Catamaran tunnel design

Discussion in 'Multihulls' started by dsigned, Aug 28, 2017.

  1. dsigned
    Joined: Aug 2017
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    Location: United States

    dsigned O.R.C. Hunter

    So, the other question I have is why there seems to be a preference for symmetrical hulls (or close to symmetrical hulls). If we think of the hulls as an aperture (like a pipe), rather than as independent (which they are not), then it seems like, for all flow rates, you would want to minimize the difference between the narrowest point and the aperture (where the bows separate the flow).

    Understandably, the water isn't constrained in the z axis the way it is in a pipe, but forcing the water down would lift the boat, and forcing it up would create drag, and it seems that in a displacement hull both of these would be somewhat undesirable.

    So the question is: is the preference for symmetry due to ease of construction? Ease of design? Stability? Convention?

    EDIT: I may have figured out the answer to this one all by myself! Seems as though the reduction in size simply accelerates the water, which then decelerates again as the aperture grows, and the drop in pressure is negligible. That's for pipes, anyhow.
    Last edited: Aug 28, 2017
  2. David J Ritchie
    Joined: Jan 2018
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    Location: Victoria BC Canada

    David J Ritchie Junior Member

    Symmetrical tunnel hulls are net worse

    An asymmetrical tunnel hull is like a monohull with a tunnel cut into it that pushes the wake out like a monohull

    But symmetrical hulls create 2 inside wakes they clash and create extra drag.

    At some scale the width of the sponsons adds too much drag for full asymmetrical but even then mostly asymmetrical is more efficient.

    All the fastest tunnel hulls are asymmetrical from turbine offshore tunnel cats to dinky little thunder cats
    dsigned likes this.
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The answer is, it depends. Certainly in planing power cats, the symmetrical form is considered more seaworthy, tracks better, basically goes where it is pointed, whereas under some circumstances, the asymmetrical form, can go feral. Obviously the race boats are all the asymmetrical form, because there is less drag, but they are typically pretty low deadrise hulls too, so there isn't a huge amount of water being displaced more to one side than the other, setting up an unwanted steering effect.

  4. David J Ritchie
    Joined: Jan 2018
    Posts: 27
    Likes: 3, Points: 3
    Location: Victoria BC Canada

    David J Ritchie Junior Member

    The deadrise angle is very high on tunnel hulls... did you mean bow hight to boat length aspect ratio?

    Would the unwanted steering effect you describe be the same or worse in a monohull?... if so i agree
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