Swain BS_36 Stability curve

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

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  1. junk2lee
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    junk2lee Junior Member

    Just some stuff I've been looking at lately.Grist for mills...
    Swain BS_36 Stability curve...I assume data derived from"additional"?

    Some additional information:

    Estimated displacement D : 8.256 tonnes
    Z coordinate of CoG : 0.880 m
    Initial transverse metacentric height ho : 1.600 m
    Maximal GZ : 1.094 m
    Heeling angle for GZ maximum : 80.0 degr
    Heeling angle at which righting lever=0 again : 180.0 degr
    Maximum dynamic heeling angle : 120.0 degr

    Swain GZcurve.jpg

    KN sin(Psi)
    0.0° 0.000
    2.0° 0.087
    5.0° 0.217
    10.0° 0.428
    15.0° 0.627
    20.0° 0.816
    25.0° 0.995
    30.0° 1.162
    35.0° 1.313
    40.0° 1.445
    45.0° 1.557
    50.0° 1.651
    55.0° 1.728
    60.0° 1.789
    65.0° 1.838
    70.0° 1.882
    75.0° 1.930
    80.0° 1.960
    85.0° 1.951
    90.0° 1.924
    95.0° 1.882
    100.0° 1.825
    105.0° 1.757
    110.0° 1.677
    115.0° 1.585
    120.0° 1.482[
    125.0° 1.369
    130.0° 1.249
    135.0° 1.123
    140.0° 0.995
    145.0° 0.864
    150.0° 0.731
    155.0° 0.595
    160.0° 0.455
    170.0° 0.185
    175.0° 0.079
    180.0° 0.000
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    And where this BS is from? Don't say Brent has calculated it.. ROFL
     
  3. junk2lee
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    junk2lee Junior Member

    Alright,I won't!:)(say, "Brent has calculated it") ...and the tenor of your comment informs me not to say who.But I do think whoever did it has the plans and the knowledge to model in 3d and took pains to be accurate.
    These are stability calculations for BS_36.Here they are.Enjoy.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Enjoy for sure.. Looks like it's a new revised model of BS_36, anyway that form stability curve is typical for "plank on edge" boat.. Wonder thou where to put those famous "structurally superior" pilge keels in a boat without pilge.. :p
     
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  5. MikeJohns
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    MikeJohns Senior Member

    What makes you think that? It's a common trap to be credulous because the output looks technical, but it doesn't make it accurate.

    The data presented is highly questionable, it looks very much like the cog figures have been simply chosen to match a desired stability curve.


    I'd like to see the CG calculation, I think you'll find that's simply wrong, look at the supposed GM of 1.6 m !

    An accurate and thorough weights and moments calc can come close to providing the CG providing the person conducting it has the experience to know what to include.

    Tad Roberts was offering to do an inclining test for Brent, that would be illustrative and it allows kG to be accurately derived.
     
  6. MikeJohns
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    MikeJohns Senior Member

    Teddy
    I agree, I think Whoever dished that up has no experience of naval architecture and it looks very fishy to me.
     
  7. Jack Hickson
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    Jack Hickson New Member

    Looks good to me. These boats are anything but plank on edge. Their modest beam and combination trunk cabin and pilothouse give a huge amount of ultimate stability.
     
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  8. MikeJohns
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    MikeJohns Senior Member

    For the viewers:

    Jack Hickson is Brent Swain.

    Brent if you really want to know you should take Tad up on his offer of an inclining test. You'll find that stability curve is junk. Anyone with a BS36 near Hobart can have the test done by me too for free.

    There's something very wrong with those figures. Send me your weights and moments calcs and I'll run it in a proper stability package free to trim and I'll check your CG.
     
  9. Willallison
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    Willallison Senior Member

    Sibqc...
     
    Last edited: Mar 9, 2011
  10. pdwiley
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    pdwiley Senior Member

    Haven't we been here before???

    Anyway the weight (displacement) is wrong. The only actual weights I've ever seen posted as per crane hook load cell were over 20,000 lbs for a 36' BS boat and I assume that was with empty tanks.

    Junk2lee, you must have *known* that nobody here was going to accept a calculation like that without all the math and assumptions used to derive it. Are you just trolling or what?

    PDW
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Sadly you are deluded.

    Given that for GZ = KN – KG.sin(theta)

    So, KG you say is:
    And KN is given by:
    So the GZ at
    2 degrees = 0.056m
    5.0 = 0.010m
    10. = -0.065m
    15 = -0.141m
    20 = -0.214m

    And so on….

    DOH:eek::eek:....only first value is correct...should read below (still not woken up):
     
  12. junk2lee
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    junk2lee Junior Member

    Hi,pdwiley.Not exactly deja vu but I, of course, recall the lack of 'em and I found some.....Boat Stability curves are not really my study,but here they are labeled "Swain36". They look similar to a keel sailboat,I think and haven't I seen similar things before that weren't accompanied by blow-by-blow diaries ...?

    I can't argue them.That's for the pundits.I'm just the newshound here.
    Oh well.Go ahead,attack the newspaper.Blame the messenger.sigh.:(
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Well, what do you expect when you stand behind the values posted:

    Yet you preface this by saying:

    See above, a step by step guide how to perform the calculation.


    The values you have posted are utter rubbish. The person who provided you with the values clearly has no idea about stabilty and you should not use any data provided by said person. EVER!

    The only "enjoyment", is seeing another person defending BS blindly after falling for the snake oil spin, AGAIN!!. After that amusement has faded, one is left feeling sad for the lack of professionalism and those wishing to follow a preacher burying their heads in the same sand as the preacher, rather than seek/listen to proper independent advice.
     
  14. pdwiley
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    pdwiley Senior Member

    Point is, you know that Brent's claims re stability are disbelieved and despite numerous requests many, many times, plus offers to do the calcs for free by a number of people, Brent has never done them or allowed them to be done on an as-built hull, now you post figures here without a source. I used to be a scientist; any set of figures that aren't traceable and reproduceable are unbelievable, it's real simple.

    What did you expect would happen??

    Post the source of the figures and you're off the hook, people can then look at who did it, how they did it and then see if their calcs come to the same conclusion. None of this is rocket science, it's basic scientific method, ditto engineering practice. It's not even BEST practice, just the minimum.

    I for one get really tired of trying to educate people, I no longer get paid to do it and I'm not real interested in trying to explain the same things to the same person/people the 3rd or 4th time. That's why I figure you're a troll, a sock-puppet or just lack the ability to learn. If this forum had a kill file a la usenet, the next post like this from you would get you residence.

    PDW
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Hahahahahaa

    In my relaxed post holiday state state, I failed to re-input each KN for each angle, I forgot to use the new one….doh!!! :eek::eek::eek:

    However what is more amusing is that you didn’t realise this!...:eek::confused:

    If you use the values you posted you get exactly that graph.

    BUT…at small angles of theata, the slope of the curve is equal to the GM at 1 radian. This is the back check to ensure the values are correct.

    So, looking at your graph….

    GZ = GM (theta)….where for small angles theta is in radians.

    Thus take 2 degree, this values (is correct) 0.056 or we can say that GM = GZ/theta

    Hence GM = 0.056/(2/57.3) = 1.6044

    This agrees with that given of 1.60

    So, if we apply real stability conditions, what happens??...in other words, are those values real or just made up, for a given design??

    Well, if a weight moves across the deck from side to side, how does this effect the GZ value?

    The new GZ = GZ(original) – (wh/W).cos(theta)


    If the boat is already heeled at 2 degrees, lets use these figures when taking an example of 1 person walking across the deck, from side to side (since we now have them). Width of deck approx 3.0m, weight of person 75kg.

    The new GZ at 2 degrees is 0.029m a drop from 0.056m!

    If we do the same but now with 2 people walking from side to side

    The new GZ at 2 degrees, with 2 ppl, is 0.002m

    If we do the same but now with 3 people walking from side to side

    The new GZ at 2 degrees, with 3 ppl, is -0.02m

    So if 3 people walk across the boat from side to side, the result is a negative GZ. The stability is reducing, and badly..

    If we look at this for the 5 degrees

    The new GZ at 5 degrees, with 3 ppl, has gone from 0.14m to 0.06m

    This simple check does not take into account any rise in KG from these people stepping on board and walking across the deck. (The KG rises to 0.95m an increase of 0.07m with 3 persons on the deck, standing up). Which also affects those figures above.

    Thus, the figures posted on first appearance seem plausible, but upon closer scrutiny (apart from the rubbish 180 degree range of stability)do not hold up. Since once a simple stability check is introduced, shows the stability to be extremely poor. Dangerously so...
     
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