Clarity wanted: heavy vs light

Discussion in 'Multihulls' started by GaryJones, Mar 22, 2019.

  1. rxcomposite
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    rxcomposite Senior Member

    Yes it is. Other parameters have been calculated beforehand and is reflected as inputs like surviving wave height, maximum wave height, ect. You are talking of different hull shapes behavior. The data reports only a hull shape that has been predetermined before like length, breadth of hull, girth distance, ect. It takes 5 to 10 spreadsheets to determine the input parameters.

    Sailing upwind or downwind. The method of calculation will not care. Only what is measurable like speed, wave height, density not to mention mode like hogging or sagging.
     
  2. rxcomposite
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    rxcomposite Senior Member

    Then it is not designed properly.
    Design engineering is for the real world. Which real world are you talking about?
     
  3. fastsailing
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    fastsailing Junior Member

    The real world, where designing for 8 kN maximum mast step loads for a 1000 kg multihull based on 30 m/s^2 vertical acceleration and mass of 200 kg for mast & rigging & sails & fittings is totally inadequate and criminally negligent. And where 60 kN breaking load including safety factors is needed based on statical loads and mast strength. Furthermore, the maximum possible loads happen, when multihull is flying a hull and there is angular acceleration (heeling) about the waterplane of the leeward hull caused by aerodynamic forces on the sails, not slamming loads. Those might be associated by the vertical acceleration of the center of gravity of the boat (for a catamaran), but not caused by slamming, it can happen even with completely flat water surface with no waves in a small lake. If the mast step would be located on the leeward hull (like in a certain type proa), the loads would reduce less than proportionally to staying base, while the vertical acceleration of the mast step might be close to zero and any calculation based on that is still totally wrong, because this claim is incorrect:
    as it suggests at zero vertical acceleration the loads are also zero, while they might be at maximum.
     
  4. fastsailing
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    fastsailing Junior Member

    What data?
    I was talking about the mathematical formula (3-10a or 3-10b) on your post 20
    Clarity wanted: heavy vs light https://www.boatdesign.net/threads/clarity-wanted-heavy-vs-light.62002/page-2#post-850911
    That formula ignores many things having a large effect on slamming loads, as I have already explained. It is not useful without knowing when it is applicable.
     
  5. rxcomposite
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    rxcomposite Senior Member

    Here is the Longitudinal Impact Coefficient table. I thought you dismissed the formula as applicable only to planing monohulls. NAVSEA LIC003.jpg
     
  6. rxcomposite
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    rxcomposite Senior Member

    Show the forum how you design this so as not to be "criminally liable".
     
  7. gonzo
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    gonzo Senior Member

    Only on the context of two boats of the same shape. A light boat will accelerate more than a heavy boat when a wave hits it. The light boat will have a livelier motion that may be more uncomfortable. The heavier boat will accelerate less and often have a more comfortable motion.
     
  8. gonzo
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    gonzo Senior Member

    How about the same responses, but quoting full sentences so the context remains. This is ridiculous and childish.
     
  9. Dejay
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    Dejay Senior Newbie

    So like you said before, slamming forces from waves would be less severe on a lighter boat that could move out of the way because of less inertia, even before the wave hits with the swell.

    So you can make a boat tougher to sink in a storm by making it lighter? Using less laminate to save weight and having the boat be less impacted by waves?

    I've been wondering why some cruising catamarans have these insanely thick laminates and some of the same size very light.

    Is there this design spiral where you add weight somewhere for some luxury, increasing slamming loads then you have to add structure adding weight again until you slip into a new class where it gets heavier and heavier?

    Or is it just that people prefer the motions and slower accelerations of a much more hefty boat and fuel money is no object?
     
  10. TANSL
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    TANSL Senior Member

    It does not have to be this way. Depends largely on the bow shapes, and general shapes, of the boat
     
  11. Dejay
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    Dejay Senior Newbie

    Well assuming identical boat and hull shape except weight.

    Would a lighter boat always get less slamming force from wave? Or would it sometimes get just as much slamming force than a heavier ship?
     
  12. fastsailing
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    fastsailing Junior Member

    Suppose you have 2 identical boats, but load them up differently to have different weight. Then you lift up both by a crane at a same altitude, and release suddenly towards a flat sea surface. The resulting forces are close to the same, but the force acting on the heavier boat acts much longer for a larger impulse required for the same change in velocity. Max pressure in both cases is dynamic pressure at the max speed downwards, but as the area effected by the pressure increases the pressure drops at a different rate due to different acceleration resulting a slightly different maximum force. The higher the height the less the difference in max force, because it approaches max pressure times total projected bottom area for both boats.
    I guess such test has something in common with slamming loads of planing boats, but not so much (if at all) with displacement hulls of a cruising catamaran used for cruising never leaving the surface, where loads are more related to variations of buoyancy than dynamic effects (impacts with water).
    Having said that it's still important to design and build an ocean going boat to withstand such slamming loads, just in case, for safety reasons.
     
    Dejay likes this.
  13. Ad Hoc
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    Ad Hoc Naval Architect

    It really does help the OP if you stick to a description that is clear and consistent.
    Yet you can't when your first opinion is counted and your only retort is just twists and turns to obfuscate the obvious and then just ad hominem posts because you are unable to retort with data, just opines.
    For example:

    and your reply:

    So which is it?

    Well, you go on to say:

    So a LIGHT boat with a HEAVY load has a full load displacement of more than a another vessel's LIGHTSHIP. So that means it is HEAVIER than the empty vessel.... :confused:

    This is more confusing than an episode of....SOAP!

    Your constant flip-flopping to suit your own narrative does not help the OP one bit.
    Just a waste of bandwidth.
     
    Last edited: Mar 31, 2019
  14. gonzo
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    gonzo Senior Member

    Once more you are misquoting to start stupid arguments. It is asinine.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    And once more you are refusing to clarify your back tracking flip-flopping statements.

    If you don't post contradictory statements, there would be no need to ask/seek clarification - which you never seem to provide.
     
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