Draft of amas and vakas on a tri

Discussion in 'Multihulls' started by MAL1968, Apr 21, 2012.

  1. MAL1968
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    MAL1968 Junior Member

    Should the draft of the amas on a tri match or be slightly shallower than the vaka? I was under the impression that they should be shallower draft to provide greater bouyancy. I designed a vaka with a 2 foot draft and then a pair of amas with a 6 inch draft at 90% length of the vakas. It somehow doesn't look right.

    Just wondering if there numbers should be close together such as the Cp and the draft depth. Perhaps a deeper draft for the amas would be necessary. Thanks for any input.
     
  2. yipster
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    yipster designer

    Been googling without much result on that
    Bigger sailing tri's have the amas V like, always one out of the water giving unbalance stationair
    Other side of the game is like incat having the mainhull/vaka ^ up out of the water most of the time or even nacelles on cats
    Like to hear more reasoning on this as well
     
  3. MAL1968
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    MAL1968 Junior Member

    I think I am going to try some dimensions that match, or are closer to the the main hull. I may be confused , however as to which hull type provides better floatation, a shallow draft hull versus a deeper draft hull. My thought pattern has been that a shallower draft at a given length would provide more floatation. The shallower draft hull, However seems to give a broad shallow plane with a very thin depth of keel, and it is this thin depth of keel that I am worried about.
    The only 2 remedies that come to mind in this are to A.) Increase the draft depth for a deeper keel or B.) raise the freeboard by another 6 to 12 inches and that would rest the keel of this ama on or just below the waterline.
     
  4. MAL1968
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    MAL1968 Junior Member

    The draft depth for the Vaka is 2 feet and for the ama was 6 inches, i think the draft needs to be closer for the ama may be 1.5 and match the Cp and block coefficient of the main hull.
     
  5. yipster
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    yipster designer

  6. MAL1968
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    MAL1968 Junior Member

    Alot of interesting ideas. I have seen everything from raked masts to aka design but no one talks about differences in drag between the vaka and amas. They talk about am floatation being 160% or higher. How is this achieved?
     
  7. Corley
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    Corley epoxy coated

    To achieve your target of 160% as an example you would want the float to achieve that displacement when fully immersed. So you take your design displacement (of the whole boat) and design the float so that when immersed with the deck level with the water it displaces 160% of the complete displacement of the craft.

    The reasons for different drafts are not as straight forward, on a boat with fairly straight beams in which you wanted to achieve the same dihedral of a curved beam design you could make the float of slightly shallower draft or you could adjust the intersection of the beam/main hull interface. Dihedral can be useful from a performance perspective as it elevates the windward float and beam to a position where they are less likely to be hit by waves which causes drag. Trimarans also generally cant the float hulls to an angle of around 15 degrees to account for their positioning when heeled this will increase the impression of reduced draft in a profile drawing.

    Float shapes vary with their intended purpose a performance focussed design would probably have a more full semicircular profile to minimise wetted surface area. A more comfort oriented design might have more of a parabolic shape which allows for a less rapid take up of buoyancy and a more comfortable motion once again everything depends on what you are trying to achieve.
     
  8. MAL1968
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    MAL1968 Junior Member

    So a float with 160% bouyancy must displace 160% of the design displacement of the whole boat when fully immersed. Is that right?
     

  9. Corley
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    Corley epoxy coated

    Thats correct.
     
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