Clarity wanted: heavy vs light

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

  1. gonzo
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    gonzo Senior Member

    The heavier boat will get higher slamming loads. If they are identical, that is the strength of all the structures is the same, the heavier boat will break up first.
     
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  2. rxcomposite
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    rxcomposite Senior Member

    A heavier boat will have less vertical acceleration (g) in the CoG. A lighter boat will get tossed around easily thus a much higher g. The scantling or strength is dependent on how heavy or light the boat is.
     
  3. gonzo
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    gonzo Senior Member

    Not really. For example, a boat of light scantlings with a very heavy load may be heavier than an empty boat with heavy scantlings.
     
  4. TANSL
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    TANSL Senior Member

    Continuation .... and a a boat of light scantlings with a very light load, or no loaded, may be heavier than an heavy loaded boat with heavy scantlings.
     
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  5. rxcomposite
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    rxcomposite Senior Member

    It is really. Vertical acceleration and displacement go hand in hand in the preliminary design to determine the operational envelope. It is an INPUT to determine scantlings. Vaccel3.jpg

    Class Rule, Heller and Jasper, or other methods like NAVSEA chart/formulas all point to displacement and vertical acceleration first to determine dynamic load necessary for the calculations of scantlings. NAVSEA002.jpg

    It is very important. In a small boat, say a sailing boat, the vertical acceleration will determine the mast load at the foot. This is called the critical mast load. With the mast and all its associated weights terminating in a small footprint, it could punch thru the deck/keel step.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Heavier than what....an apple?

    Without context, it is a meaningless statement.

    Exactly...ad infinitum.
    At least Tansl understands the pointless assertion.
     
  7. gonzo
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    gonzo Senior Member

    Heavier than the lighter one. The context is a comparison between two boats. Seems like once more you are starting stupid arguments.
     
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  8. fastsailing
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    fastsailing Junior Member

    In the first post of this thread the context was defined as: "2 identical 50ft catamarans in shape and form"
    So not just any 2 boats. And later on it was also defined that both boats initially move at the same speed, when a gust occurs.
    Is your statement
    intended for that context, or something else?
    I'm just seeking for clarification, not arguing anything.
     
  9. fastsailing
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    fastsailing Junior Member

    Interesting, but your second attachment is intended for planing monohulls, not for displacement sailing multihulls. It doesn't even care if certain speed in a certain sea state is upwind or downwind. In a real world direction of velocity of the boat relative to wave train has plenty of affect on slamming loads, not just speed and wave height.

    What does kv longitudinal impact coefficient (fig 3-8) depend on?
    (Hull beam, beam between chines, B/T ... ?)
    And what value does it have for your example cat in the first attachment?

    Can you show your calculation for that result?
    It seems not including mass of the sails (not having the same maximum acceleration for the very small time interval in question) the boat must have 10...100 g of vertical acceleration for you statement to be correct.
    Can you also show comparison with maximum static mast compression and buckling load of the mast section?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Are you sure???.., because you seem, as always, at odds with your own statements - mostly when someone points them out and then you bleat and moan about it.
    So, what did you say?

    Ok...that is what is being discussed - 2 boats.

    Eerr....How can one be heavier if they are identical?? :confused::confused:

    So like this boat, this is heavier:

    [​IMG]

    As noted:

    Since you're saying the QM2, because it is heavier will:

    Hmmmm.....ok.carry on wasting bandwidth now.
     
  11. rxcomposite
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    rxcomposite Senior Member

    Fastsailing- The first figure is an ouput of the software plotting the V acceleration of the same cat boat same conditions, same speed. Only the displacement was changed (exagerated a little bit). The data for the cat is there and the operating conditions (surviving wave height, speed, ect) are specified. The formula can be programmed into excel. I have that.

    The second image maybe for a monohull but can apply to catamaran as well when determining dynamic load. I posted it just for comparison of different methods using the same principle. As a rule, I don't mix and match different rule.

    For the third question, You may consult Skenes method published by Yades, If you used by other methods generated in the operating conditions, you will find the max of vertical acceleration. Now for a sailboat with a mast and sail, the weights of the mast, sail, riggings,blocks, are all forces. Great for static load but multiply that total amount by the g and you will arrive at the pressure bearing down on the mastplate. No, you don't need 100 g's. Typical, you will run into 2.7-2.9 g's in some operating conditions. It can be substantial if your cat is just two hulls connected by a crossbeam with the mast sitting in the center of the mastbeam.

    For the longitudinal impact coefficient, I cannot describe right now the exact meaning unless I open my book or spreadsheet. But there is a plot of vertical acceleration that increases towards the bow, away from LCG. I think I have something similar that I have posted before. NAVSEA has something similar graph. Depends on what method I am using.
     

    Attached Files:

  12. fastsailing
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    fastsailing Junior Member

    Weights of the mast, sail, riggings, block are almost irrelevant in order to determine static mast loads at the step. The relevant static loads are mostly caused by tension of shrouds, forestay, mainsheet, and halyards unless they terminate at the mast above the step. For a 1000 kg multihull, the static weight of the mast & rigging can be less than 1 kN, while the static load at the step can be more than 30 kN. If newtonian vertical acceleration would be 30 m/s^2, (proper acceleration then being 39.81 m/s^2) then dynamic addition to the mast & rigging weight would be 3 kN, and the total increased from 30 kN to 33 kN. 10% or less is a significant increase, but certainly not determining the load at the mast foot like you claimed:
    It would have been totally different case if the step had 30 g vertical acceleration due to slamming, but I have never experienced such and you calculations did't suggest that either, which I wanted to verify.
    The safety factors alone would easily take care of such a small ~10% increase. There are much greater variation of static loads due to trimming of sails or adjusting running backstays if used, or shroud tensioners if used for the same purpose.
    The sails will not accelerate equally as the mast, and any dynamic weight increase of the sails will reduce the sheet and staying loads, because a multihull is not a perfectly rigid object.
    Further more if slamming loads are initiated at or near the bow, forestay will reduce tension first, reducing mast loads at the step before step accelerates upwards.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Not so that is why it is included in the design practice. Ignoring it can be disastrous. It can be substantial with the g or load factor. The stay(s) is an added force, not weight and is about as much as the weight of the mast/associated weights. This time, it is a force, not weight and the negative and positive g forces will apply. Do not also confuse material safety factor to load factor.

    I am not claiming anything. Just going by the rudimentary rules calculations. And there is a lot. You just have to read.

    Here: Почему ломаются балки катамаранов? (Судостроение / Технологии) - Barque.ru http://www.barque.ru/shipbuilding/1967/why_break_beams_of_catamarans
     
    Last edited: Mar 29, 2019
  14. fastsailing
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    fastsailing Junior Member

    The second image gives a formula for vertical acceleration, which, according to the information provided by you, is independent on many factors that in a real world have a very significant effect on slamming loads.
    These include:
    direction of motion of the boat relative to the direction of motion of the waves.
    beam between chines of a planing monohull
    deadrise angle of a planing monohull
    waterline beam of a hull of a high speed displacement catamaran
    shape of all the cross sections of hulls from bow towards the stern

    For the same speed, displacement, lwl, sea state and direction of motion, the narrower the hulls are, the less vertical acceleration due to slamming there will be. The ultimate example being a large (relative to wave height) SWATH type catamaran.
    Therefore the real question is, when does the given formula give the correct order of magnitude of vertical acceleration?
    Surely, it is limited to something it is initially intended for, not applicable for all crafts out there.
    Just compare a flat bottom wide planing hull with rectangular cross sections and narrow planing V-hull of high deadrise angle, both having same speed, displacement and lwl, and it becomes obvious why that is the case.
     

  15. fastsailing
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    fastsailing Junior Member

    3 kN and 30 kN are not in the same order of magnitude. The step must be designed to deal with maximum buckling load of the mast tube, which can be 60 kN for the same boat with 100 kg mast & rigging weight. There will never be more load than that, regardless of the vertical acceleration.
    A sailing vessel which meets all the rules, but fails during normal sailing in normal conditions is worthless for its use. You have to do the actual real world engineering too, not just look at all the rules, especially if they do allow designing for only 8 kN (=200 kg * 40 m/s^2) mast step loads when combined mass of mast & rigging & sails & etc is 200 kg for a 1000 kg multihull.
     
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