Trimaran with accomodation in the amas

Discussion in 'Multihulls' started by eiasu, Nov 23, 2012.

  1. mcarling
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    mcarling Junior Member

    This is what my intuition says should happen when the polar moment of inertia is increased by moving weight from the vaka to the amas (while keeping the center of mass unchanged). I would expect the accelerations to reduce but the amplitudes to increase (certainly in a calm see and I guess in rough seas also), though I would expect that the increased amplitudes would be negligible because rough seas won't roll won't roll the vaka of a trimaran out of the water.

    I thought "stability" in the context of the rolling of a vessel was defined as high acceleration and low amplitude (in other words closely following any waves).

    I would expect moving weight from the vaka to the amas (while keeping the center of mass unchanged) would result in more waves crashing over the amas. Is this what you meant? I'm having trouble seeing how more waves could get on the deck of the vaka as a consequence of such a weight redistribution.
     
  2. rberrey
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    rberrey Senior Member

    I should have also added to keep the weight centered , not bow or stern, but same thing for the vaka. Rick
     
  3. cavalier mk2
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    cavalier mk2 Senior Member

    "I would expect moving weight from the vaka to the amas (while keeping the center of mass unchanged) would result in more waves crashing over the amas. Is this what you meant? I'm having trouble seeing how more waves could get on the deck of the vaka as a consequence of such a weight redistribution."

    That is what I mean but I was also thinking of wing deck boats where it is all connected. Spreading the weight out to the amas actually makes the tri handle more like a catamaran with the spread out weight distribution. It works for them and can work for a tri. Ed Horstman actually pointed this out to me, I think the actual words were, "Think about it dummy, if it works for a cat spreading the load won't hurt a tri but with a tri you have a choice about the kind of motion you want. "
    With weight in the amas they rise slower and immerse deeper in the wave so the crest can be past before the lift off angle arrives. I've put some weight out there cruising and found it dampened the roll. Centered weight lets the tri react to the surface of the water more quickly. If it is spread out for and aft it will go through the waves versus over them with the weight centered. Every boat handles differently, it is important to move things around to see how what you sail handles best in different conditions and be open to adjusting your load under way for peak performance. Moving the sail bags can be a easy way of adjusting trim etc....
     
  4. cavalier mk2
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    cavalier mk2 Senior Member

    "I thought "stability" in the context of the rolling of a vessel was defined as high acceleration and low amplitude (in other words closely following any waves)."

    Here what you achieve is a more level platform with a dampened motion in the waves. This can be better for rig efficiency, and keeping a long lever arm. If you are heeled on the face of a wave the lever arm is shorter.

    We cruise with most of the weight in the center hull in our 37' but find some weight in the amas does dampen motion.
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    We should also make a note that ama shape will have a large effect on motion with load too. A V shape ama will have more gradual wave immersions while a round bottom ama will be much more abrupt.
     
  6. mcarling
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    mcarling Junior Member

    I had in mind something like a Dragonfly with only trampolines, cables, and the akas connecting the amas to the vaka. I should also have considered wing deck boats when I was contemplating the meaning of your statement. Please accept my apologies.

    Thank you for confirming that my intuition and understanding of the simple physics involved is not faulty. It always seemed to me that the oft-repeated advice "don't put anything heavy in the amas" was either based on superstition or on an unstated premise. I'm relieved to have my understanding confirmed by a professional boat designer. You made my New Year's Day. Thank you.

    In the context of a trimaran, I guess the dampened rolling from shifting some weight to the amas would make for more comfortable rolling. The buoyancy of the amas puts an (elastic) limit on the roll amplitude, while the reduced acceleration of the roll should be pure benefit to the comfort of the passengers.

    Yes, and I believe that will be true for the windward ama lifting out of the water more gradually/abruptly with a V-shape/round bottom.

    This leads me to something else I don't fully understand. From what I've seen, sailing trimarans typically have ama keels of uniform draft from the bow to the midship section, then gradually rising keels (diminishing draft) from the midship section to the stern. I believe the primary consideration in this design is minimizing resistance to forward motion. It seems to me that an ama keel profile nearer to that of a conventional monohull (more V-shaped looking from abeam) would also provide for more gentle insertion of the amas into the water during rolling, for the same reasons as a V-shape as seen from astern. Would that be possible without increasing resistance to forward motion?
     
  7. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I can understand how having extra weight in the amas will dampen roll, but having extra weight in the amas also reduces buoyancy -- which might increase the chance of their digging in and precipitating a roll-over.

    Likewise, I understand how a cross-sectional vee-bottoms ease the immersion of the amas, but I also wonder if flatter bottoms would resist immersion better in case of a wave breaking abeam, and also allow the whole boat to slide sideways easier, if needed?

    I'd like to know the optimum ama shape for a cruising trimaran, both cross-section, longitudinal section, and profile shape, particularly those shapes that can be developed for plywood.

    New Year's Greetings to All :)
     
  8. mcarling
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    mcarling Junior Member

    Assuming a symmetric port/starboard weight distribution, I think that effect would be exactly balanced by the equal and opposite effect on the other ama. The latter would be kept from lifting by its additional weight just as much as the former would tend to sink due to its additional weight. Correct?
     
  9. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    I'm not a NA so my ideas may be completely wrong. My approach is to take an idea to an extreme and see if that doesn't shed light on the topic.

    In the case of putting weight in amas, mentally I load them up with weight such that they don't have any buoyancy. What happens? Then, mentally, I make the amas out of 'unobtainium' so they have no weight whatsoever. Which of these two cases would result in a more stable, roll-resistant platform?

    I conclude that the lighter and more buoyant the amas are the harder it will be to roll the boat. But extra that buoyancy may also result in a harsher ride.
     
  10. mcarling
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    mcarling Junior Member

    I'm not a naval architect either, but I did study some physics. Any redistribution of weight that does not move the center of mass cannot change the righting arm (assuming the akas are perfectly rigid), but would change the polar moment of inertia. Moving weight out of the amas into the vaka (keeping the center of mass constant) will reduce the polar moment of inertia. Reducing the polar moment of inertia would make it easier to roll the boat.

    The mental exercise of shifting all the weight into and out of the amas is a good approach, but I don't think you correctly thought through what would happen under those imaginary circumstances.
     
  11. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Again, I'm quickly getting out of my depth in the engineering...

    I agree with your conclusions regarding changing the distribution of mass and how it affects the polar moment of inertia.

    What is less clear to me is your assertion that moving mass into or out of the amas has no change on the righting arm? If so much mass was moved into the amas that they had neutral buoyancy, they would no longer be able to be the pivot of the righting arm (except, perhaps briefly, through inertia) -- instead the vaka would become the pivot. That's why I thought it important to maintain as much buoyancy as possible in the amas, at least if you're trying to prevent a wave/wind induced roll-over.
     
  12. mcarling
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    mcarling Junior Member

    The vaka is always the pivot -- unless the vaka is flying, which is normally something to be avoided. If the vaka is flying, then the pivot is an ama, but then I believe the righting arm is still unchanged (so long as the ama is the pivot) because the CG is unchanged.

    If you make the ama non-buoyant (which is a radical discreet design change, not merely a continuation of moving some weight from the vaka out to the amas) and the vaka is flying then, yes, the righting arm would change. However, I wouldn't call a boat with two non-buoyant lateral extensions a trimaran. A trimaran has three hulls, by definition. Buoyancy is, in my opinion, an essential feature of a hull.
     
  13. Richard Woods
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    Richard Woods Woods Designs

    You have to be careful to differentiate between outrigger weight and outrigger displacement (or buoyancy). Also between stability and boat motions. All of which sometimes lead to contradictory design parameters.

    A tightrope walker is more stable when carrying a pole PROVIDING he doesn't rock it. But once in motion it will be very hard to stop.

    When sailing in a cross sea you often get rhythmic rolling and being in phase with the wave crests can lead to very uncomfortable motions. Think of it as the difference between pitching (which all boats do) and hobbyhorsing (which better designed boats, by and large, don't do)

    There are many trimarans sailing with less than 100% buoyant outriggers

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  14. cavalier mk2
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    cavalier mk2 Senior Member

    "This leads me to something else I don't fully understand. From what I've seen, sailing trimarans typically have ama keels of uniform draft from the bow to the midship section, then gradually rising keels (diminishing draft) from the midship section to the stern. I believe the primary consideration in this design is minimizing resistance to forward motion. It seems to me that an ama keel profile nearer to that of a conventional monohull (more V-shaped looking from abeam) would also provide for more gentle insertion of the amas into the water during rolling, for the same reasons as a V-shape as seen from astern. Would that be possible without increasing resistance to forward motion?"

    I've seen amas that cover the whole spectrum depending on the design. On performance boats the profile you are describing is too get buoyancy forward quickly to prevent diagonal capsize. The sharp bows ease the transition and allow wave penetration. Many tris have more rocker to minimize light air wetted surface. The older Newicks as a example have the bows very high to avoid tripping and a banana keel profile. allowing a clean exit. The rocker provides a progressive increase in buoyancy as you surmised.

    The ama rule of thumb on sterns is to have a double ended shape for amas that immerse for less drag and a transom on round high buoyancy amas that stay on the surface for a clean break.Interestingly current thinking is moving toward more v ama sections with foils to reduce the shock loads that have caused structural problems. Shapes that ease the transitions will of course be easier on the structure, rig and crew.
     

  15. cavalier mk2
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    cavalier mk2 Senior Member

    Outriggers are interesting in that they work without any real buoyancy. They have neutral weight in the water and ballasted weight as they lift.
     
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