A Cat Hull profile

Discussion in 'Multihulls' started by L. Hutchins, Sep 30, 2006.

  1. L. Hutchins
    Joined: Aug 2006
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    L. Hutchins Junior Member

    Hello All.

    My recently acquired A-Cat is going into the shop soon so I have the opportunity to change things over the winter. It is a 1983 ( I think) design by Gougeon Brothers Inc. It has a rocker which is placed far aft and sweeps up dramatically near the transom. Newer designs seem to be much flatter in this area. Is there any advantage to be gained by adding to the boat in this area to flatten the profile? Is the weight penalty just too big to bother?

    See Attached picture.

    Leigh
     

    Attached Files:

  2. farjoe
    Joined: Oct 2003
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    farjoe Senior Member

    I wonder why none reacted to this very interesting topic.

    From a non expert point view it would seem that a straight keel from bow to transom would be ideal for load carrying purposes as well as minimising pitch.

    This would tend to make tacking very hard so a sharp rocker at the transom as suggested and perhaps a shallower rocker at the bow would allow the crew to move back to clear the bow during a tack and speed up the tack.
     
  3. Retired Geek
    Joined: Mar 2006
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    Retired Geek Junior Member

    Other rocker designs


    Attached are some current design lines plans, being Flyer and the last Boyer designed hull so you have a reference to what goes reasonably well now. At a regatta last weekend, hulls with similar shapes to yours were sailing competitively against Flyers, more so in the heavier air, so there is not much in it really in terms of performance gains, is the driver skills are left out of the equation. At a guess I'd say get a new sail, perhaps a carbon mast and beams and you'll see way more gains than messing with the hull shape.
     
  4. Retired Geek
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    Retired Geek Junior Member

    the other hull

    this is flyer
     
  5. Retired Geek
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    Retired Geek Junior Member

    another try

    another shot at attaching smaller files
     

    Attached Files:

  6. L. Hutchins
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    L. Hutchins Junior Member

    Thank you very much!

    Now some other questions. If I replace the main and rear beam with carbon where should I put them? Right now the main beam is 6" ahead of the center of the boat. It seems to me (from pictures) that beams and rigs are moving back and rudders are getting longer and more highly loaded to increase the efficiency of the entire boat.

    Allot of pictures show a twisted boat under load (windward hull bow up). Is this a problem? It seems that the dual forestay arrangement contributes greatly to this. Would it be smart to create a tall bridal system to make sure that both hulls were sharing some of the load from the main sheet? A tall bridal would not bend the hulls together very much but would also ensure that the leeward hull had some pull on it.

    Lastly foil profiles. Do you know what is being used down there in NZ and AUS? I have been advised that a NACA 0012 is a good choice and I recently saw a post from Steve Clark who likes a NACA 63-010. Do you use different foils for rudder and main foil?

    All replies appreciated.

    Leigh
     
  7. Retired Geek
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    Retired Geek Junior Member

    If I was you, I'd leave the beams where they are unless you want to rebuild all the supports under them (requires removing the deck at a guess).

    Hull racking once you get to a crbon beam isn't a big issue so long as they are attached firmly (as in glued in place or taped in place).

    63-010's are fine as a section and you'll probably find someone like waterrat or similar that make the boards and rudders for a price.
     

  8. Erwan
    Joined: Oct 2005
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    Erwan Senior Member

    Hi Retired Greek, Hi Leigh

    Thank you very much for posting Flyer/Goodall hull sections, I was curious to see that for a while.
    Did you already try to use these hull offsets with Michlett/Godzilla software ? I had the opportunity to observe real life hull performance with F18 and it seems to validate and confirm optimization outcomes provided by Léo in his very interesting workpapers available on Cyberiad.net
    So I think it could be very insightfull to run these 2 hull shapes with this great software.

    Leigh, for your forestay issue, I can tell you that a few yaers ago, Goodall/Boyer A cat had 2 single forestays but a stiff hull platform compared to Bim Italian hull platforms which used a single forestay for the first meter on the top and then splitted in 2 . It seems to be less stressing for the platform. Conversly, Marstrom with his full carbon ultrastiff hulls also used the Bim solution. I hope it could be helpfull.
     
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