored on the pontoon :)
    Lister
     
  8. Tad
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    Tad Boat Designer

    :p.......damm flying boat makes a tremendous racket about 6 times a day........
     
  9. HReeve
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    HReeve Junior Member

    Maybe they are two of the three squatters that have been removed in the past months. 3 down, 20+ to go.
     
  10. Wynand N
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    Wynand N Retired Steelboatbuilder

    Winners know when to stop....apparently losers not:rolleyes:

    When will BS stop chasing his own tail. This fabled claim has been beaten to death in the past, and this postmortem proved that the body is indeed dead dead and still old BS Jack wants to make it run:confused:

    One question BS Jack; would you say that since the majority of boats in the world designed by qualified naval architects and boat designers and who's stability curves does not even come to your fabled 175 degree inverse stability claim, are incapable of designing proper and stable boats?
    Or are you the only person that have such powers and all the others are dumb?

    Your answer would be interesting if you have the balls to answer on the question.

    Your design(?) is limited by the shape of the plates used and rooted in the length and width of plates per side and the result is of course that silly narrow hull that takes shape from that. No way can you predict the actual shape if you have to do a new "design" - the result is only available after the plates are slit and forced together. This is also the very reason there are no lines per say for any of your boats. And lines are the essence of any hull design....
    With basic lines you can do preliminary hydrostatics and also a static stability curve - all this before the steel is ordered!. If some of the data eg; displacement or stability curve issues are in question or not to the design brief, a few changes to the lines and new calculations again until its OK.
    As said, you have no control over your design and the result is basically a hit and run affair. And that bring me back to the question posed to you.

    Since most designers basically uses the same method I described for preliminary design study doing a new boat, and can control the stability of the hull designed, why would all their hulls not even come close to your wet finger in the wind guessing method and fabled 175 degree claim:?:
     
  11. Jack Hickson
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    Jack Hickson New Member

    Wynand
    Most designers design 36 footers with far more than 10ft 6 inch beam. Few have wheelhouses. As Marchage points out, that is the main source of ultimate stability problems, along with flush decks.. If you check older boats with far more moderate beam, and relatively higher trunk cabins, you will find that an AVS of around 175 degrees was far more common. You will never get that on a 36 with 12 foot beam and a flush deck.
    As Angus primrose pointed out ,a few inches reduction in beam gives you a huge improvement in self righting ability. 18 inches on a 36 is a huge reduction in beam .
    Tad's post 141 shows an AVS of over 160 degrees. How does that compare with a Farr38 at around 130 degrees and most newer boats with a similar AVS? Over 160 is not good enough , but 130 is?
    The difference ? Excessive beam, and lack of cabin and wheelhouse buoyancy. .
    The shape of the plate edges determines the shape of the hull, absolutely. You could weld up a hundred hulls, and as long as the plates which make them up are all identical, the hulls would all be identical. I have measured several ,and all were identical, within 1 /16th of an inch .That is a simple fact of geometry. It is impossible for them to be anything but identical, just as it is geometrically impossible for 6 absolutely square plates, all joined corner to corner to be anything but square. The same rule applies to more complex shapes.
    There were two BS 36's in Silva Bay most of the winter, visible from Tad's front window. The one in Degnen is as finished as she will get, the owner considers her finished.. She is a twin keeler. For a single keeler ,there is one in Ladysmith Maritime Society's dock, if she hasn't left for the summer ,and another at the Nanaimo Yacht club dock.
     
  12. Jack Hickson
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    Jack Hickson New Member

    You can go buy a beer in the pub there and get the same view. Much cheaper.
     
  13. Jack Hickson
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    Jack Hickson New Member

    Take away the weight of the mast from its volume, and you have it's remaining buoyancy.
    Center that halfway along its length (24 feet) and you have the leverage of the buoyancy on the hull in ft lbs.
    Simple
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    The internet is great leveller pointing out inaccuracies in peoples myopic view of boats. A simple search of 36footers yields more than throw away statements, but actual facts.

    So, do these yachts with more than 10’6” beam have cockpits/wheelhouses?

    Beam 11.1 with cockpit/wheelhouse
    http://www.boatquest.com/Sail/Mumm/Category/Length/13932/Feet/USD/1/boats.aspx#Specification

    And another with 11.6 beam and yup cockpit/wheelhouse
    http://www.boatquest.com/Sail/Morgan/Category/Length/35490/Feet/USD/1/boats.aspx#Specification

    another with 12ft beam and even bigger cockpit/wheelhouse
    http://www.boatquest.com/Sail/Gozzard/Category/Length/39641/Feet/USD/1/boats.aspx#Specification

    or this one with 12 ft beam and cockpit/wheelhouse
    http://www.boatquest.com/Sail/Spirit/Category/Length/90944/Feet/USD/1/boats.aspx#Specification

    or this 11.11 beam with cockpit/wheelhouse
    http://www.boatquest.com/Sail/Catalina/Category/Length/128497/Feet/USD/1/boats.aspx#Specification

    and another:
    http://www.boatquest.com/Sail/Dehler/Category/Length/125466/Feet/USD/1/boats.aspx

    and another:
    http://www.boatquest.com/Sail/Hunter/Category/Length/129387/Feet/USD/1/boats.aspx#Specification

    the list is endless…..and yes, they do have them, despite protestations and diversion from BS that they don’t.


    So what about older boats, here’s one 30 years old:
    http://www.boatquest.com/Sail/Hunter/Category/Length/140698/Feet/USD/1/boats.aspx#Specification

    Or this one nearly 40 years old with 11’6” beam
    http://www.boatquest.com/Sail/Morgan/Category/Length/35490/Feet/USD/1/boats.aspx#Specification

    or this 30 year old with 11 foot beam
    http://www.boatquest.com/Sail/Hunter/Category/Length/140698/Feet/USD/1/boats.aspx#Specification

    and this 30 year old at 11.11 beam
    http://www.boatquest.com/Sail/S-2/Category/Length/144686/Feet/USD/1/boats.aspx#Specification

    and so on…

    Wait..all those older boats are greater than 10’6”, in fact they are the same as those new boats all basically have the same beam.

    So we have older boats, with cockpits/wheelhouses and new ones too, all with roughly the same beam.

    Your point is what?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks BS I have done that. The internet is great, isn’t it? :p

    So let us have a look at the mast, is it sealed as you say, well, let’s look at one of the origami sites building your boats:

    http://origamiboats.webs.com/apps/photos/photo?photoid=15971946

    Oh..look, it is not sealed….huh???

    But wait you say..

    Here, look, this picture shows the opposite...
    Mast from Origami site.jpg

    So, the mast is not sealed and those look like halyards or wires run inside the mast.....they followed your exact guidance, are they wrong?...care to explain?
     
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