ABS - Vertical acceleration

Discussion in 'Class Societies' started by AndreO, Oct 18, 2011.

  1. AndreO
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    AndreO New Member

    Hi. I am studying the ABS HSC rules 2001, and I found that the vertical acceleration is too high (68 g's - It is not possible to tolerate) for a hull dimensions of: Lw= 9 m. ; Bw= 2.98m. ; V = 55kts, h1/3=4 m., running trim=4° and a displacement of 5.3 tons.

    And this high value, increases the bottom slamming pressure and therefore all the design process.

    Please anyone can tell me about this result?

    Thanks a lot
     
  2. Petros
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    Petros Senior Member

    I am not familar with the rules sited, but I have studied similar requirments in German Lloyds. Most of the rules result is similar design loads and the Deck loads were rather high, but not anywhere near that high. I can not think of any moving vehicle or vessel that would ever use numbers anywhere near that high, it would never float.

    That can not be correct, you are off by an order of magnitude at least. Could that be kg/m2? or some other unit of measure? Better double check your calculations or your assumptions, better yet post the formula.

    You should end up with deck loads in the 200 lb/sq ft range (about 98 kg/m2)
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Can you post some of the product values you obtained too.

    I have used this before and not found problems. However i prefer DNV rules. ABS produce heavy structures and the plan approval also don't understand "high speed" philosophy, they still want "over engineered" because it looks better!!. They all tend to be big ship rule guys.
     
  4. AndreO
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    AndreO New Member

    Thanks for your answers

    Really this is a very high value and I am thinking if this rules are design for long vessels, but I am in the application limits (L<130 m) *ABS HSC 2001 Part 1 Section 1*

    Attached the equations from original ABS HSC 2001 rules for your consideration

    If anyone knows other explanation for this,

    I appreciate the attention
     

    Attached Files:

  5. Ad Hoc
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    Ad Hoc Naval Architect

    Yes.

    Do you really want to run at 55knots in a boat of 30 feet in 13foot waves??!!!
     
  6. DUCRUY Jacques
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    DUCRUY Jacques Junior Member

    Hello,

    I am not sure that the formula of ABS HSC for calculating the vertical acceleration is correct for boat under 20 m.

    I believe ISO give another formule for little boat (in metric units) :

    Vertical acceleration = 0.5 V/Displacement^0.17 = 6.4 g

    Pressure : (0.1 * 5300)/(9 * 3) * (1 + 6.4) = 145 kN/m²

    This seem more realist ...
     
  7. fcfc
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    fcfc Senior Member

    Are you sure ISO is still valid above 50 kts ???

    I would guess strange results come from rules used outside their scope.
     
  8. AndreO
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    AndreO New Member

    Hi

    I am studying the boat in the worst condition: Maximum velocity and high waves. Obviously, if I had a reduction in this values, the vertical acceleration decreases. And then ¿What is the sea condition and the velocity adecuate to enter in the ABS equation?

    Thanks
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    That is for you decided, it is called "designing". The boats structure reflects what the SOR (Statement of Requirements) of the boat has to satisfy.

    So, the question you need to ask yourself is this:

    Can you or would you, run a vessel of just 30feet at 55 knots in waves of 13 feet, roughly half its length.??

    If you are unsure, then your SOR is insufficient for your design, i.e., it has no meaning. Then all you're doing is just playing with numbers without comprehending the meaning and ramifications on design.
     

  10. Alik
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    Alik Senior Member

    The formula for nCG used in all those standards (ISO, ABS HSC, etc.) is Savitsky-Brown formula for monohull planing craft. One should see what are the limitations of that formula. Normally design accelerations for commercial craft are below 2g and for rescue craft below 8g.
     
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