24ft Bluewater cat sketch

Discussion in 'Multihulls' started by dsigned, Dec 18, 2018.

  1. dsigned
    Joined: Aug 2017
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    dsigned O.R.C. Hunter

    Loosely inspired by MingMing II, I wanted to make something that would be a good cruising boat underway, and in a variety of conditions. I've come to believe that most modern cats are overly beamy, so I tend to aim for a 3:1 L:B ratio as opposed to a 2.2:1. The idea is to have a cat that's sea kindly, quick, and that can fit into a standard monohull berth. This cat in particular is roughly the same dimensions as a J-24 (albeit quite a bit lighter).

    Screenshot_2018-12-18_20-32-24.png

    You'll notice the bottom looks flat from the side. This is somewhat deceptive as the overall shape isn't supposed to be flat. I think it would be interesting to test a couple of different hull types to see which was more pleasant to sail. The first would be something akin to an axe bow (that's the complicated version). The other would be something akin to a Bolger Sneakeasy hull, but with a little more of a lifted rear to help keep the boat from hobby horsing.

    Screenshot_2018-12-18_20-33-03.png

    Same cat from above. It has a full queen bed in front of the pilothouse, with the top of the sleeping area functioning somewhat like a desk for the captain. You'll notice the hulls aren't nearly as pointy as is typical. That's for two reasons. First, to make it easier to construct. Second, to maximize interior volume while minimizing the overall length and width. The goal would be to ape the shape of large cargo carrier vessels. I'd be interested to know what the drag penalty is for not going with a pointier bow.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    The drag penalty from close-spaced hulls would be worth investigation, and the safety of a narrow beam cat under sail ought to concern you too.
     
  3. dsigned
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    dsigned O.R.C. Hunter

    Drag penalty is significantly less than "being a monohull", so whatever penalty there is won't outweigh the advantages versus a beamy cat (cost of ownership or seakindliness depending on what kind of cat you're comparing it with). Also, the hull interference will be less than you think because if the boat is heeling even 5 degrees, one hull is generating substantially less wake (and is substantially less subject to wave interference). I'd love to see the standard wave interference diagrams for the hulls of a catamaran at a "normal" level of heeling sailing upwind (or on a broad reach).

    The safety of a catamaran as far as beam is concerned is a function of the sail, which would be sized according to righting moment, not length. So again, not an actual problem. If it's helpful, don't think of it as a narrow beam cat. Think if it as a cat with 8ft beam and roughly 750 lbs of hull acting 3.5 ft from the center. Size sail appropriately for expected conditions. It should still have greater righting moment than the aforementioned J-24, and be lighter. So if we assume a similar sail area, it will be faster.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Your rig will have to be greatly reduced compared to a wide beam cat, though, to have comparable resistance to capsize, so you are losing a lot of sail drive there.
     
  5. dsigned
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    dsigned O.R.C. Hunter

    It will be exactly the same as a wide beam cat with the same righting moment.
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    The wide beam cat with similar demi-hulls will have much greater righting moment, though. Sail cats don't have wide beam for no reason.
     
  7. dsigned
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    dsigned O.R.C. Hunter

    Yes, and a wide beam cat with similar demi hulls wouldn't fit into a standard monohull slip, would be significantly more expensive to build, significantly harder to handle, and wouldn't be trailerable without some sort of way to make the cat narrower.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    The idea of a trailerable cat with a powerful sail rig makes me nervous.
     
  9. Niclas Vestman
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    Niclas Vestman Junior Member

    The Gougeon 32' cat is close to 3:1 ratio footprint. Google it. A lot of interesting stuff avaliable online.
     
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  10. dsigned
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    dsigned O.R.C. Hunter

    It doesn't have to be "powerful" by catamaran standards. Stick a J-24 rig on it and it would still be faster and more stable than J-24. And again, I think it's better to compare it to another cat with similar beam. A Richard Woods Saylon 20 has a beam of 8 ft and about 20 sq meters of rig. A J-24 has a 24 sq meter rig. So just imagine a slightly longer Saylon with slightly more canvas.
     
  11. dsigned
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    dsigned O.R.C. Hunter

    I really like the Gougeon 32. But they're pretty rare, expensive and don't have much in the way of accommodations. None of which is a knock against the design. It does exactly what it was designed to do (which was primarily showcase the fiberglass techniques used in its construction). It also somewhat belies the idea that a higher length to beam ratio is "slow".
     
  12. Niclas Vestman
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    Niclas Vestman Junior Member

    Yes, the G32 has proven it self. Also the most comercially succesful cruising cat of all times, by a large margin has a unusually narrow footprint. 2,5:1. The Gemini, 105/M/MC/X and the 35. Kurt Hughes also has a new 26' quite narrow concept in the world.
     
  13. Niclas Vestman
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    Niclas Vestman Junior Member

    I think one of the biggest issues to solve is that none of the mentioned cats is intended for ocean crossing (even if skilled risktakers have crossed the Atlantic on small catamarans and even beach cat type vessels). Also as you mentioned, there are bridgedeck issues. Gemini has very low clerance and barely full standing headroom. And that is a 50% longer boat than the proposed. Most small cat interiors consist of standing height in the hulls where combined with bridgedeck. But only enough headroom for sitting or sleeping in the central bridgedeck area.
     
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  14. Niclas Vestman
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    Niclas Vestman Junior Member

    I guess accomodation issues make most opt for a trimaran for up to 30'. Most considder there to be a very clear and well founded length limits for where each form factor makes sense. On the other hand there is some discusion if a mini transat scow bow, with concave bottom shape, might be a future hybrid between mono and cat. That to me is a very interesting idea.
     
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  15. Richard Woods
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    Richard Woods Woods Designs

    My Saylon design is a Cat D boat, the lowest possible suitable for lakes, rivers, the ICW in the USA etc. I wouldn't even take it across the English Channel, nor to the Bahamas from Florida (which is a much shorter and easier trip). It is simply too small and narrow

    Lots of the G32 have capsized BTW

    RW
     
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