Weight Distribution

Discussion in 'Stability' started by nimblemotors, Feb 22, 2012.

  1. nimblemotors
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    nimblemotors Senior Member

    Where is the best place to have the weight in a multihull?
    I get that as low as possible is the first consideration, but what about in 2d?
    In the center, or out in the corners?
    From what I've read so far, a multihull is very hard to turn over sideways,
    but it can pitchpole over the front. So where is the best place to have weight to prevent this?
     
  2. jehardiman
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    jehardiman Senior Member

    The center of weight should be over the center of buoyancy......

    Now that the obvious is stated, you should keep weight out of the ends and concentrated amidships to reduce inertia loads. This will reduce structural loads and overall weight.
     
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  3. baeckmo
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    baeckmo Hydrodynamics

    Yup, and it means that the hull will respond in resonance to smaller waves with higher frequency, thus avoiding resonance with higher waves, containing higher energy levels. In short; it makes for a safer boat.
     
  4. michael pierzga
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    michael pierzga Senior Member

    Very hard to steer a boat with weight in its ends. Bow down is particularly tiresome . You turn port and the front of the boat keeps going straight on. Imagine trying to precisly control the movement of a 5kg chunk of lead on the end of a two meter pole . Multiply times a million course corrections and your hands will grow blisters.

    Keep all fixed ballast in the middle of the boat...down low....and dont exceed the designed displacement

    On a monohull you shift movable ballast aft when running downwind to force the bow up , keeping the bow from grabbing the back of a waves and auguring in. .

    The object of weight distribution and movable trim ballast..crew, sails, cases of beer... is to encourage the front of the boat....the part of the hull doing sea keeping duty, to respond to the stern, the part of the boat with the rudder. .

    Dont know how a multi behaves but Id expect the same.
     
  5. CatBuilder

    CatBuilder Previous Member

    Like has been said, the c.o.b. is the best place. This is nearly always just aft of mid ships on a multihull. If it is a catamaran, I find I get the best ride having the weight also down in the hulls amidships.

    You don't have to position anything to avoid pitch pole. Your extra stuff on a multihull should simply not be so heavy as to cause that great a change in trim. If it is, better to leave it on a dock.

    There is a cheat. Look at your waterline, then place the weight and observe the change. Your goal is to keep the waterline perfectly level and perfectly parallel to the water. If it goes out of whack, the weight is in the wrong place or you need to shift something else.
     
  6. michael pierzga
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    michael pierzga Senior Member

    The secret to good handling is to follow the designers plans to the letter.

    Unfortunately with even with good design and good building the stern of a boat gains weight and sinks .

    You live , hang tenders, store Rum and add goodies in the stern of boats.

    Do you add ballast forward to compensate for this midlife bulge ? is the million dollar question.
     
  7. nimblemotors
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    nimblemotors Senior Member

    Having read through hours of archive posts, the obvious is not obvious!
    I know for cars we want weight low and centered, but boats who knows. Power boats have a lot of weight in the stern. I suppose that is not for stability but to increase forward motion. But another puzzle is that I've seen (deadly catch show) fishing boats will extend long booms out the sides of the boats and hang weights on them to stablize them. I'd guess this works because the weight isn't out to the side, but in fact, is lowered down into the water, so adding weight below?

    So would it make sense to do the same thing for a multihull, and have a chain or even a long moveable dagger board with big weight bulb on the bottom (like some monohull sailboats have) that can be lowered down in rough weather?

    Another related question is would it make sense to be able to flood water tight compartments in the hulls to add more weight to the boat to add stability when riding out a storm? Or leave it bobbing higher in the water?
     
  8. daiquiri
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    daiquiri Engineering and Design

    If by "weight" the OP intends the CoG it's also the only possible place... ;)
    But I am not actually sure what is meant by "weight".
     
  9. gonzo
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    gonzo Senior Member

    Boats with light ends have higher acceleration, which is a more uncomfortable motion. Spreading the weight will make the motion slower, but the slamming loads will increase.
     
  10. jehardiman
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    jehardiman Senior Member

    ...sigh... it's always bad when you have to explain a joke. Anyway for a staticly free floating body the center of weight and center of buoyancy are always in a vertical line. So it's an old NA joke.

    Cats work because they are light wave followers, and while you can tune thier response somewhat with weight distribution, this generally leads to larger loads and crossdeck slamming. For reasonable boat sized cats you cannot de-tune them sufficiently for short crested seaways anyway, so your are left with the snap motion problem gonzo refers to. The paravanes used on fishing boats that you refer to are realisticly not using weight to damp motions, but rather flow. While they appear to be heavy relative to a man, thier weight and mass relative to the ship is inconsquential in changing the ship's roll or pitch response.

    If what you really want is low response to waves, then you will have to go to other hull-forms, but those are fraught with thier own design implications (i.e. why you don't see SWATHs less than 60m).
     
  11. nimblemotors
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    nimblemotors Senior Member

    If I could please request less TLA, it would help.
    By "weight" I mean mass.
    A real world example/question is where should I put the engine(s) in a catamaran? One large engine in the middle, two smaller ones, in the middle of the hulls, or the sterns, or four smaller ones, two in the bows, two in the sterns, etc. (the cost of engines is not relevant to the question).
    Same would go for fuel tanks or water tanks. Where is best to put them to:
    A) the boat survive a huge storm in the middle of the ocean
    B) have humans "survive" the motions of the boat

    I can see with the weight (ok, mass) centered, the cat will be more prone to "hobbyhorse". Bad for people, good for boat?
     
  12. gonzo
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    gonzo Senior Member

    I think the engines should be installed where the designer said. Usually there is a degree of adjustment on all boats with loading, but it shouldn't be extreme. I know this is somewhat vague, however, most designers will indicate the limits.
     
  13. mydauphin
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    mydauphin Senior Member

    CATS should be as light as possible, but the strength and all the running gear has to be in the right place. If you have to worry about it then you dont know how to make it work. Do you know what size engines you need for your size cat? Is you CAT designed to have engines in Hull or center? What about rigging and mast designs? These are not optional questions? You need a designer. Hint: Sailboats and Multi-hulls are a lot hard to design correctly than a monohull powerboat.
     
  14. nimblemotors
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    nimblemotors Senior Member

    Indeed, if I have to ask, then I must not know. nice discussion, thanks.
     

  15. CatBuilder

    CatBuilder Previous Member

    Wait a second... is this line of questioning about that older Wharram? Is that the boat you are talking about?

    If so, you should just look at other Wharrams of the same design if you are really asking where to put engines.

    If that was more of a hypothetical question, keeping weight low in the hulls, way outboard and near the point where the hull is deepest and fattest is the best place if you have to add any. That's maximum stability (adds a little tiny bit to righting moment).

    Again, just watch the waterline when you do add weight. If it goes out of level, you added it to the wrong spot. It's really that simple, but keep it light or you'll be lucky to break monohull speeds.
     
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