Stability Index help

Discussion in 'Stability' started by Mik the stick, Dec 7, 2012.

  1. Mik the stick
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    Mik the stick Senior Member

    I am a retired engineer (electronic) and have a interest in aircraft design and boat design. I have bought Dave Gerr's book on propellers.

    From Westlawn articles I got the formulas to work out roll period. From this forum I learned the CSF and Brewers Motion comfort formula.I know how to calculate Stability index but I don't fully understand it. Posts in this forum show some sail boats have SI between 0.47 and 0.96.

    Does a lower SI number mean the boat is more stable in bad weather.

    My main interest is in WW2 boats/ships. I calculated a Fairmile B motor launch to have an SI of 0.57 even if my calculations are not to accurate I think that's not bad. If under 1 is good enough for offshore work The B could operate anywhere in the world which of course it did.
    mik.
     
  2. Mik the stick
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    Mik the stick Senior Member

    Found an answere of sorts

    I found a pdf file on this forum in it it say the limiting GM is the value required to pass IMO regulation which I know nothing about. It goes on to say a rule of thumb for required GM is 20%Hw(wave height). My calculation for GM was 6.44 so 6.44/0.2 = 32ft. My Beaufort scale chart gives force 9 probable wave height of 23ft and maximum of 33ft. Force 8 as 18ft and 25ft.

    Obviously there is no guarantee you won't capsize a boat even in good weather, with a skilled captain I feel a Fairmile B could survive a force 9 gale, a force 8 would probably not be to much of a problem.

    I'm sure I read somewhere MTB's could operate in force 4 to 5 but it can't have been comfortable.
    Mik
     
  3. TANSL
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    TANSL Senior Member

    GM has nothing to do with the wave height. Its value depends on the position c. of gravity of the boat and the metacentre, which depends, in turn, on the moment of inertia of the waterline, and displacement to that waterline.
    Even if you find a lot of information on the web, it is not always easy to understand it and can not interpret correctly. Best would be to consult with an expert.
     
  4. Mik the stick
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    Mik the stick Senior Member

    What I was trying to say was the formula uses GM to predict the size of wave which will capsize your boat. I know I'm not an expert but research has taught me RAISING OR LOWERING GM EFFECTS STABILITY.

    Having said that I estimated a 10 foot rowing boat with 2 on board to have a GM of about 3.5 for a capsize wave height of around 17.5 feet. I was in an 8 foot dingy on the river Forth (only the once). That experience makes a nonsense of all my calculations and almost everything I have written thus far.

    Adding 100 pounds for a small mast and sail the 10 foot boat would have a CapsizeRiskFactor of 2.17 with 2 on board, not safe offshore.
     
  5. TANSL
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    TANSL Senior Member

    The transversal equilibrium of any ship depends on the interaction between the heeling moments and righting moments. These have to do with the heeling or righting arm. And the latter is precisely the value of GZ at each heeling angle. Of course, everything is related, but the GZ depends on the relative position of G and the center of buoyancy, which have nothing to do, if I'm not mistaken, with the metacentre.
     
  6. Mik the stick
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    Mik the stick Senior Member

    What I want to do is compare boats/ships. I don't need formulas to tell me that it is going to be much more comfortable in a 500 ton ship than a 68 ton one in any weather.
    I suppose what I want to know is if two ships have an SI of 0.56 and 0.47 respectively is the ship with SI = 0.47 the most stable of the two, and at what point (SI=1.2 perhaps) does the SI number reflect a ships unsuitability for use in bad weather.
     
  7. TANSL
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    TANSL Senior Member

    First you would have to define what you mean by being "more stable". Stability is one thing, and comfort is another.
    When you define these concepts, we can continue to exchange views.
    In a sense, one can say that a ship is just as stable whatever the weather does. But, of course, with rough seas will move more than with calm seas, however stable or unstable to be boats.
    There are boats that have a very short period of balance, that is, the boat is very fast retrieves a heeling moment. Some might think that is "very stable", and, maybe, he is right, but the boat can be very uncomfortable, due to the rapidity of their movements.
    Neither a ship of 500 tons has reason to be less stable or more stable than one of 60 tons.
    So, please clarify what you're talking about.
    Regards
     
  8. Mik the stick
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    Mik the stick Senior Member

    I have copied two boat specs from another forum, looking at them the blue boat is biggest it also has the best Motion comfort ratio and Capsize safety factor of the two. Blue boat has the higher SI which equals 1 versus 0.47. Both boats are rated for 7 meter waves and force 10. Is the blue boat safer in bad weather? What SI would be unsuitable for Force 10 and 7 meter waves? Red boats Capsize Safety factor of 2.39 would indicate offshore racing. would be dangerous. what do you think.

    RED's BASIC DATA:
    Overall Hull Length = 11,98 m
    Length Waterline = 10,57 m
    Flooded Buoyancy (Y/N) = N
    Beam Waterline = 3,27 m
    Beam = 4,1 m
    Tc (HD) = 0,55 m
    T = 2.38 m
    Displacement MSC = 5165 kg
    Displacement Max = 6190 kg
    Ballast = 1300 kg (guess; does not influence any ratio or factor other than W/Disp)
    Height of CE above DWL = 6,87 m
    Height of CLR below DWL = 0,95 m
    Angle of vanishing stability, Avs = 123 deg
    Downflooding angle Afl = 137 deg
    GZ at downflooding angle = -0,23 m
    GZ at 90 degrees = 0,58 m
    Sail Area = 71,1 sq.m
    Area to flooding (Agz) = 74,46 m.deg (The one to be used in this case, as Afl >Avs)
    Area to Avs = 75,6 m.deg

    RED's 'OLD' RATIOS AND PARAMETERS
    Length/Beam Ratio L/B = 2,75 -- being L = (Lod+Lwl)/2
    Ballast/Disp Ratio W/Disp = 0,25
    Displacement/Length Ratio D/L = 121,99
    Sail Area/Disp. Ratio SA/D = 24,18
    Power/ Disp. Ratio HP/D = 4,74 HP/ton
    Hull speed HSPD = 7,89 Kn
    Potential Maximum Speed PMS = 9,78 Kn
    Velocity Ratio VR = 1,24
    Capsize Safety Factor CSF = 2,39
    Motion Comfort Ratio MCR = 15,2
    Heft Ratio HF = 0,5
    Angle of Vanishing Stability AVS = 116 º
    Roll Period T = 1,93 Sec
    Roll Acceleration Acc = 0,3 G's
    Stability Index SI = 0,47
    Upright Heeling Moment UHM = 19635,03 Ft*pound
    Heeling Moment at 1º HM1º = 1306,88 Ft*pound
    Dellenbaugh Angle DA = 15,02 º

    BLUE's BASIC DATA
    Overall Hull Length = 12,2 m
    Length Waterline = 10,52 m
    Flooded Buoyancy (Y/N) = N
    Beam Waterline = 3,2 m
    Beam = 3,65 m
    Tc (HD) = 0,80 m
    T = 2,00 m
    Ballast = 3600 kg (guess; does not influence any ratio or factor other than W/Disp)
    Displacement MSC = 10319 kg
    Displacement Max = 11769 kg
    Height of CE above DWL = 6,65 m
    Height of CLR below DWL = 0,8 m
    Angle of vanishing stability Avs = 147 deg
    Downflooding angle Afl = 114 deg
    GZ at downflooding angle = 0,4 m
    GZ at 90 degrees = 0,67 m
    Sail Area = 72,5 sq.m
    Area to flooding (Agz) = 57,42 m.deg
    Area to Avs = 66,83 m.deg (The one to be used in this case, as Avs>Afl)

    BLUE's 'OLD' RATIOS AND PARAMETERS.
    Length/Beam Ratio L/B = 3,11 -- being L = (Lod+Lwl)/2
    Ballast/Disp Ratio W/Disp = 0,35
    Displacement/Length Ratio D/L = 247,21
    Sail Area/Disp. Ratio SA/D = 15,54
    Power/ Disp. Ratio HP/D = 2,37 HP/ton
    Hull speed HSPD = 7,87 Kn
    Potential Maximum Speed PMS = 8,26 Kn
    Velocity Ratio VR = 1,05
    Capsize Safety Factor CSF = 1,69
    Motion Comfort Ratio MCR = 35,35
    Heft Ratio HF = 1,24
    Angle of Vanishing Stability AVS = 123 º
    Roll Period T = 3,66 Sec
    Roll Acceleration Acc = 0,07 G's
    Stability Index SI = 1
    Upright Heeling Moment UHM = 19074,33 Ft*pound
    Heeling Moment at 1º HM1º = 697,53 Ft*pound
    Dellenbaugh Angle DA = 27,35 º
     

  9. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Sorry friend Mik, but I would be a fool if I dared to say what ship is safer. Safety depends on several things,besides the cluster of figures you have shown me. I have not, nor I can have it, the remotest idea.
    I do not know if what you want is that I was amazed by the amount of data you have, but you will not get your purpose. One have to know and study many aspects of a ship to know whether or not it is safe. When you learn this, we will continue talking.
     
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