AMA displacement

Discussion in 'Multihulls' started by rapscallion, Jul 2, 2011.

  1. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    I have a question:

    When calculating the max AMA displacement, does one assume the ama is just about completely submerged?
     
  2. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Well, you can't have any more displacement than that... ;)
     
  3. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    Ama

    ========================
    You know this but I'll run thru it anyway. Ama designed displacement depends on the design of the whole boat:
    1) Boat will not fly main hull, ama immersion up to max means its time to reef or depower. You can design for a different cutoff point than max immersion.
    -
    2) Boat will fly main hull, but w/o foils. Ama displacement must be the same as
    the weight of the whole boat+crew+stores and designed for low wetted surface/low wave drag. In practice this means something on the order of the total buoyancy of the ama being around 200% of that required to float the boat. Thats just a number-the actual ama design should meet the low wetted surface/low wave drag criteria with substantial reserve buoyancy.
    Pitch characteristics of the ama become very important when flying the main hull as well. Rocker of the main hull and overall beam become very,very important when designing to fly the main hull with or without foils: if main hull rocker is too great or overall beam too little then the sailing angle with the main hull 1" above the water will be too great. There are some tri's theoretically "designed" to fly the main hull that have a nominal sailing angle of 26 degrees-too much! USA 17 flew the main hull at 13 degrees ,if I remember correctly.
    -
    3) Boat will fly main hull and ama will use lifting foil to reduce the weight the ama itself has to carry with buoyancy. High performance only. ORMA tri's were designed to allow ama foil to carry up to 70% of the total sailing weight with 30% left for ama bouyancy which was used as the primary means of pitch control when flying the main hull. Total ama buoyancy could be reduced to below 100% though for ocean going boats staying around 200% is probably better. There are new systems that, if they work, will change this configuration ,improve pitch control and allow earlier takeoff.
     
  4. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    "does one assume the ama is just about completely submerged?"

    It's the total displacement of the completely sunk ama. It means something, but as a % what that is, is going to be highly variable. Mostly of significance when discussing stuff within a type. Consider a single change, beam, that alone would affect the meaning of the ama displacement.
     
  5. sailsocal
    Joined: Jun 2006
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    sailsocal Junior Member

    Amas + foils

    There's a new patent that addresses this issue:

    http://www.google.com/patents?id=_8...ook_result&ct=result&resnum=1&ved=0CCgQ6AEwAA

    Boat builders interested in licensing this design can contact the patent holder at www.QuadMarine.com
     
  6. Doug Lord
    Joined: May 2009
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    Doug Lord Flight Ready

    hydrofoil altitude control

    ======================
    Very interesting-thanks. It appears to be more of a unique method of hydrofoil altitude control. Be interesting to see if it was actually responsive enough to do that. One problem that is immediately apparent is the use of four foils, another is that the forward and aft foils are under a form of active control. In many previous hydrofoil arrangements 2,3, and 4 foils the aft foils are not controlled by the altitude control system. However, there have been suggestions made(by Tom Speer,no less) that a mixed control of the aft foil on a bi-foiler might be beneficial. We'll see-I wish them luck!
     
    Last edited: Jul 8, 2011
  7. sailsocal
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    sailsocal Junior Member

    The patent states that the concept is not specific to a multihull with 4 hydrofoils, so you could have passive foils aft, and active foils forward. A trimaran configuration could be especially interesting, with minimal-buoyancy outside amas, and a larger central hull for buoyancy at rest.
     
  8. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    The earliest generation of trimarans had amas whose maximum displacement was less than the weight of the boat, but this is way too low. Today, I believe the rule of thumb is a cruising tri should be on the order of 130% of the weight, and more like 200% of the weight for a racing tri.

    The reason why an ama needs to have more displacement when fully immersed than the weight of the boat is because different parts of the ama are immersed at different times. Going upwind, the load on the boat is mostly sideways, and the ama will be more or less evenly immersed along its length. But going downwind, the load is more forward, and when the boat is in a bow-down attitude much of the aft part of the ama is out of the water.

    Once the bow of the ama goes under, the centroid of the waterplane starts to move aft rapidly with a small increase in bow-down pitch trim. This is unstable behavior, because any additional loading on the ama comes from further aft, which only tends to depress the bow further. But if you design the shear of the ama to take into account the maximum bow-down attitude, you can make the design much more forgiving.

    A multihull footprint plot like this one can help you determine the appropriate distribution of buoyancy in the ama.
    [​IMG]
    The blue lines show the center of buoyancy for different combinations of heel and pitch trim. The green lines are the equivalent center of gravity locations corresponding to Shuttleworth's stability indices for a trimaran. The red line shows the location of the center of buoyancy when the bow of the ama is fully immersed.

    You should decide what the requirements are in terms of the limiting conditions for the boat, and then use those requirements to determine the amount of buoyancy and the distribution of the buoyancy in the amas. From that, you can shape the sections to minimize the wetted area and provide the vertical distribution of the reserve buoyancy to match the desired pitch trim.
     
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  9. rapscallion
    Joined: Oct 2006
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    rapscallion Senior Member

    Sweet Graph Tom!
    It really helps put the elongated wave piercing hulls into perspective.
     

  10. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If the ama is planing, the dynamic lift needs to be calculated.
     
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