Designing a solo 16' Cruising Cat(amaran)

Discussion in 'Boat Design' started by LP, Sep 11, 2015.

  1. claydog
    Joined: Dec 2010
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    claydog Junior Member

    Very common in my neck of the woods for pontoons.

  2. LP
    Joined: Jul 2005
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    LP Flying Boatman

    All good comments, Guys. I really appreciate everyone's thoughts here.

    Here is an initial stability study for Soloist.

    Yellow is my initial design.

    Orange is with an eight foot beam.

    Red is with roughly 8'4" beam. I'm leaving room for a rub strip.

    Blue and Green are a couple of mono designs I'm playing with.

  3. Ilan Voyager
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    Ilan Voyager Senior Member

    "Why not 20 instead of 18......why not 22 instead of 20........ It's a viscous circle and I'm too early in the design to start pushing outside my intended design envelope. I'm building a 24' mono right now where a 20-21'er might have been enough. As you know boats cube with length, but maybe not quite so if it is only the amas that are growing. 16 is my chosen length....for now. 18' is not unreasonable and even 20 if I were only extending the amas, it would do wonderful things with regard to accommodations and rig. It would move the CLR forward and give me better placement for the rig. Still very early in the process."

    Experience and calculations. Law of scales; volumes increase at cube, surfaces at square... After all I'm naval engineer, I have done miles and miles on small cats from force 0 to 6-7, I have designed a good dozen of small multis so I got some idea of what does not work. Curiously I agree with a bunch of pro NA of multis.
    A slow multihull it's the worst thing, if you sacrifice habitability compared to a mono of same size, it's in the purpose of going fast. Fast is fun. A slow cat that stays scotched in light weather because it has too much wetted surface and weight, and not enough sail is a pain. Worst if it tacks like a sick snail. 90% of coastal cruising is done in light weather, sometimes with tides and currents, and always being too close to the coast. On sea the land is the danger. You have to optimize the cat for that.
    If you build a boat with a resale value close to zero, you make it simple, light and fast to build. Unless if you like to spend years in your shop glassing, sanding, puttying and resanding the corners.

    18 feet instead of 16 can be done with the same width and similar hardware. With care with a very similar weight. BUT you have added 2 feet in the middle, it's a huge increase in volume with little weight added, that helps a lot in a cat.
    2 feet in a small cat it's about 1 knot of mean speed at force 2-3 with a better longitudinal stability. Add an increased security as now you can handle securely a force 4, a common situation in summer under a big cumulonimbus. The problem on small cats resides in the diagonal stability and longer is better.
    It's a size where you can get tons of used high quality hardware in excellent shape because there are plenty of regatters who renew the hardware. As you are using a trailer 18 or 16 does no change to put it in the water, you have received excellent counsels about that. So it's ridiculous to do not go to 18 feet. Except if you have by religion a restriction about the cipher 18.
    18', 4'6" is a good a common size for doing other than making circles in front of the beach. Not too small, not too big. You can sail it alone, a crew won't be a problem. Ask yourself why they are so many guys on 18 feet cats.

    "I would like to hear why it is useless and dangerous; emphasis on dangerous. I just can't agree with the tent notion. Apologies."
    Useless because you get a bulbous appendices which it is too small for giving a real shelter on such a small boat. Note that no professional NA of good reputation like Woods, Hughes, Kelsall or others propose plans of a 16 or 18 cruising catamaran. The smallest is at 21 feet, and there are excellent reasons for that, it's the size where you begin to be able to put accommodations for 2 persons without jeopardizing the sailing qualities and security... Note that a 21 feet trailerable Acorn by Woods weights just above 800 pounds, has a displacement of 1600 pounds and uses a Hobie 18 rig. It's simple to corners. Have a look on the PDF of the Acorn I have joined. It's made by a pro. Look at the scantlings...And over all it's a safe true small cruising cat, you can handle a bad wind or a short thunderstorm so common at the end of the afternoon in summer without calling the coast guards, and you can sail. It's not a box disguised in catamaran.

    1- windage, enormous surface on a very small boat. Cats have already a lot of windage, no need to add more on a poor little thing. And FLAT, rounded is nice but it the best way to end in the water.
    2- Non structural useless weight at the wrong place, giving a higher center of gravity. On a big cat, yes it can be used as structural beam, no need on a small cat.
    3-it interferes with an efficient sail plan, and obliges to put it higher or to sacrifice the sail surface.
    4-it interferes and restrains the crew movements. On a 16 or 18 feet cruising cat the crew represent at least 1/4 to 1/3 of the total displacement, and has to move constantly to keep the lateral and longitudinal equilibrium if they want to go faster than an optimist. No hope to stay static on a small cat.
    If you want to go upwind you have to move the crew forward and put the weight in alignment with the center of sail. It's primordial in coastal cruising to have good performances upwind, that can save your life.

    "but if I can get double the speed of a mono of the same length"
    Nope with your actual design and options or you have to compare with very old designs. You will have the greatest difficulties to follow a modern 18' coastal cruising mono of the French school. These little beasts weight 800-1000 pounds, use a spi, a full battened mainsail, do plane easily, tack in seconds and go very well upwind. Have room for two. And are simpler and cheaper to build...

    I do understand your desire to do your own design. But for that, you need theoretical knowledge and most important practical knowledge in building and navigation. And that it's acquired by sailing on similar boats. If you feel easy on a 18 feet beach cat with a solid 20-25 knots wind and having fun making runs and jumping the waves, you can go to next step, you know what is needed to sail a small cat, sport or cruising safely far from the assistance boat of the club. You know how to get speed and you know the limits.

    Before spending hours on a dead end design have a look on what propose reputable NAs on small cruising cats.

    Or follow my advice with a modified Hobie 18 or similar. With a tent like did N. Herreshoff on Amarillys. He had some knowledge about boats and coastal cruising...I can insure you that such a simple modified Hobie is great fun. Very safe as you can, with a some tricks, right up it alone. We capsized it voluntarily, until we were able to get it We did many times from 20 to 100 NM a day, dry and comfortably sat with chair-backs, yes!!! And the night a good tent with 6 feet headroom, 2 hammocks and as we are hedonist French, the cooler with the wines, bread, cheeses and smoked duck. And a good hot Ethiopian coffee at the end...

    Attached Files:

  4. LP
    Joined: Jul 2005
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    LP Flying Boatman


    Terrific exposé. Thanks for taking the time to write such a lengthy reply. You just may have convinced me to stretch the design to 18'. I would admit that in the whole build scheme, the extra work and materials in minuscule and the benefits are worthy.

    What was your 4'6" reference? I can't connect it with any of your other comments. Thanks.

  5. Ilan Voyager
    Joined: May 2004
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    Ilan Voyager Senior Member

    You're welcome, sorry I made a typo mistake it's 8'6" or 102" =2.59m. it's simply the maximum width of a trailer in a lot of countries. Check your local laws to not have a (bad) surprise. Most 18" cats are around 2.28m to 2m50 (I prefer metric...).
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